JOURNALVOL. 33, NO.2

OF THE AMERICAN WATER RESOURCES ASSOCIATIONAMERICANWATERRESOURCES ASSOCIATION APRIL 1997

STREAMFLOW MEASUREMENT USING SALT DILUTION IN TUNDRASTREAMS, NORTHWEST TERRITORIES, CANADA'

Chris Spence and Michael McPhie2

ABSTRACT: With the recent increased exploration and miningactivity in the Northwest Territories, there has been growing inter-est in streamfiows. However, streamfiow monitoring in Canada'snorth is limited, especially in the central Northwest Territorieswhere the exploration activity is concentrated. To complicate mat-ters, the standard approach of measuring discharge with currentmeters or weirs is often inadequate or prohibitively expensive, asmany streams in the region are shallow, braided and rocky. Inresponse, alternative techniques such as salt dilution can be used.A salt tracer's competenco in turbulent and rocky channels makesit ideal for discharge measurements in these situations. This papersummarizes the work performed by Indian and Northern AffairsCanada and Canamera Geological Ltd. staff in determining thestream discharge of a lake outlet using a potassium chloride (KC1)tracer. A variety of streamfiow measurement methods were per-formed and compared to determine the viability and rigor of thedilution method. Results suggest the dilution method comparesfavorably to other measurement techniques both in accuracy andoperational ease.(KEY TERMS: Northwest Territories; streamfiow measurement;salt dilution; instrumentation.)

INTRODUCTION

With the recent increased exploration and miningactivity in the Northwest Territories, there has beengrowing interest in regional streamfiows. However,streamflow monitoring in Canada's north is limited,especially in the central Northwest Territories' SlaveGeological Province where the exploration activity isconcentrated. The standard approach of measuringdischarge with current meters or weirs is oftenimpracticable or prohibitively expensive as manystreams in the region are shallow, braided and rocky.Alternative techniques such as tracers, including elec-trolytic solutions, fluorescent dyes and radioisotopes,

may prove more suitable due to their competence inturbulent and rocky channels.

The recent intensive exploration activity for dia-monds in the Slave Geological Province has led to the'discovery of a number of diamond-bearing kimberlite"pipes". One of these pipes underlies Ranch Lake,located two kilometers southwest of the study area.Pre-development plans for Ranch Lake have resultedin the study area lake, Nisha, being designated as apotential mine tailings deposit area. This paper sum-marizes the work performed by Indian and NorthernAffairs Canada and Canamera Geological Ltd. staff indetermining the stream discharge of the Nisha Lakeoutlet. The purpose of such work was twofold. Toassist with the design of runoff controls and tailingsmanagement for Nisha Lake, lake discharge datamust be collected and an open water rating curveestablished. The second goal was to determine if saltdilution is a viable and rigorous method for measur-ing discharge for the purposes of constructing a ratingcurve for tundra streams in the Northwest Territo-ries.

STUDY AREA

The study stream is located at a remote site in theSlave Geological Province, in the central portion ofthe Northwest Territories at 65 16.11'N and 11130.05'W (Figure 1). The stream originates from NishaLake, and constitutes the only outlet from this region-ally above average sized lake. The Nisha Lake sub-basin is part of the much larger Coppermine River

IPaper No. 96016 of the Journal of the American Water Resources Association (formerly Water Resources Bulletin). Discussions are openuntil October 1, 1997.

2Respectively, Hydrologist, Water Resources Division, Indian and Northern Affairs, Box 1500, Yellowknife, Northwest Territories X1A 2R3;and Canamera Geological Ltd., Suite 540, 220 Cambie Street, Vancouver, B.C. V6B 2M9 (presently with Project Development Group, RoyalOak Mines, P.O. Box 2010, Timmins, Ontario P4N 7X7).

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drainage basin that drains approximately 330 kilome-ters to the northwest at the hamlet of Kugluktuk, onthe shores of the Coronation Gulf.

Nisha Lake has one outlet stream that flows in anorth-westerly direction. Typical of many streams inthe region, the waterway that constitutes the NishaLake outlet is a complex set of small streams thatform a diffuse drainage pattern through an area oflow relief (Figure 2). These characteristics make con-ventional means of measuring streamflow difficult.However, at one point these small streams do con-verge to form a steep channel (Figure 3), where it ispossible to gauge stream discharge with a currentmetre. As the objective of the project was to deter-mine if salt dilution is a viable and rigorous methodfor measuring stream discharge in these diffusestreams, salt dilution measurements were made alonga reach which began in the diffuse channels andended in the steep channel.

Figure 2. The Diffuse Drainage Patternof the Nisha Lake Outlet.

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Figure 1. Study Site Location.

Streamfiow Measurement Using Salt Dilution in Tundra Streams, Northwest Territories, Canada

QA•R2"3•S12

n(1)

The Manning roughness coefficient, n, in this studyis assumed to be equal to 0.05. The average value forn for natural streams is about 0.035 (Linsley, Jr. etal., 1982). A value of 0.05 represents the high end ofthe scale for a steep stream with rocky beds similar tothe steep reach shown in Figure 3 (Church andKellerhals, 1970).

Salt Dilution Applications

Current Meter Measurements

Current meter measurements were made on threeoccasions from late June to early July. An cross-section for current metre measurement was chosenacross the steep reach of the Nisha Lake outlet. Thecross-section was divided into ten cells, each 1 meterin length, within which velocity, depth and dischargewere measured. Velocity measurements were obtainedat sixth-tenths depth in each cell. Stream dischargewas calculated as the sum of discharge in each cell.During each measurement two other cross-sectionswere also measured within the steep reach to confirminitial stream discharge results.

Slope Area Calculation

The slope-area calculation method of estimatingflow is an application of hydraulic principles. Thisapproach is most frequently used to estimate the peakdischarge of a stream (Church and Kellerhals, 1970;Linsley, Jr. et al., 1982). The slope, S, and cross-sectional area, A, of the steep reach (Figure 3) weremeasured and an estimate of channel roughness, n,determined. The hydraulic radius, R, was calculatedfrom a mean of several sections of the stream (Churchand Kellerhals, 1970), and the area of these sectionswas computed. Discharge was calculated, in m3/s,using the Chezy-Manning formula:

The basic principle of salt dilution is the conserva-tion of mass of some form of tracer (Kite, 1994). If asalt solution is instantaneously added to a stream ofunknown discharge, Q, at some point downstream,the observed salt concentration, c(t), will rise fromzero to some peak value and then fall back to zero asthe "solution wave" passes. At each instant of time, t,during the passage of the wave, the stream water willcontain a quantity, S, of the original solution. Themean concentration of the solution will be:

c(t)=S/Q. (2)

When the entire wave, uniformly mixed across thechannel, passes the point downstream so that:

S Qf'c(t)dt (3)

stream discharge can be approximated by:

Q = S.

where the concentration values are measured by elec-trical conductivity readings which are then trans-formed to concentration via a calibration (Church andKellerhals, 1970).

Conductivity and salt concentration were calibrat-ed by first mixing an initial 100 milliliter 20 percentsolution. This 100 milliliters is the primary solutionand has a relative concentration of 1.0. The primarysolution was then diluted by placing 10 milliliters intoa 1 liter flask and filling the remainder volume withstream water. This new, secondary solution has a rel-ative strength of 0.01. Next, 20 liters of stream waterwas placed in a calibration tank partially submergedinto the stream to minimize temperature changes inwhich background temperature and electrical conduc-tivity was measured. Secondary solutions of 2, 5, 10,25, 50 and 100 milliliters were then added to the

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Figure 3. Steep Channel Reach of theNisha Lake Outlet Stream.

MEASUREMENT TECHNIQUES

(4)

Spence and McPhie

calibration water, noting concentration and measur-ing conductivity and temperature each time. The con-ductivity-concentration relationship was determinedby plotting measured conductivity against relativeconcentration and performing a regression analysis(Church and Kellerhals, 1970). As electrical conduc-tivity can vary with temperature, a temperature-correction factor was also determined by simultane-ously measuring temperature and conductivity in alarge (30 liter) sample of stream water with a con-stant conductivity. This temperature correction factorwas applied to the final conductivity readings.

If a quantity of solution is added to a stream, theamount of this solution which passes through a givensection downstream can be determined as a relativeprobability of the water which passed through theinjection point, subsequently passing through thepoint downstream. For an accurate measurement offlow, the discharge-weighted probabilities must beequal everywhere at the measurement point, so themeasurement is a true reflection of the total amountof water flowing downstream (Church and Kellerhals,1970). For dilution discharge measurements, thismeans the injected solution must flow a distancebefore it becomes completely mixed into the stream-flow and a measurement of the solution is made.There have been a number of methods derived fordetermining this mixing length, Lm, using channelproperties. Hull (1962) recommended:

Lm =BQ3 (5)

where B is equal to mean river width and Q is an esti-mate of discharge. Kite (1993), using data from Patraand Bhunia (1984), found the equation:

Lm =260.V (6)

where A is the cross sectional area of the stream, tobest fit a data set of calculated and measured mixinglengths. However, neither of these approachesaccount for turbulent dispersion. Church and Keller-hals (1970) summarize the development of:

(u.w2)Lm

(4. d)(7)

where v is the mean velocity of the stream, w is themean width and d is lateral dispersion. An estimateof dy can be determined by injecting a quantity of flu-orescent dye into a stream and timing the period ittakes for the dye to cover the entire width of thestream. For midstream injection:

. (8)

Mixing length was determined using Equations (7)and (8). A survey of the stream was conducted inorder to determine w and v. A fluorescent dye solutionwas injected at a midpoint across the stream. Theperiod it took for the dye to reach both banks (t) wastimed with a stopwatch. Once the mixing length wasdetermined, suitable sites for the injection point andmeasurement site were selected.

The reach within which dilution measurementswere made began at an injection point in the diffusechannels and ended at a measurement point in thesteep channel. To create a well-defined solution wave,the required increase in conductivity should beapproximately 50 percent of background conductivityof the stream (Kite, 1993). This required roughly 1liter of 20 percent potassium chloride (KCI) solutionper cubic meter of expected discharge (Church andKellerhals, 1970). KC1 was chosen as it is considereda standard in salt dilution work as well as being inex-pensive. This solution was then injected into thestream as close to the center of flow as possible. Asthe two sites were at least the mixing length apart,placement of a conductivity probe at the measure-ment site in a specific part of the stream cross-sectionwas not important, but it was placed in a strong-flowing section of the stream in order to allow propersampling by the probe. The solution wave was thenrecorded at one-second intervals and averages loggedevery three minutes by a Campbell Scientific data log-ger. Once the solution wave had passed, conductivitymeasurements were downloaded and converted toconcentration values, plotted against time and dis-charge calculated (Figure 4). In total, six measure-ments were made over the open water season. Todefine preliminary rating curves, stage was measuredusing a pressure transducer in a pool immediatelybelow the measurement site (bottom center of Fig-ure 5) where water levels were deeper and much moreconstant than those in the stream proper.

RESULTS AND COMPARISON

Discharge measurement results are summarized inTable 1. A peak discharge value of 4.57 m3/s wasobtained from the Manning formula. This result issimilar to the 4.83 m3/s from the salt dilution methodand 4.2 m3/s from the current meter. It is expectedthat the peak flow of the spring freshet occurred veryclose to June 23. If this is the case, the salt dilutionmeasurement appears to be compare favorably with

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Streamfiow Measurement Using Salt Dilution in Thndra Streams, Northwest Territories, Canada

Figure 4. Graph fllustrating July 27 Conductivity Measurements, the SolutionWave Plotted Against Time and Discharge Calculations.

both the current meter and Manning's formula.Except for the July 26 measurement, the currentmeter produced results consistently lower than thesalt dilution method or slope area calculation. Theselower measurements may be due to the shallow depthof the stream. Conventional current meters canunderestimate streamfiows in depths of less thanabout 15 centimeters of water (Church and Keller-hals, 1970). Thirty percent of depth measurementsalong the cross-sections were below this 15 centimetercriterion.

Date

Q(slope-area)

(m3/s)

Q(dilution)

(m3/s)

Q(metered)

(m3Is)Depth

(m)

June 23 4.57 4.83 4.2 0.437

July 11 1.36 .89 0.426

July 13 1.45 .422

July20 1.00 .417

July 26 0.47 0.50 0.42

July 27 0.35 0.414

Rating curves were derived for both the meteredand salt dilution measurements (Figure 6). The num-ber of measurements taken, especially with the cur-rent metre, suggests these curves should beconsidered rudimentary and subject to further mea-surement. It would have been ideal for more measure-ments to be included to improve these curves.However, budgetary limitations and a short openwater season did not allow for more than three sitevisits. Nonetheless, the curves presented in Figure 6provide preliminary estimates of the rating curve forthe Nisha Lake outlet.

CONCLUSION

The results imply that the salt dilution method canwork well and provides a viable choice in measuring

>

Q

-u0

C-)

1.0

0.8

0.65<

0.400

0.2

06030

Timeminutes

TABLE 1. Summary of Discharge Measurements.

Figure 5. Lower Reach of Nisha Lake Outlet Showing PoolBelow Measurement Site Within Which a Pressure

Transducer Was Placed for the Measurement of Stage.

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Figure 6. Nisha Lake Outlet Open Water Rating Curves Derived fromCurrent Meter and Salt Dilution Measurement Methods.

streamfiow in remote tundra streams. While the num-ber of coincident observations does not permit a sta-tistical comparison, the dilution method compareswell visually against the current meter. The differ-ence between the higher estimates of flow from thesalt dilution method and the lower current metremeasurements may account for the underestimationexpected in the measurements from the current metrethat occurred due to the shallow depths in the stream.The peak stream discharge as measured with thedilution method also agrees closely with the theoreti-cal peak discharge as calculated with the Manningformula.

By the beginning of August, stream levels haddropped and exposed enough of the bed above thewater level that the authors believed the currentmetre was no longer able to accurately measure dis-charge. While the results are not shown here due to alack of comparative data, dilution methods were stillused to measure streamflow into August. This isimportant because streamfiow data for small streamsare scarce in the Northwest Territories. To be able tomeasure flows in small isolated streams and collectdata irrespective of stream cross-sectional character-istics provides decision makers with much neededinformation.

Due to a short open water season and the high costof access, only three coincident measurements weremade. The study stream is an arctic-nival stream(Church, 1974). and experiences high flows for two

weeks of the year during the spring snowmelt, quicklydeclines to low flow and drops to zero flow by freezeup in late September. The authors intent was to mea-sure each significant phase in annual flow - freshet,declining summer flows and low baseflow becauseaccess to the study stream was only possible by float-plane at a high cost. For the purposes of water man-agement, three measurements well timed over thesummer was the most efficient use of taxpayer andcorporate dollars. The remaining salt dilution mea-surements were made by the probe and data loggerleft on site, as an exploration camp employee injectedthe salt solution.

The only drawback of the salt dilution method isthe need for complete mixing of the salt solution intothe stream. Average tundra streams likely require amixing length of 150-400 meters. Many streams inthis region are much shorter. If the required mixinglength is not present, it is recommended a more tradi-tional measurement approach be used.

This rigorous and cost-effective method to measurestreamfiow in tundra streams could be repeated byother agencies and industry. It could be used inassessing mineral exploration and development in theNorth and make environmental studies easier andmore cost effective. Better and more accurate environ-mental reviews of proposed mining and explorationactivities could, in turn, promote sustainable prac-tices in the mining industry.

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Streamfiow Measurement Using Salt Dilution in 1indra Streams, Northwest Territories, Canada

LITERATURE CITED

Church, M., 1974. Hydrology and Permafrost with Reference toNorthern North America. In: Permafrost Hydmlogy Proceedingsof Workshop Seminar 1974, Canadian National Committee forthe International Hydrological Decade, Ottawa, Ontario, pp. 7-20.

Church, M. and R. Kellerhals, 1970. Stream Gauging Techniquesfor Remote Areas Using Portable Equipment. Inland WatersBranch, Department of Energy, Mines and Resources, TechnicalBulletin No. 25.

Hull, D. E., 1962. Dispersion and Persistence of Tracers in RiverFlow Measurements. International Journal of Applied Radiationand Isotopes 13:63-74.

Kite, G., 1993. Computerized Streamfiow Measurement Using SlugInjection. Hydrological Processes 7:227-233.

Kite, G., 1994. Measuring Glacier Outflows Using a ComputerizedConductivity System. Journal of Glaciology 40(134):93-96.

Linsley, Jr., R. K., M. A. Kohler, and J. Paulhus, 1982. Hydrologyfor Engineers (Third Edition). McGraw-Hill, New York, NewYork.

Patra, A. K. and A. K. Bhunia, 1984. Discharge Measurements inHilly Streams by Dilution Method. Journal of Civil Engineering65:19-65.

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