thermal characteristics of wisconsin headwater streams

Transactions of the American fisheries Society 123:641-656, 1994American Fisheries Society

Thermal Characteristics of Wisconsin Headwater StreamsOccupied by Beaver: Implications for Brook Trout Habitat

GIL McRAE1

Minnesota Cooperative Fish and Wildlife Research Unit2

University of Minnesota Department of Fisheries and Wildlife1980 Folwell Avenue. St. Paul. Minnesota 55108, USA

CLAYTON J. EDWARDSU.S. Forest Service. North Central Forest Experiment Station

Post Office Box 898. Rhinelander. Wisconsin 54501. USA

Abstract.—Expansion of populations of beaver Castor canadensis in northern Wisconsin hasraised concerns over warming of coldwater fish habitats as a result of impoundments created bythe mammals. We examined temperature with a network of electronic thermographs that recordedhourly water, air, and soil temperatures on four headwater streams occupied by beaver duringsummer 1990 and 1991. Stream temperatures followed air temperatures, even near groundwatersources. There was no consistent relationship between size or number of beaver impoundmentsand the degree of downstream warming. Large impoundments, although often warming down-stream temperatures slightly, dampened temperature fluctuations immediately downstream. Localgroundwater inflow and vegetative and topographic shading also dampened warming by impound-ments. Several beaver impoundments were removed to evaluate ensuing temperature changes.Removal of beaver dams did not generally reduce the difference between upstream and downstreamtemperatures; in some cases dam removal increased the warming rate. Direct thermal benefits ofdam removal in headwater streams may be outweighed by the potentially disruptive effects on thecomposition of fish and invertebrate communities downstream. It is suggested that managementfocus on relating topographical and geographical attributes to the potential for substantial ground-water discharge and to suitable summer temperatures for coldwater species such as brook troutSalvelinus fontinalis.

Maximum summer water temperature is the (Barnes and Wagner 1981), provides habitat forsingle most important factor limiting the geo- beaver Castor canadensis, and beaver have re-graphic distribution of brook trout Salvelinus fan- sponded by colonizing headwater streams in largetinalis (MacCrimmon and Campbell 1969). In numbers (Avery 1992). Beaver impoundmentsWisconsin, at the southern edge of the brook trout's provide habitat for other wildlife, particularly wa-endemic range, thermal constraints restrict the terfowl (Renouf 1972; Brown and Parson 1979).species to headwater streams, where they rely on Warming of downstream reaches by beavergroundwater discharges for maintenance of suit- ponds is commonly cited as a detrimental effectable temperatures (Becker 1983; Meisner 1990). of beaver on trout populations (Knudsen 1962;Habitat destruction due to logging, agricultural Avery 1992). Stream temperatures were typicallydevelopment, and pollution, and the subsequent not the primary interest in studies of beaver andstocking of more tolerant brown trout Salmo trut- trout interactions, however, and the literature hasta and rainbow trout Oncorhynchus mykiss, have established no clear relationship between differentfurther restricted the range of brook trout in the sizes or numbers of impoundments and the degreestate (Brasch et al. 1958; Avery 1983). The prox- of stream warming. After removal of beaver damsimity and abundance of aspen Populus tremu- on a headwater stream in northeastern Wisconsin,hides, a tree species that rapidly becomes estab- Avery (1992) observed 0.6°C (after 1 year) andlished following the removal of old-growth forests 2.5°C (after 4 years) reductions in the difference____ between peak summer temperatures at stations in

the headwaters and those at the mouth. Evans1 Present address: Department of Zoology, North Car- (1948) measured temperatures at the inlets and

76 Tl S^Universily' Ralcigh' North Carolina 27695' outlets of beaver ponds in northeastern Minnesota2 Cooperators: Minnesota Department of Natural Re- during July and 1 week in August. Of the 10 ponds

sources. National Biological Survey, University of Min- included in the study, eight had higher tempera-nesota, and the Wildlife Management Institute. tures at the outlet than at the inlet (increases of

641

642 Me RAE AND EDWARDS

3-14°C), but two ponds had consistently lowertemperatures at the outlet than at the inlet; de-creases were ascribed to the shading effect of veg-etation overhanging the outlet. Rupp (1954) com-pared water temperatures before and after beaverdam removal in and at two distances downstreamfrom five beaver ponds on Sunkhaze Stream,Maine. He recorded average surface water de-creases of 7.2°C at the pond sites, 6.7°C at 46 mdownstream from the dams, and 2.8°C at 402 mdownstream from the dams. Patterson (1951) col-lected temperatures in August 1949-1950 at theoutlets of nine tributaries to the Peshtigo River,Wisconsin, including two streams (Rock and Hal-ley Creeks) in our study. The streams containingbeaver ponds averaged 6.7°C warmer than streamswithout ponds. The streams without ponds wereshorter (mean length, 1.9 km) than those withponds (mean length, 4.8 km); shorter springstreams are more likely to be influenced bygroundwater and less likely to be warmed beforereaching the mouth (Ward 1985). Salyer (1935)observed that beaver ponds of 0.2 ha or more innorthern Michigan often caused a rise in the tem-perature of a stream, but that elevated tempera-tures rarely persisted more than 400 m down-stream from the pond.

We conducted a detailed temperature study onfour headwater streams in northern Wisconsin oc-cupied by beaver with the following objectives:(1) to examine the relationship between air, soil,and stream temperatures; (2) to evaluate the ther-mal effects of existing beaver impoundments; and(3) to evaluate the effect of dam removal on streamtemperatures. Our analyses focused on within-yearcomparisons to provide insight regarding thermalhabitat suitability for brook trout in terms of thespecies' lethal temperatures, preferred tempera-tures and metabolic optima (Fry 1971).

Study SiteFour streams were examined in the Peshtigo

River watershed in the Nicolet National Forest innortheastern Wisconsin (Figure 1). Rock Creek ispredominantly a sedge meadow (Carex sp.) andlowland shrub stream divided into east and westbranches, with a total stream length of 11 km. Thecreek flows through a wide valley (0.25-0.75 km)over much of its length with an average gradientof 2.4 m/km. In June 1991, the average maximumdepth in Rock Creek (exclusive of impoundments)was about 0.5 m, and average discharge near themouth was 0.2 m3/s. The entire watershed (17km2) is forested with a mixture of hardwoods and

conifers; aspen is particularly abundant in mostareas. Beaver have been reestablished in RockCreek since the early 1950s (Patterson 1951); in1990-1991, several large beaver ponds were pres-ent on both branches. In 1990, the west branchcontained one 9-ha pond and an additional damwhich, despite having been recently breached, im-pounded a small pool. The east branch had fourmajor impoundments. The two uppermost damsimpound 3-5 ha each; two large ponds (10-12 haeach) occupied much of the middle portion of theeast branch (Figure 1).

Halley Creek flows through a relatively narrowvalley (average gradient, 2.5 m/km) and is wellshaded over much of its 7.6-km length (Figure 1).In June 1991, maximum depth was about 0.3 m(exclusive of impoundments), and average dis-charge at the mouth was 0.2 m3/s. The vegetativecover consists of mixed hardwoods and conifers,and aspens are abundant near the stream. Betweenstudy sites H4 and H6, the valley widens consid-erably and there is little vegetative canopy. Por-tions of Halley Creek upstream from the upper-most impoundment had no surface water flow aftermid-July, although flow was substantial in Mayand early June. Numerous springs enter HalleyCreek, several of which flow intermittently aboveground through dense stands of northern whitecedar Thuja occidentalis before reaching thestream.

No Name Creek 1 (average gradient, 3.3 m/km)flows into the Rat River and was sampled onlyduring 1991 (Figure 1). The creek is 1.6 km longand was dominated by a 12-ha beaver pond im-mediately upstream of site N1A. Upstream fromthis pond, the stream is rarely confined to thechannel and is extensively braided.

No Name Creek 2 (average gradient, 3.0 m/km)empties into Otter Creek and was also sampledonly in 1991. The creek is 3.2 km long and wellshaded only in the vicinity of site N2A; there islittle vegetative shading over the remainder of thecreek due to a sequence of four beaver ponds be-tween sites N2A and N2C (Figure I).

There were resident brook trout populations inRock and Halley creeks, according to unpublishedelectrofishing surveys conducted by the WisconsinDepartment of Natural Resources (WDNR) in1985-1986 and the U.S. Forest Service in 1992-1993. Most brook trout in Rock Creek were foundabove the uppermost dam on the east branch andnear the mouth. Brook trout were found through-out the middle and lower sections of Halley Creek.No other salmonids were found in these surveys

THERMAL EFFECTS OF BEAVER DAMS 643

32'30- 88°30<00'

E. Rock Creek

Thermograph Sites:n WaterA Air0 SoilBeaver Ponds:O > 8 hectaresO 4 - 8 hectareso < 4 hectares

N1A* To Peshtigo

No Name Creek #1

Otter Creek

FIGURE 1.—Beaver impoundments and thermograph locations in the Nicolet National Forest, Wisconsin. Sitesenclosed in parentheses were sampled only in 1990; those with an asterisk were sampled only in 1991. Blackenedimpoundments were removed.

and fisheries information was not available for theother two study streams. There is no record ofslocking in any of the four streams, but brooktrout, brown trout, and rainbow trout have beenslocked in the Peshtigo River (WDNR 1980).Fishing pressure has been low on all the sludysireams due lo limited access.

MethodsData collection and estimation. —Temperatures

were collected with chart-type thermographs (Ryanmodel J-90) and electronic recording ihermo-graphs (Ryan model RTM daialoggers) pro-

grammed for 1-h sampling intervals. The chart-type Ihermographs were used on Rock Creek(stations R1-R6) in 1990, whereas the remainderof the data were collected with dataloggers. Bothtypes of thermographs were accurate to 0.3°C, ac-cording to laboratory calibrations, and recordedto the nearest 0.1 °C.

Thermographs were placed in steel canisters onihe siream boliom generally immediately up-stream and downstream of beaver impoundmenis(Figure I). The lack of a well-defined stream chan-nel often caused us to include areas of braidedstream as well as one or more beaver ponds be-tween upstream and downstream thermograph


pairs. Soil temperature measurements were takenat two stations during 1990-1991, one at lowerRock Creek and the other at upper Halley Creek(Figure 1); thermistors were inserted 1 m belowground level in a well-shaded area along eachstream. Air temperatures were recorded by therm-istors attached to the north side of a tree or shrub1 m above ground level close to the stream mon-itored.

Combined topographic and vegetative shading(percent) was estimated with the stream segmentshade model developed by Bartholow (1989).Based on the output of the shade model, each airand stream site was classified into one of four shadecategories: A = 0-24%, B = 25-49%, C = 50-74%,and D = 75-100%. The model uses latitude, streamazimuth, stream width, topographic angle, andseveral vegetation-related variables (height, crown,offset, density) as input variables. Stream lengthsand stream azimuths, which refers to the generalorientation of a stream reach with respect to duesouth (Bartholow 1989), were measured from 7.5-minute topographic maps. Impoundment areaswere estimated from a combination of field ob-servations and high-resolution aerial photo-graphs. Meteorological data were obtained from aU.S. Forest Service weather station (monitoreddaily at 1300 hours) 15 km northwest of the studyarea. Stream flow was calculated from streammorphology data and velocities measured with aMarsh-McBirney velocity meter by the methodof Platts et al. (1983). The remaining input vari-ables were measured or estimated in the field ac-cording to methods described by Bartholow (1989).

On August 14-15, 1990, one dam immediatelyupstream of site R5 on Rock Creek was removedwith explosives and 12 smaller dams (generallyless than 3 m wide x 0.5 m high) were removedby hand between sites R5 and R6. Between July15 and July 17, 1991, 36 beaver dams were re-moved with explosives from Halley Creek, andseveral smaller dams, many of which had beensubmerged in the ponds, were removed by hand.The impoundment immediately downstream ofsite H5 (Figure 1) was the only beaver pond leftundisturbed on Halley Creek. Beaver were keptoff both Rock and Halley Creeks during the studyperiod by trapping.

Four temperature ranges were defined to assessthermal habitat suitability for brook trout (Ra-leigh 1982): a lower range (<11°C), an optimalrange (11-16°C), an upper range (17-23°C), and alethal range (>24°C). The total number of hoursin which stream temperatures fell into each of these

ranges was calculated for each stream and springstation.

Water-air-soil temperature associations.—Therelationship between air, stream, and soil tem-peratures was examined through multiple-corre-lation analyses with 1991 data and subsequenttesting with an F-test. The 1991 data were moresuitable for these analyses because air tempera-tures were available from several sites with vary-ing degrees of shading. Local differences in airtemperature were taken into account in the cor-relation analyses by grouping each water stationwith an air station of similar characteristics (shadecategory and elevation).

Water temperature extremes typically lag 1-4 hbehind air temperature extremes due to the highspecific heat of water relative to that of the at-mosphere (Ward 1985). We characterized thedelayed response of water temperature to air tem-perature by calculating coefficients of determina-tion between air temperatures and water temper-atures lagged by successive 1-h intervals. The lagcorresponding to the maximum coefficient of de-termination for the water-air relationship (i.e., themagnitude of the phase shift between the two tem-perature traces) was used as a measure of the re-sponsiveness of the stream to air temperature ata particular site. Smaller phase shifts indicatedthat water temperature patterns closely mimickedair temperature patterns. The hourly water tem-perature data were then shifted backwards from0000 h by the amount of the phase shift, and 24-hmeans were calculated from the shifted "daily"temperatures. These shifted 24-h-period meanswere used in the correlation analyses with un-lagged daily mean air and daily mean soil tem-peratures as independent variables. Means froma 24-h period, rather than hourly temperatures or24-h maxima, were used in the correlation anal-yses for the following reasons. First, hourly tem-perature data exhibit a strong systematic effect (acharacteristic sinusoidal daily pattern) that can af-fect the precision of the estimated regression co-efficients. Secondly, although daily maximumstream temperatures are more important than dai-ly means in regard to coldwater fish habitat, weconsidered daily means more appropriate for usein correlation analyses because the relationshipbetween daily maximum stream temperature andair temperature is nonlinear (Theurer et al. 1984).

Temperature patterns and dam removal ef-fects.—Three response variables—daily meantemperature, daily maximum temperature, and thenoon rate of heating (°C/h; see definition below)—


were used to examine temperature patterns amongwater stations on each creek and water tempera-ture changes associated with dam removal. Thesemeans were not the phase-shifted means used inthe correlation analyses, but those calculated frommidnight to midnight. In addition, the differences(downstream minus upstream) in the three re-sponse variables associated with station pairs werealso used as response variables. Each station pairwas formed from thermographs separated by dif-ferent distances and numbers of impoundments(Table 1). Daily mean and maximum air temper-atures were used as response variables to examinedifferences associated with air temperature sta-tions. To control for day-to-day variation, we usedblocked analyses of variance (ANOVA, with daysas blocks) to quantify relationships among streamand air temperature sites. Following ANOVA, allpairwise differences were subjected to the Bonfer-roni significant difference test (BSD; Miller 1 98 1 ).

Temperature changes associated with dam re-moval were examined with two sets of two-wayfactorial ANOVAs. The first assessed the impactof dam removal on the sites along Halley and low-er Rock Creeks by fitting separate sets of n x 2factorial models (n sites x 2 treatments [beforeand after dam removal]) for each creek with thethree response variables mentioned above. Thesecond assessed the impact of dam removal onthe differences (downstream minus upstream) inthe three response variables associated with fourstation pairs on Halley Creek (Table 1). This sec-ond set of ANOVAs, for which the differencesassociated with the station pairs were responsevariables, consisted of three 4 x 2 factorial mod-els (4 station pairs x 2 treatments). Analyses wereconducted with the SAS®3 System, and statisticalsignificance was assessed at P < 0.05 for all tests.

Modeling of daily stream temperature cycles. —Daily stream temperature typically follows a si-nusoidal pattern due to the high specific heat ofwater, which buffers against rapid temperaturefluctuations. A sinusoidal time series model (Bing-ham et al. 1982; Chatfield 1989) was used to de-scribe this diel pattern. Hourly stream tempera-tures were modeled as

23y, ->+

i-O(i)

3 Reference to trademarks and trade names of man-ufacturers does not imply government endorsement ofcommercial products.

TABLE 1.—Distances between stations and total areasof beaver impoundments associated with thermographstation pairs on four northern Wisconsin streams. Sitelocations appear in Figure 1.

Station pair

Distancebetweenstations Impounded Shade

(km) area (ha) categories11

R2-R1R3-R2R5-R4H3-H2H4-H3H5-H4H6-H5N2B-N2AN2C-N2B

0.81.40.80.81.31.51.30.91.3

61987

1513101416

A-BA-AA-AC-CA-CA-AA-AA-DA-A

* Categories: A, 0-24%; B, 25^9%; C, 50-74%; D, 75-100%.

y, = water temperature at hour /;Mj = daily mean water temperature for day j\Aj = amplitude of temperature variation for

day./ (roughly one-half the range);o> = angular frequency (radians);// = hour(/ = 0, 1, . . . , 23);

</>7 = acrophase or time of maximum temper-ature on day y;

c>i = random error.Substituting o> = 2ir/24, the frequency correspond-ing to a period of 24 h, and expanding the cosineterm allows expression of equation ( 1 ) in linear-ized form:

23 »](2)

Equation (2) may then be rewritten by substituting

and7, =

and adding an additional term (Xv/, where v, = //- 11.5) that forces each daily temperature traceto match up more closely that of the next day. Insimplified notation, the final model has the form

yf = MJ + Xv/ + ffxi + (3)


TABLE 2.—Correlations of lagged water temperatures with air and soil temperatures at each stream thermographsite for 1991. The phase shifts indicate the number of hours water temperature was lagged to achieve maximumR2 in the water-air correlation, and they include differences in sampling times. All R2 values are significantlydifferent from zero (F-tests, P < 0.05) except those shown as NS.

Station combinationWater

RlR2R3R4R6R7R8Hl>Hlb

H2a

H2b

H3«H3b

H4a

H4b

H5a

H5b

H6a

H6b

H7a

H7b

Sla

Slb

S2a

S2b

N1ANIBN2AN2BN2C

Air

AlA3A3A3A3A2A4A5A5A5A5A5A5A5A6A5A6ASA6ASA7ASA6ASASA8A8A9A9A9

Soil

GlGlGlGlGlGlGlG2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2G2

Period of recordJul 19-AugOSJul 18-Sep ISJul 18-Sep ISJul 18-Sep ISJul 18-Sep ISJun22-Sep 15Jul 18-Sep ISJun 05-Jul 14Jul 18-Aug 19Jun 05-Jul 14Jul 18-Aug 19Jun 05-Jul 14Jul 18-Aug 19Jun 05-Jul 14Jul 19-Aug 19Jun 05-Jul 14Jul 19-Aug 19Jun 05-Jul 14Jul 19-Aug 19Jun 05-Jul 14Jul 18-Aug 05Jun 30-Jul 14Jul 19-Aug 19Jun 05-Jul 14Jul 18-Aug 19Aug07-Sep 15Aug07-Sep 15Aug07-Sep 15Aug07-Sep 15Aug07-Sep 15

Phaseshin (h)

2.33.54.64.63.73.71.84.15.14.34.33.92.93.41.23.11.92.58+c

2.8.7.1

0.1.4.4.5

0.81.76.63.3

Airpartial R2

0.850.790.590.080.680.720.750.470.480.460.420.220.240.470.180.780.490.800.570.840.780.660.670.650.710.730.780.820.510.34

Soilpartial R2

0.05NSNS

0.30NS

0.07NS

0.28NS

0.27NSNSNS

0.23NS

0.04NSNS

0.16NSNSNSNSNS

0.12NSNS

0.06NSNS

Multiple R2

0.900.790.590.380.680.790.750.760.480.740.420.220.240.700.180.820.490.800.730.840.780.660.670.650.840.730.780.880.510.34

a Before dam removal.b After dam removal.c The water-air R2 at this site increased as lags increased from 0 to 8 h; statistics listed are for the 8-h lag.

Using equation (3), we estimated the rhythmcharacteristics (the set of coefficients X, ft, and ydescribing the shape of the temperature trace) foreach day by ordinary least-squares regression* Thusthe shape of the temperature pattern for day j atany given location is accurately characterized, andthe temperature (y,) at any hour (//) may be ap-proximated from the ordinary least-squares esti-mators of X, /?, and 7. Hourly temperatures ap-proximated with equation (3) rarely differed morethan 0.1 °C from the observed temperatures. Be-sides simplifying data visualization and manage-ment, the time series model also allowed us tocalculate a rate of heating at any given hour bytaking the first derivative of equation (2) with re-spect to // (dy/dt), which is equivalent to the slopeof the line tangent to the temperature curve athour /. We chose to use the rate of heating at solarnoon (midway between sunrise and sunset) as avariable for comparison among stream thermo-

graph sites, because the rate of temperature changewas usually near its maximum at that time.

ResultsWater-Air-SoU Temperature Associations

Although there was little variability in meandaily air temperatures among the nine air stations(mean, 16.5°C; SD, 0.69) during mid-July to mid-September 1991, there were significant differencesin daily maximum air temperatures (F-test; P <0.0001) associated with shading differences. Meandaily maxima at air sites in shade categories Aand B (sites Al, A3, A6, and A7) were more than2°C higher than those in shade categories C andD (sites A2, A4, A5, A8, and A9). Soil tempera-tures remained within a range of 12-14°C for muchof summer 1991; site Gl averaged about 0.5°Ccooler than site G2.


Halley Creek July 7 - August 29.1990 Rock Creek June 29 - September 15 ,1990

No Name Creek 1 June 19 - September 15.1991No Name Creek 2 June 29 - September 15.1991

N1A N1B N2A N2B N2CSite

24

21 •

18 •

15 -

12 -

B A A A A A

llilllR1 R2 R3 R4 R5 R6

Site

Rock Creek June 1 - September 15,1991

0.80.6 I

°'4 I0.2 ^

0.0 —

R2 R3 R4 R6 R7Site

0.0 3:

D Average Maximum G Rate of Heating

FIGURE 2.—Average daily mean and maximum stream temperatures and average noon rate of heating at thestudy sites. Letters above bars represent shade categories (A = 0-24%, B = 25-49%, C = 50-74%, D = 75-100%),and error bars indicate one-half of the simultaneous 95% confidence intervals derived from Bonferroni significantdifferences. Sites are listed in downstream order from left to right and locations appear in Figure 1.

Air temperature accounted for an average of63% of the variability in lagged water temperature(exclusive of site H6 after dam removal; mean R2,0.63; SD, 0.20; Table 2). The phase shift betweenwater and air temperature ranged from 0.1 to 6.6h (exclusive of site H6 after dam removal; mean,2.81 h; SD, 1.49). The water-air phase shift couldnot be determined at site H6 following dam re-moval because dramatic cooling effectively damp-ened the sinusoidal nature of the water tempera-ture trace (discussed below). Phase shifts wereparticularly low and coefficients of determinationrelatively high for the two spring sites (SI, S2),where the entire flow was derived from subsurfaceinflow exposed to the atmosphere for a fairly shortperiod of time. The addition of soil temperaturesgenerally improved the fit of the multiple-corre-lation model for stations in the upper reaches ofRock and Halley Creeks (Rl, HI, H2, and H3),but offered little improvement for stations in thelower reaches of these creeks or for any of thestations on either of the No Name creeks. Onlyone station, R4 on the west arm of Rock Creek,showed a stronger association between water and

soil temperatures than between water and air tem-peratures (Table 2).

The removal of dams on Halley Creek was as-sociated with an increase in the responsiveness ofwater temperature to air temperature at sites H3,H4, H5, and H7, as indicated by the decreasedphase shifts (Table 2). The correlation betweenwater temperature and air temperature was sub-stantially less following dam removal at three sitesin the midreaches of Halley Creek (H4, H5, andH6). Only site H6 showed a higher correlationbetween air and soil temperatures after dam re-moval than before. Dam removal had no sub-stantial effect on either the phase shift or the fit ofthe correlation model at the two spring sites, whichare off the main channel of Halley Creek (Figure1).

Beaver Impoundments and Thermal RegimesLimited sampling on Halley Creek in 1990 in-

dicated that the thermal character of the mid-reaches was similar to that of the upstream section(Figure 2) despite the presence of five beaver pondsbetween sites H2 and H4. Daily mean and max-


O

Throughout Summer0.4

0.2

CDO

0.0

-0.2

(O

-0.4 3:R2-R1 R3-R2 R5-R4 H4-H3

R2-R1b R3-R2b H3-H2 H5-H4

Station Pair

H6-H5 N2C-N2B

N2B-N2A

Ten Warmest Days

O

IQ£I8.Q>

R2-R1 R3-R2 R5-R4 H4-H3 H6-H5 N2C-N2BN2B-N2AR2-R1 R3-R20 H3-H2 H5-H4

Station Pair

| D Average U Maximum B Rate of Heating |

FIGURE 3.—Average differences between downstream and upstream sites for thermograph station pairs on threeof the study streams throughout summer 1991 and on the 10 warmest days during the period of record. Error barsindicate one-half of the simultaneous 95% confidence intervals derived from Bonferroni significant differences (barswith asterisks are not significantly different from zero, P > 0.05). Superscript "a" on a station pair indicates 1990records; superscript "b" indicates 1991 data. Impounded areas and distances between stations are listed in Table1 and periods of record appear in Figure 2.

imum stream temperatures at these two sites weregenerally only 1-2°C higher than those at the springsite, S2. However, temperatures had increasedsubstantially, and the stream was warmed at near-ly twice the rate of the midreaches (as indicatedby the rate of heating: Figure 2) by the time thewater reached site H6.

The general thermal pattern was similar in RockCreek in 1990 and 1991 (Figure 2). In the east

branch, temperatures increased gradually fromsites Rl to R3, and the rate of heating at site R3,which was immediately downstream of a series ofthree impoundments, was significantly lower (BSD,P < 0.05) than that at site R2 immediately up-stream of the ponds. Stream temperatures in theupper west arm of Rock Creek (site R4) were sig-nificantly lower than those below the confluenceof the two branches (sites R6-R8) during both


years, and sites R4 (1990) and R5 (1990-1991)warmed at significantly lower rates than stationselsewhere on the creek.

Stream temperatures at the two stations belowthe large pond on No Name Creek 1 were verysimilar in summer 1991 (Figure 2). However, thedownstream station wanned at a slower rate thanthe upstream station.

In No Name Creek 2, temperatures were sig-nificantly higher at site N2B, which was directlybelow a pair of beaver ponds, than at either theupper (N2A) or lower (N2C) station (Figure 2).However, the downstream site (N2C), situatedimmediately below a similar pair of ponds, wassignificantly cooler than either of the upstreamstations. In general, the sites on No Name Creek2 warmed relatively slowly (<0.2°C/h), and theupstream station warmed slightly faster than theother sites.

Examination of the differences in temperatureassociated with nine station pairs (Table 1) re-vealed no clear relationship between either thedistance between stations or the intervening im-poundment area and the degree of downstreamwarming (Figure 3). This conclusion held true evenwhen a subset of the 10 warmest days (as definedby maximum daily air temperature) were consid-ered. Station pairs R3-R2 and N2C-N2B, despitebeing separated by similar distances and im-poundment areas, displayed opposite patterns ofdifferences in daily average and maximum tem-peratures. A similar phenomenon occurred withthe R2-R1 and the H3-H2 station pairs. Threestation pairs contained sites in different shade cat-egories (Table 1); in each case the downstream sitewas more shaded than the upstream site. In twocases (R2-R1 and N2B-N2A) temperatures weresignificantly higher downstream of the impound-ment^), whereas the stations in the third pair (H4-H3) were statistically identical both on the warmestdays and throughout the summer of 1991 (Figure3).

Only the Rock Creek station pairs, in which thedownstream stations were significantly warmer onaverage than those upstream throughout the sum-mers of 1990-1991, consistently showed warmingbelow impoundments (Figure 3). Differences inthe three response variables associated with theR2-R1 station pair were greater on the warmerdays, but average daily temperatures associatedwith the R3-R2 station pair were actually lowerdownstream during the warmest periods. Differ-ences associated with the R5-R4 station pair dis-played a pattern similar to those of the R3-R2

pair, with higher average temperatures and lowerrates of heating downstream of the impoundment.

The relationships within the four station pairson Halley Creek were less consistent than thoseon Rock Creek (Figure 3). In the H3-H2 stationpair, mean daily downstream temperatures aver-aged more than 3°C lower than the upstream sta-tion despite the two intervening impoundments.The stations of the H4-H3 station pair, which wereseparated by three impoundments, showed no sig-nificant differences in the temperature variables.The two station pairs on the lower reaches of Hal-ley Creek, H6-H5 (separated by two impound-ments) and H7-H6 (with no intervening impound-ment) showed significant downstream warmingand increases in the rate of heating.

The station pairs of No Name Creek 2 showedopposite temperature relationships even thoughthe distances between stations and the interveningimpoundment areas were similar. The middle sta-tion (N2B) warmed less rapidly than the upstreamstation (N2A), and there was no significant differ-ence in the heating rate between the middle andlower (N2C) sites.

Dam RemovalRemoval of the dam on the west branch of Rock

Creek in 1990 had no identifiable effect on themean daily average or maximum temperatures atsites R4 or R5 (Figure 4). However, daily averageand maximum stream temperatures decreased by1.6°C and 2.3°C, respectively, at site R6 followingdam removal. The heating rate at site R4, whichwas upstream of the former pond, increased bymore than a factor of seven following dam re-moval, but it decreased by more than 0.2°C/h atsite R6.

Average daily mean and maximum stream tem-peratures were cooler at all sites in the period fol-lowing the large-scale dam removal on HalleyCreek in 1991 (Figure 4). These differences weregenerally less pronounced in the upper reaches(upstream of site H3) than in the lower portion ofthe creek. Average heating rates before and afterdam removal were not significantly different forthose stations upstream of H5, but there were largedeclines at sites H6 and H7, where the averagevalues were 0.8°C/h and 0.2°C/h lower, respec-tively, in the period following dam removal.

Removal of the dams on Halley Creek was as-sociated with a decrease in the temperature dif-ferences in the H3-H2 and H5-H4 station pairs(Figure 5). However, the differences associated withthe H4-H3 and H6-H5 station pairs increased

650 Mi RAE AND EDWARDS

24

~ 21

Rock Creek Haliey Creek

ft

JillSite

H1 H2 H3 H4 H5 H6 H7

Site

| D Dams Intact M Dams Removed |

FIGURE 4.—Average daily mean and maximum stream temperatures and average noon rate of heating beforeand after beaver dam removal on Rock and Haliey Creeks, Wisconsin. Sampling dates were June 24-August 13,1990 (dams intact), and August 16-September 15, 1990 (dams removed), for Rock Creek, and June 5-July 14,1991 (dams intact), and July 18-August 19, 1991 (dams removed), for Haliey Creek. Sites are listed in downstreamorder from left to right, and error bars represent one-half of the simultaneous 95% confidence intervals derivedfrom Bonferroni significant differences.

slightly in the period after dam removal. The dif-ferences in average heating rate associated withthe Haliey Creek station pairs differed little beforeand after dam removal, except the downstreamstation of the H6-H5 pair warmed much less rap-idly than did the upstream station in the periodfollowing dam removal.

Thermal Habitat SuitabilityStream temperatures in Rock Creek were most

commonly in the 17-23°C range, although the two

uppermost stations on the east branch (sites Rland R2) maintained temperatures in the optimumrange for extended periods of time (Table 3). In1990, lethal temperatures occurred for at least partof the day at sites R2 and R6 several times in lateJune and early July, and temperatures remainedin the lethal zone for an entire 24-h period at siteR3 in early July. In 1991, the warmer of the twoyears, lethal temperatures occurred for at least partof the day at all sites in Rock Creek (Table 3).Elevated temperatures tended to persist longer at


D Dams IntactB Dams Removed

H3-H2 H4-H3 H5-H4Station Pair

H6-H5

FIGURE 5.—Average differences between downstream and upstream sites for four thermograph station pairs onHalley Creek before (June 5-July 14) and after (July 18-August 19) beaver dam removal on Halley Creek in 1991.Error bars represent one-half of the simultaneous 95% confidence intervals derived from Bonferroni significantdifferences. Impounded areas and distances between stations appear in Table 1.

site R3 and at the stations below the confluenceof the two branches (sites R6-R8). In general, thestations directly below impoundments (R2, R3,and R5) tended to maintain temperatures withina narrower range than did those stations imme-diately above ponds.

Temperatures at sites R4, R5, and R6 were gen-erally cooler and more variable following removalof the dam on Rock Creek in 1990. The percentageof time in which temperatures were in the opti-

mum zone increased substantially at these threestations following dam removal.

Stream temperatures in Halley Creek tended tobe lower and less varible in the middle portion ofthe creek (Table 3). In 1990, the two upper sta-tions (sites H2 and H4) maintained temperaturesin the optimum range for much of the summer,while temperatures at site H6 fluctuated in the 17-23°C range for much of July and August. Beforedam removal on Halley Creek in 1991, the two

652 McRAE AND EDWARDS

TABLE 3.—Percentage of time summer stream tem-peratures fell into one of four temperature ranges in fourheadwater streams in northeastern Wisconsin. The op-timal zone for brook trout is 11-16°C, and sustainedtemperatures above 24°C are lethal. Site locations appearin Figure 1 and sampling periods appear in Figure 2.

Thermo-graphsite

Total hourssampled

Percent of total hoursin temperature range (°Q:

<11 11-16 17-23 £24

Rock Creek 1990RlR2R3R4«R4b

R5»R5b

R6a

R6b

Total

It8961.8961,8961,104

7201,104

7441,104

74411,208

1 680 280 80 313 480 00 200 60 220 27

3169896949

10080887871

0330000602

Rock Creek, 1991RlR2R3R4R6R7R8Total

2,5682,5682,5682,5682,5682,5682,568

17,976

0 280 220 40 100 80 90 100 13

6972848984828380

36

1218977

Halley Creek, 1990H2H4H6Total

1,0081,0081,0083,024

11 891 990 254 71

00

7525

0000

Halley Creek, 1991Hla

Hlb

H2«H2b

H3«H3b

H4«H4b

H5»H5b

H6*H6b

H7«H7b

Total"Totalb

960792960792960792960792960792960792960792

6,7205,544

0 70 320 211 471 64

19 560 62

26 610 23

15 610 240 840 241 470 329 55

86647851352538137624731671516635

7411000010305121

No Name Creek 1, 1991N1ANIBTotal

2,1362,1364,272

0 20 40 3

778079

211618

No Name Creek 2, 1991N2AN2BN2CTotal

1,8961,8961,8965,688

0 270 00 510 26

73984973

0201

' Before dam removal.b After dam removal.

sites in the middle portion of the creek (H3 andH4) generally remained within the optimum zonelonger than the other stations. In 1991, lethal tem-peratures persisted for an extended period of timeonly at sites HI and H7, the uppermost and low-ermost sites, respectively. Lethal temperatures didnot persist for an entire 24 h period at any of thestations in Halley Creek in 1991.

Removal of the beaver dams on Halley Creekin 1991 affected the thermal character of stationsH6 and H7 most profoundly; the dramatic coolingeffect at site H6 resulted in a substantial increasein the duration of optimal temperatures (Table 3).These changes occurred even though the large im-poundment immediately downstream of site H5was left intact.

Periods of several days in which temperaturesreached the lethal zone were common at both sta-tions in No Name Creek 1 in 1991 (Table 3). Thethermal character of these stations was nearlyidentical throughout the summer, and tempera-tures reached the optimum zone briefly and onlyat night on a few occasions in late July, late Au-gust, and early September.

In No Name Creek 2, the upper (N2A) and low-er (N2Q stations generally showed cooler and morevariable thermal patterns than did the middle sta-tion (N2B; Table 3). Lethal temperatures werereached only for very short periods at site N2Bearly in the summer. Temperatures at site N2Bwere very stable, remaining in the narrow rangeof 17-23°C throughout most of the summer.

DiscussionAs water moves downstream, its temperature

seeks an equilibrium with air temperature, a pro-cess influenced by local environmental factors suchas stream shading and subsurface inflow (Sullivanet al. 1990). The rate at which water temperaturechanges as it approaches equilibrium depends onstream size (Edinger and Geyer 1968). Because ofthe high specific heat of water, large volumes ofwater change temperature relatively slowly.Downstream of large beaver impoundments, thestream channel was often wider and deeper thanin areas without beaver ponds. The lower rates ofheating associated with the stations immediatelydownstream of large impoundments (R3, R4, andN2B; Figure 2) may reflect the buffering influenceof increased volume. These same three stationsalso exhibited a great degree of thermal consisten-cy (Figure 2; Table 3) throughout the summer andwere less responsive to air temperature in general,


as indicated by the longer phase shifts and lowerair partial R2 values in Table 2. The bufferingeffect of increased volume is particularly impor-tant in low-flow situations. Anderson and Miya-jima (1975) constructed pools on a Coloradostream to buffer water temperature in periods oflow flow. Stream temperatures were 0.6-2.2°Clower and peak temperature duration (above22.2°C) was substantially shorter in the pools thanin upstream riffles.

The opposite effect of reduced volume is evi-dent in the correlation analyses for the two springsites on Halley Creek. The small volumes of waterin the spring channels, which were generally lessthan 1 m wide and 0.5 m deep, were very respon-sive to changes in air temperature, as indicated bythe short phase shifts and relatively high air partialR2 values (Table 2). Although water at these siteswas derived entirely from groundwater, it was rap-idly warmed by the atmosphere upon emergingabove ground. The lack of a ground term effect inthe correlation models for the two spring stations(Table 2) also supports the contention that theinfluence of the spring source diminishes quickly.Thus the cooling effect of concentrated ground-water discharges may not persist in downstreamreaches unless additional subsurface inflow or suf-ficient shading exists to temper local air temper-atures.

Air temperature is the single most importantdeterminant of stream temperature in the absenceof other thermal inputs (Bartholow 1989), and itis particularly important for small exposed streams(Ward 1985). This relationship is complicated inheadwater streams in summer because cooler sub-surface inflow typically contributes a substantialpercentage of total flow. We found generally strongpositive relationships between lagged water andair temperatures (Table 2) indicating that air tem-perature plays a dominant role in regulating streamtemperatures even when the thermal regimes aredriven by groundwater discharge. Addition of theground temperature term improved the fit of thecorrelation models only for sites at the upperreaches of Rock and Halley Creeks (Rl and HI-FI 3). Given the location of these generally well-shaded stations, it seems likely that a higher por-tion of the base flow at these sites was due togroundwater inflow that had not yet warmed tosurrounding temperatures.

Local differences in the degree of shading,groundwater inflow, and stream volume make itdifficult to generalize about the effect of beaverimpoundments on stream temperature, even

within the scope of a single headwater stream. Eachof the study streams had fairly unique longitudinaltemperature patterns. In Rock Creek, tempera-tures tended to be fairly constant among ther-mograph stations, gradually increasing in dailymaxima from headwaters to mouth (Figure 2).However, average maximum daily temperaturesdiffered by only 1.3°C between Rl and R8throughout summer 1991, representing an averageincrease of only 0.03°C/100 m of stream. This rateof increase is less than that observed between thetwo sites on No Name Creek 1, which were notseparated by any beaver ponds (Figure 1). In termsof daily maxima, site N1B averaged 0.7°C warmerthan N1A throughout the summer of 1991, whichtranslates to an average increase of 0.1 °C/100 mof stream.

Halley and No Name 2 Creeks generally exhib-ited a greater degree of thermal heterogeneity thanthe other two creeks. The uppermost stations weregenerally warmer than the lower stations on bothHalley and No Name 2 Creeks. The relatively poorcorrelations between air and water temperaturesat sites H1-H4 and N2B-N2C indicate that someother factor, most likely subsurface inflow, ac-counted for a large portion of the variability inwater temperature in the midreaches of HalleyCreek and the lower part of No Name Creek 2.These observations corroborate those of Threinenand Poff (1963), who noted that thermal discon-tinuity is common in Wisconsin trout streams.

The longitudinal temperature patterns in thestudy streams agree well with inferences based onsurficial geology. The amount of groundwater dis-charge to a stream depends on the area contrib-uting recharge to the aquifer and the rate of re-charge (Todd 1983). In northern Wisconsin,aquifers are often found along watercourses whereglacial meltwaters removed fine material and leftbehind permeable deposits of sand and gravel.These materials have high hydraulic conductivi-ties and thus recharge and release groundwaterrapidly (Todd 1983; Simpkins et al. 1987). In manylow-lying areas, these alluvial deposits are over-lain by a layer of peat, which tends to form wheregroundwater inflow far exceeds the outflow. Theseareas of peat, which generally lie at or near thewater table (Simpkins et al. 1987), proved to beexcellent indicators of groundwater sources, andthey related fairly well to the longitudinal tem-perature patterns on the study streams. Site R4on the west arm of Rock Creek, which is underlainby a large bed of peat (Simpkins et al. 1987),showed a stronger association between water and


ground temperatures than between water and airtemperatures (Table 2). Each of the lower two siteson No Name Creek 2 are underlain by beds ofpeat (Simpkins et al. 1987), which implies thatsubstantial groundwater input may have mitigat-ed any warming effect due to the four impound-ments. Similarly, large expanses of peat in themidreaches of Halley Creek (between H2 and H5)indicate that these stations were likely receivingsubstantial amounts of subsurface inflow in ad-dition to that originating from the two spring sites.

The potential for evaluating brook trout habitatsuitability based on geological principles has beenexamined in detail by Dean et al. (1991). Theseresearchers coined the term "geofisheries" to de-scribe an algorithm that determines brook trouthabitat suitability based on the geological struc-tures from which groundwater is derived. We didnot attempt such an evaluation in the presentstudy; however, the paradigm followed by thistechnique has potential applicability in the man-agement of trout streams occupied by beaver. Ageofisheries perspective could be used to identifythose streams or sections of streams that are likelyto have stable groundwater input throughout thesummer and thus are more likely to support brooktrout populations. Identification of combinationsof features, particularly the contiguous occurrenceof large areas of unsorted glacial till, which areimportant recharge areas, and well-sorted alluvi-um, which form excellent conduits to the streambeds (given sufficient hydraulic gradient), as wellas expanses of peat would indicate areas with thehighest potential for stable groundwater flows. Thethermal effect of beaver impoundments could thenbe more accurately evaluated in relation to thelocation of coolwater discharges.

The tendency for brook trout to seek out areasof cooler inflow for spawning (Brasch et al. 1958;Webster and Eiriksdottir 1976; Becker 1983) andto avoid high temperatures (Huntsman 1942;Gibson 1966) has been well documented. In thepresent study, we found maximum daily temper-ature differentials above and below beaver im-poundments to be quite variable, ranging from-2.1°C for the H3-H2 station pair to more than4.5°C for the H5-H4 station pair (Figure 3). Thesetemperature differentials appeared to be indepen-dent of intervening impoundment area and dis-tance between stations. In such heterogenous ther-mal environments, at least equal concern shouldbe given to the potential for beaver dams to blockaccess to coldwater refuges during the summer.Identification of these coolwater refuges through

a process analogous to the geofisheries algorithmwould undoubtedly lead to more focused brooktrout management on streams with substantialbeaver activity.

Removal of beaver dams, particularly those es-tablished for several decades, can have profoundeffects on the ecology of downstream reaches. Stockand Schlosser (1991) reported that a catastrophiccollapse of a beaver dam and ensuing flood re-sulted in more than a 90% decrease in benthicinsect density downstream and altered the struc-ture of the fish community by causing a short-terminflux of pond species. Mean insect density 1 dafter the collapse (1,632/m2) was only 8% of thepre-flood density, and 60 d after the flood meaninsect density in downstream riffles was only 62%of pre-flood values. These results, combined withthose of the present study, suggest that losses indownstream diversity may outweigh the thermalbenefits of dam removal in many cases. The large-scale dam removal on Halley Creek was associ-ated with a substantial thermal decline (>2.5°Cchange in average daily maxima) only at site H6.The temperature decline at site H6 occurred eventhough the large impoundment below site H5 wasleft intact. The localized nature of the temperaturedecline suggests that the cooling effect of subsur-face inflow, which may have been suppressed bythe pond upstream of H6, could have been re-sponsible for this anomaly. This suggestion is sup-ported by the presence of a band of peat that ex-tends from Valley Lake, a seepage lake about 0.5km south of H6 (Figure 1), to Halley Creek justupstream of H6. If this speculation is correct, thesame degree of cooling at sites H6 and H7 mayhave been attained by removing only the dam im-mediately upstream of H6, rather than all the damsupstream of H5.

Temperature comparisons associated withbeaver dam removal in this study were confound-ed with seasonal effects as air temperatures de-creased and soil temperatures increased towardthe end of the summer. Average daily maximumair temperatures (measured at A2) were slightlyless during the period following dam removal(mean, 21.6°C; SD, 3.49) than before (mean,22.0°C; SD, 2.65), but the difference was not sta-tistically significant (r-test, P = 0.18). Average dai-ly soil temperatures at Gl were slightly but sig-nificantly higher (/-test, P = 0.0001) after damremoval (mean, 11.7°C; SD, 0.54) than before(mean, 11.0°C; SD, 0.56). These factors, along withothers associated with seasonality such as decreas-ing day length, likely accounted for a portion of


the temperature decreases following dam remov-al.

The headwater streams in this study represent-ed typical, heterogenous thermal environmentsdominated by groundwater inflow and regulatedby local air temperatures. This heterogeneitycaused the thermal effect of beaver impoundmentsto be highly site dependent. Our results suggestthat large ponds act as thermal buffers, raisingdownstream water temperatures slightly in somecases, but also dampening the diel fluctuation. Itwas also demonstrated that dam removal was gen-erally not effective in reducing downstream tem-peratures substantially. However, following care-ful study of the geological and topographicalattributes of the watershed, judicious removal ofa few key beaver impoundments may have a strongeffect on downstream temperatures without theloss of diversity associated with large-scale damremoval. A thorough understanding of the attri-butes leading to surface expressions of ground-water and subsequent identification of the areasthat are important thermal refuges for trout willenhance the information base necessary for man-agement of beaver, trout, and waterfowl popula-tions along headwater streams.

AcknowledgmentsWe thank R. Newman, J. Perry, C. Bingham,

M. Henry, and C. Coutant for their reviews of themanuscript and J. Carter, K. Thiel, and D. Smithfor their assistance in field work. Funding for thisresearch was provided by the U.S. Forest Servicethrough a cooperative agreement with the Min-nesota Cooperative Fish and Wildlife ResearchUnit of the University of Minnesota.

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thermal characteristics of wisconsin headwater streams

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