10

Small Streams and WetlandsProvide Beneficial EcosystemServices

Natural processes that occur in smallstreams and wetlands provide humanswith a host of benefits, including flood control,maintenance of water quantity and quality, andhabitat for a variety of plants and animals. Forheadwater streams and wetlands to provide ecosys-tem services that sustain the health of our nation’swaters, the hydrological, geological and biologicalcomponents of stream networks must be intact.

Small Streams and WetlandsProvide Natural Flood ControlFloods are a natural part of every river. In timespast, waters of the Mississippi River routinelyovertopped its banks. Floodwaters carried thesediment and nutrients that made theMississippi Delta’s soil particularly suitable foragriculture. But floods can also destroy farms,houses, roads and bridges.

When small streams and wetlands are in their nat-ural state, they absorb significant amounts of rain-water, runoff and snowmelt before flooding.However, when a landscape is altered, such as by alandslide or large forest fire or a housing develop-ment, the runoff can exceed the absorption capac-ity of small streams. Moreover, the power ofadditional water coursing through a channel canchange the channel itself. Humans often alter bothlandscape and stream channels in ways that resultin larger and more frequent floods downstream.

A key feature of streams and rivers is their shape.Unlike a concrete drainage ditch, a naturalstreambed does not present a smooth surface forwater flow. Natural streambeds are rough andbumpy in ways that slow the passage of water.Particularly in small narrow streams, friction pro-duced by a stream’s gravel bed, rocks, and dams ofleaf litter and twigs slows water as it moves down-

stream. Slower moving water is more likely to seepinto a stream’s natural water storage system-its bedand banks-and to recharge groundwater. Slowermoving water also has less power to erode streambanks and carry sediment and debris downstream.

In watersheds that are not carefully protectedagainst impacts of land development, stream chan-nels often become enlarged and incised fromincreased runoff. Changed channels send waterdownstream more quickly, resulting in moreflooding. For example, after forests and prairies inWisconsin watersheds were converted to agricul-tural fields, the size of floods increased. Thischange in land use had altered two parts of theriver systems’ equation: the amount of runoff andshape of the stream channel. Cultivation destroyedthe soil’s natural air spaces that came from wormburrows and plant roots. The resulting collapse ofthe soil caused more rainfall to run off into streamsinstead of soaking into the ground. Additional sur-face runoff then altered the stream channels,thereby increasing their capacity to carry large vol-umes of water quickly downstream. These largervolumes flow downstream at much higher velocity,rather than soaking into the streambed.

Urbanization has similar effects; paving previ-ously-vegetated areas leads to greater storm runoff,which changes urban stream channels and ulti-mately sends water more quickly downstream.Covering the land with impermeable surfaces,such as roofs, roads, and parking lots, can increaseby several times the amount of runoff from a rain-storm. If land uses change near headwater streams,effects are felt throughout the stream network. Inan urban setting, runoff is channeled into stormsewers, which then rapidly discharge large volumesof water into nearby streams. The additional watercauses the stream to pick up speed, because deeper

A headwater stream

channel near Toledo, OH

relocated to accommodate

development.

Photo courtesy of

Marshal A. Moser

11

water has less friction with the streambed. Thefaster the water moves, the less it can soak into thestreambed and banks. Faster water also erodeschannel banks and beds, changing the shape of achannel. The effect is magnified downstream,because larger rivers receive water from tens, some-times hundreds, of small headwater basins. Whensuch changes are made near headwater streams,downstream portions of the stream network expe-rience bigger and more frequent flooding.

As regions become more urbanized,humans intentionally alter manynatural stream channels by replacingthem with storm sewers and otherartificial conduits. When larger,smoother conduits are substitutedfor narrow, rough-bottomed naturalstream channels, flood frequencyincreases downstream. For example,three decades of growth in stormsewers and paved surfaces aroundWatts Branch Creek, Marylandmore than tripled the number offloods and increased average annualflood size by 23 percent.

Small Streams and WetlandsMaintain Water SuppliesHeadwater systems play a crucial role in ensuringa continual flow of water to downstream freshwa-ter ecosystems. Water in streams and rivers comesfrom several sources: water held in the soil, runofffrom precipitation, and groundwater. Watermoves between the soil, streams and groundwater.Wetlands, even those without any obvious surfaceconnection to streams, are also involved in suchexchanges by storing and slowly releasing waterinto streams and groundwater, where it laterresurfaces at springs. Because of these interactions,groundwater can contribute a significant portionof surface flow in streams and rivers; conversely,surface waters can also recharge groundwater. Ifconnections between soil, water, surface waters,and groundwater are disrupted, streams, rivers,and wells can run dry. Two-thirds of Americansobtain their drinking water from a water systemthat uses surface water. The remaining one-thirdof the population relies on groundwater sources.

The quality and amount of water in both of thesesources respond to changes in headwater streams.

USGS estimates that, on average, from 40 to 50percent of water in streams and larger rivers comesfrom groundwater. In drier regions or during dryseasons, as much as 95 percent of a stream’s flowmay come from groundwater. Thus, the rechargeprocess that occurs in unaltered headwaterstreams and wetlands both moderates down-stream flooding in times of high water and main-

tains stream flow during dry seasons.

Headwater streams and wetlands havea particularly important role to playin recharge. These smallest upstreamcomponents of a river network havethe largest surface area of soil in con-tact with available water, thereby pro-viding the greatest opportunity forrecharge of groundwater. Moreover,water level in headwater streams isoften higher than the water table,allowing water to flow through thechannel bed and banks into soil andgroundwater. Such situations occurwhen water levels are high, such as

during spring snowmelt or rainy seasons. Duringdry times, the situation in some reaches of thestream network, particularly those downstream,may reverse, with water flowing from the soil andgroundwater through the channel banks and bedinto the stream. This exchange of water from thesoil and groundwater into the stream maintainsstream flow. However, if land-use changes increasethe amount of precipitation that runs off into astream rather than soaking into the ground, therecharge process gets short-circuited. This increasedvolume of stream water flows rapidly downstreamrather than infiltrating into soil and groundwater.The consequence is less overall groundwaterrecharge, which often results in less water instreams during drier seasons.

Therefore, alteration of small streams and wetlandsdisrupts the quantity and availability of water in astream and river system. Protecting headwaterstreams and wetlands is important for maintainingwater levels needed to support everything from fishto recreational boating to commercial ship traffic.

“ALTERATION OF

SMALL STREAMS

AND WETLANDS

DISRUPTS THE

QUANTITY AND

AVAILABILITY OF

WATER IN A

STREAM AND

RIVER SYSTEM.”

12

Small Streams and Wetlands TrapExcess SedimentHeadwater systems retain sediment. Like the flowof water, movement of sediment occurs through-out a river network. Thus, how a watershed ismanaged and what kinds of land uses occur therehave substantial impact on the amount of sedi-ment delivered to larger rivers downstream.Increased sediment raises water purification costsfor municipal and industrial users, requires exten-sive dredging to maintain navigational channels,and degrades aquatic habitats. Intact headwaterstreams and wetlands can modulate the amount ofsediment transported to downstream ecosystems.

Runoff from rain, snowmelt andreceding floodwaters can wash soil,leaves and twigs into streams, wherethe various materials get broken upinto smaller particles or settle out. Ifnatural vegetation and soil cover aredisturbed by events and activitiessuch as fires, farming or construc-tion, runoff increases, washingmore materials into streams. At thesame time, the increased velocityand volume of water in a streamcause erosion within the streambedand banks themselves, contributingadditional sediment to the streamsystem. Moreover, the faster, fullerstream can carry more and largerchunks of sediment further downstream.

One study found that land disturbances such asurban construction can, at minimum, double theamount of sediment entering headwater streamsfrom a watershed. A Pennsylvania study showedhow, as a 160-acre headwater watershed becamemore urbanized, channel erosion of a quarter-mile stretch of stream generated 50,000 addi-tional cubic feet of sediment in one year-enoughto fill 25 moderate-sized living rooms. In a non-urban watershed of the same size, it would takefive years to generate the same amount of sedi-ment. Such studies demonstrate that landscapechanges such as urbanization or agriculture, par-ticularly without careful protection of headwaterstreams and their riparian zones, may cause manytimes more sediment to travel downstream.

EXCESS SEDIMENT IN DOWNSTREAMECOSYSTEMS COSTS MONEYKeeping excess sediment out of downstream riversand lakes is one ecosystem service intact smallstreams and wetlands provide. Once sedimentmoves further downstream, it becomes an expen-sive problem. Too much sediment can fill up reser-voirs and navigation channels, damage commercialand sport fisheries, eliminate recreation spots, harmaquatic habitats and their associated plants and ani-mals, and increase water filtration costs.

Additional sediment damages aquatic ecosys-tems. Sediment suspended in the water makes itmurkier; as a result, underwater plants no longer

receive enough light to grow. Fishthat depend on visual signals tomate may be less likely to spawn inmurky water, thereby reducing fishpopulations. High levels of sedi-ment suspended in water can evencause fish kills. Even as it settles tothe bottom, sediment continues tocause problems because it fills theholes between gravel and stonesthat some animals call home,smothers small organisms thatform the basis of many food webs,and can also smother fish eggs.

Getting rid of sediment is expensive.For example, keeping BaltimoreHarbor navigable costs $10 to $11.5

million annually to dredge and dispose of sedimentthe Patapsco River deposits in the harbor.

SMALL STREAMS AND WETLANDS RETAINSEDIMENTHeadwater streams and wetlands typically trap andretain much of the sediment that washes into them.The faster the water travels, the larger the particles itcan carry. So, natural obstructions in small streams-rocks, downed logs, or even just a bumpy streambottom-slow water and cause sediment to settle outof the water column. Wetlands, whether or not theyhave a surface connection to a nearby stream, areoften areas where runoff slows and stops, droppingany debris the water may be carrying. Because head-water streams represent 75 percent or more of totalstream length in a stream network, such streams andtheir associated wetlands retain a substantial

“INTACT HEADWATER

STREAMS AND

WETLANDS CAN

MODULATE THE

AMOUNT OF

SEDIMENT

TRANSPORTED TO

DOWNSTREAM

ECOSYSTEMS.”

13

amount of sediment, preventing it from flowinginto larger rivers downstream.

Even ephemeral streams can retain significantamounts of sediment. Such small headwaterstreams expand and contract in response to heavyrains. During expansion, a stream flows over whatwas a dry or damp streambed. Most of the water atthe leading edge of a growing stream, called the“trickle front,” soaks into the streambed and doesnot carry sediment downstream. In a small water-shed near Corvallis, Oregon, researchers found that60 to 80 percent of sediment generated from forestroads traveled less than 250 feet downstream beforesettling out in stream pools. Headwater streams canstore sediment for long periods of time: research inOregon’s Rock Creek basin found that headwaterstreams could retain sediment for 114 years.

Natural Cleansing Ability ofSmall Streams and WetlandsProtects Water Quality Materials that wash into streams include every-thing from soil, leaves and dead insects to runofffrom agricultural fields and animal pastures.One of the key ecosystem services that streamnetworks provide is the filtering and processingof such materials. Healthy aquatic ecosystemscan transform natural materials like animal dungand chemicals such as fertilizers into less harm-ful substances. Small streams and their associatedwetlands play a key role in both storing andmodifying potential pollutants, ranging fromchemical fertilizers to rotting salmon carcasses,in ways that maintain downstream water quality.

EXCESS NUTRIENTS CAUSE PROBLEMS INRIVERS AND LAKESInorganic nitrogen and phosphorus, the mainchemicals in agricultural fertilizers, are essentialnutrients not just for plants, but for all livingorganisms. However, in excess or in the wrongproportions, these chemicals can harm naturalsystems and humans.

In freshwater ecosystems, eutrophication, theenriching of waters by excess nitrogen and phos-phorus, reduces water quality in streams, lakes, estu-aries and other downstream waterbodies. Oneobvious result is the excessive growth of algae. More

algae clouds previously clear streams, such as thosefavored by trout. In addition to reducing visibility,algal blooms reduce the amount of oxygen dissolvedin the water, sometimes to a degree that causes fishkills. Fish are not the only organisms harmed: someof the algae species that grow in eutrophic watersgenerate tastes and odors or are toxic, a clear prob-lem for stream systems that supply drinking waterfor municipalities. In addition, increased nitrogencan injure people and animals. Excess nitrogen inthe form called nitrate in drinking water has beenlinked to “blue baby disease” (methemoglobinemia)in infants and also has toxic effects on livestock.

HEADWATER STREAMS TRANSFORM ANDSTORE EXCESS NUTRIENTSHeadwater streams and associated wetlands bothretain and transform excess nutrients, thereby pre-venting them from traveling downstream. Physical,chemical and biological processes in headwaterstreams interact to provide this ecosystem service.

Compared with larger streams and rivers, smallstreams, especially shallow ones, have more waterin physical contact with a stream channel.Therefore, the average distance traveled by a parti-cle before it is removed from the water column isshorter in headwater streams than in larger ones. Astudy of headwater streams in the southernAppalachian Mountains found that both phos-phorus and the nitrogen-containing compoundammonium traveled less than 65 feet downstreambefore being removed from the water.

Stream networks filter and

process everything from

leaves and dead insects to

runoff from agricultural

fields and animal pastures.

Without such processing,

algal blooms can ruin living

conditions for fish and the

quality of drinking water.

Here, algae overtakes a lake

in Iowa. Photo courtesy of

Lynn Betts, USDA NRCS

14

In headwater streams and wetlands, more water is indirect contact with the streambed, where most pro-cessing takes place. Bacteria, fungi and other microor-ganisms living on the bottom of a stream consumeinorganic nitrogen and phosphorus and convert theminto less harmful, more biologically beneficial com-pounds. A mathematical model based on research in14 headwater streams throughout the U.S. shows that64 percent of inorganic nitrogen entering a smallstream is retained or transformed within 1,000 yards.

Channel shape also plays a role in transformingexcess nutrients. Studies in Pennsylvania have shownthat when the forest surrounding headwaters isreplaced by meadows or lawns, increased sunlightpromotes growth of grasses along stream banks. Thegrasses trap sediments, create sod, and narrow thestream channel to one-third of theoriginal width. Such narrowingreduces the amount of streambedavailable for microorganisms thatprocess nutrients. As a result, nitrogenand phosphorus travel downstreamfive to ten times farther, increasingrisks of eutrophication.

Streams do not have to flow year-round to make significant contribu-tions to water quality. Fertilizers andother pollutants enter stream sys-tems during storms and other timesof high runoff, the same times thatephemeral and intermittent streamsare most likely to have water and process nutrients.Federal, state and local programs spend consider-able sums of money to reduce non-point sourceinputs of nutrients because they are a major threatto water quality. One principal federal program,the EPA’s 319 cost-share program, awarded morethan $1.3 billion between 1990 and 2001 to statesand territories for projects to control non-pointpollution. Failure to maintain nutrient removalcapacity of ephemeral and intermittent streamsand wetlands would undermine these efforts.

Wetlands also remove nutrients from surface waters.Several studies of riparian wetlands have found thatthose associated with the smallest streams to be mosteffective in removing nutrients from surface waters.For example, headwater wetlands comprise 45 percentof all wetlands able to improve water quality in four

Vermont watersheds. Another study found that wet-lands associated with first-order streams are responsi-ble for 90 percent of wetland phosphorus removal ineight northeastern watersheds. Such studies demon-strate that riparian wetlands, especially those associ-ated with small streams, protect water quality.

As land is developed, headwater streams are oftenfilled or channeled into pipes or paved waterways,resulting in fewer and shorter streams. For example,as the Rock Creek watershed in Maryland wasurbanized, more than half of the stream channel net-work was eliminated. In even more dramatic fash-ion, mining operations in the mountains of centralAppalachia have removed mountain tops and filledvalleys, wiping out entire headwater stream net-works. From 1986 to 1998, more than 900 miles of

streams in central Appalachia wereburied, more than half of them inWest Virginia.

If headwater streams and wetlands aredegraded or filled, more fertilizerapplied to farm fields or lawns reacheslarger downstream rivers. These largerrivers process excess nutrients from fer-tilizer much more slowly than smallerstreams. Losing the nutrient retentioncapacity of headwater streams wouldcause downstream waterbodies to con-tain higher concentrations of nitrogenand phosphorus. A likely consequenceof additional nutrients would be the

further contamination and eutrophication of down-stream rivers, lakes, estuaries and such waters as theGulf of Mexico.

Natural Recycling in HeadwaterSystems Sustains DownstreamEcosystemsRecycling organic carbon contained in the bodiesof dead plants and animals is a crucial ecosystemservice. Ecological processes that transform inor-ganic carbon into organic carbon and recycleorganic carbon are the basis for every food web onthe planet. In freshwater ecosystems, much of therecycling happens in small streams and wetlands,where microorganisms transform everything fromleaf litter and downed logs to dead salamandersinto food for other organisms in the aquatic foodweb, including mayflies, frogs and salmon.

“IF HEADWATER

STREAMS AND

WETLANDS ARE

DEGRADED OR

FILLED, MORE

FERTILIZER APPLIED

TO FARM FIELDS OR

LAWNS REACHES

LARGER DOWN-

STREAM RIVERS.”

15

Like nitrogen and phosphorus, carbon is essentialto life but can be harmful to freshwater ecosystemsif it is present in excess or in the wrong chemicalform. If all organic material received by headwaterstreams and wetlands went directly downstream,the glut of decomposing material could depleteoxygen in downstream rivers, thereby damagingand even killing fish and other aquatic life. Theability of headwater streams to transform organicmatter into more usable forms helps maintainhealthy downstream ecosystems.

HEADWATER STREAM SYSTEMS STORE ANDTRANSFORM EXCESS ORGANIC MATTERIntact headwater systems both store and processorganic matter in ways that modulate the release ofcarbon to downstream lakes andrivers. Headwater systems receive largeamounts of organic matter, which canbe retained and transformed intomore palatable forms through decom-position processes. This organic mat-ter is anything of biological origin thatfalls into, washes into or dies in astream. Plant parts, such as leaves,twigs, stems and larger bits of woodydebris, are the most common of theseitems. Another source of organicmaterial is dead stream organisms,such as bits of dead algae and bacteriaor bodies of insects and even largeranimals. Waste products of plants and animals alsoadd organic carbon to water. Water leaches dissolvedorganic carbon from organic materials in a streamand watershed like tea from a tea bag.

Much of the organic matter that enters headwatersystems remains there instead of continuing down-stream. One reason is that the material often entersheadwater streams as large pieces, such as leaves andwoody debris, that are not easily carried down-stream. In addition, debris dams that accumulate inheadwater streams block the passage of materials.One study found four times more organic matteron the bottoms of headwater streams in forestedwatersheds than on the bottoms of larger streams.

Another reason material stays in headwater streams isthat food webs in small streams and wetlands processorganic matter efficiently. Several studies have foundthat headwater streams are far more efficient at trans-forming organic matter than larger streams. For exam-

ple, one study showed that, for a given length ofstream, a headwater stream had an eight-fold higherprocessing efficiency than a fourth-order channeldownstream. Microorganisms in headwater streamsystems use material such as leaf litter and otherdecomposing material for food and, in turn, becomefood for other organisms. For example, fungi thatgrow on leaf litter become nutritious food for inverte-brates that make their homes on the bottom of astream, including mayflies, stoneflies and caddis flies.These animals provide food for larger animals, includ-ing birds such as flycatchers and fish such as trout.

HEADWATER SYSTEMS SUPPLY FOOD FORDOWNSTREAM ECOSYSTEMSThe organic carbon released by headwater streams

provides key food resources for down-stream ecosystems. Headwaterecosystems control the form, qualityand timing of carbon supply down-stream. Although organic matteroften enters headwaters in largeamounts, such as when leaves fall inautumn or storm runoff carries debrisinto the stream, those leaves anddebris are processed more slowly. As aresult, carbon is supplied to down-stream food webs more evenly over alonger period of time. Forms of car-bon delivered range from dissolvedorganic carbon that feeds microor-

ganisms to the drifting insects such as mayflies andmidges that make ideal fish food. Such insects arethe preferred food of fish such as trout, char andsalmon. One study estimated that fishless headwa-ter streams in Alaska export enough drifting insectsand other invertebrates to support approximatelyhalf of the fish production in downstream waters.

Processed organic matter from headwater streamsfuels aquatic food webs from the smallest streams tothe ocean. Only about half of all first-order streamsdrain into second-order streams; the other half feeddirectly into larger streams or directly into estuariesand oceans, thus delivering their carbon directly tothese larger ecosystems. The health and productivityof downstream ecosystems depends on processedorganic carbon-ranging from dissolved organic carbonto particles of fungus, and leaf litter to mayflies andstoneflies-delivered by upstream headwater systems.

“THE ABILITY OF

HEADWATER STREAMS

TO TRANSFORM

ORGANIC MATTER

INTO MORE USABLE

FORMS HELPS

MAINTAIN HEALTHY

DOWNSTREAM

ECOSYSTEMS.”

16

Headwater Streams MaintainBiological Diversity

HEADWATER HABITATS ARE DIVERSEHeadwater streams are probably the most variedof all running-water habitats; they range fromicy-cold brooks tumbling down steep, boulder-filled channels to outflows from desert springsthat trickle along a wash for a short distancebefore disappearing into sand. As such, headwa-ter systems offer an enormous array of habitatsfor plant, animal and microbial life.

This variation is due to regional differences inclimate, geology, land use and biology. Forexample, streams in limestone or sandy regionshave very steady flow regimes compared withthose located in impermeable shale or clay soils.Plants or animals found only in certain regionscan also lend a distinctive character to headwaterstreams. Regionally important riparian plants,such as alder and tamarisk, exercise a stronginfluence on headwater streams. Headwaterstreams in regions with beavers are vastly differ-ent from those in regions without beavers.

Environmental conditions change throughout astream network. In wet regions, streams grow largerand have wider channels, deeper pools for shelter,and more permanent flow as they move down-stream. In arid regions and even humid regionsduring dry periods, headwater streams may becomesmaller downstream as water evaporates or soaksinto a streambed. Because marked changes in envi-ronmental conditions can occur over very short dis-tances, conditions required by a headwater speciesmay exist for as little as 100 yards of stream.Consequently, local populations of a species mayextend over just a short distance, particularly inspring-fed headwaters with sharp changes in envi-ronmental conditions along the length of a stream.

With this variety of influences, headwaterstreams present a rich mosaic of habitats, eachwith its own characteristic community of plants,animals, and microorganisms.

HEADWATER SYSTEMS SUPPORT A DIVERSEARRAY OF ANIMALS AND PLANTSThere has never been a complete inventory ofthe inhabitants in even a single headwaterstream, much less surveys across many types ofheadwaters that would permit a thorough under-standing of biodiversity in headwater streams.Nevertheless, it is clear that individual headwaterstreams support hundreds to thousands ofspecies, ranging from bacteria to bats.

The species in a typical headwater stream includebacteria, fungi, algae, higher plants, invertebrates,fish, amphibians, birds and mammals. Headwaterstreams are rich feeding grounds. Large amounts ofleaves and other organic matter that fall or blowinto streams, the retention of organic matter in a

Top right: A hydrobiid snail

[Pyrgulopsis robusta] found

in the headwaters of the

Snake River in Wyoming.

Photo courtesy of

Dr. Robert Hershler

Center: Caddis flies and

other aquatic insects spend

their larval stage in

streams, feeding on the

algae, vegetation and

decaying plant matter. The

Brachycentris, a caddis fly

found in headwater

streams of eastern North

America, constructs a

protective case out of twigs,

leaves and other debris.

Photo courtesy of

David H. Funk

Bottom: American

dippers rely on headwater

streams for sustenance,

walking along stream bot-

toms and feeding on insect

larvae and crustaceans

among the rocks of the

streambed. This American

dipper was photographed at

Tanner’s Flat, just east of Salt

Lake City. Photo courtesy of

Pomera M. France

Top left: Populations of the

ellipse mussel

(Venustaconcha ellipsi-

formis) have disappeared

from many of its native

Midwestern headwaters.

Photo courtesy of Kevin

Cummings, Illinois Natural

History Survey

17

channel or debris dams, and the high rates of plantand algal growth in unshaded headwaters all supplyfood sources for animals such as caddis flies, snailsand crustaceans. These animals become food forpredators such as fish, salamanders, crayfish, birdsand mammals, which, in turn, become prey forlarger animals, including herons, raccoons andotters. Many widespread species also use headwa-ters for spawning sites, nursery areas, feeding areas,and travel corridors. Thus, headwater habitats areimportant to species like otters, flycatchers, andtrout, even though these species are not restrictedto headwaters. The rich resource base that headwa-ters provide causes the biotic diversity of headwaterstreams to contribute to the productivity of bothlocal food webs and those farther downstream.

Diversity of headwater systems results in diverseheadwater plants and animals. Many of thesespecies are headwater specialists and are mostabundant in or restricted to headwaters. Forexample, water shrews live along small, coolstreams, feed on aquatic invertebrates, and spendtheir entire lives connected to headwater streams.Because different headwaters harbor differentspecies, the number of headwater-dependentspecies across North America is far greater thanthe number of species in any one headwater.

Headwater specialists often have small geographicranges. These species, many of which are imperiled,include: species of minnows, darters, and topmin-nows in southeastern springs and brooks; aquaticsnails in spring-fed headwaters in the Great Basin,the Southeast, Florida, and the Pacific Northwest;crayfish in small streams from Illinois andOklahoma to Florida; and salamanders and tailedfrogs in small streams, springs, and seeps in theSoutheast and Pacific Northwest. Two factors con-tribute to specialists’ small ranges: their limited abil-ity to move between headwaters and high diversity ofheadwater habitats. Unlike mobile animals, such asmammals and birds, fully aquatic animals like fishand most mollusks cannot move from one headwa-ter stream to another. As a result, local evolution mayproduce different species in adjacent headwater sys-tems. Moreover, environmental conditions often dif-fer greatly between adjacent headwater streams andeven within the course of a single stream. For exam-ple, in a spring-fed headwater stream in westernPennsylvania, one species of caddis fly inhabits head-

waters starting at the spring and going downstreamabout 200 yards. A different species of caddis flyinhabits the stream after that point.

Animals may use headwater streams for all or partof their lives. Although many fish species live exclu-sively in headwater systems, others use headwatersonly for key parts of their life cycle. For example,headwaters are crucial for the diversity of salmonstocks in the Pacific Northwest because salmonspawn and rear in headwater streams. In other partsof the country, trispot darters, brook trout andrainbow trout spawn in small streams. Young cut-throat trout use shelter formed by streams’ debrisdams but move onto larger portions of a streamnetwork as they mature. Intermittent streams canoffer special protection for young fish, because thesmall pools that remain in such streams often lackpredators. Still other fish species use headwaterstreams as seasonal feeding areas.

Both permanent and intermit-tent streams provide valuablehabitat for microorganisms,plants and animals. Generally,biodiversity is higher in perma-nent streams than in intermittent streams, butintermittent streams often provide habitat for dif-ferent species. Some species that occur in bothtypes of streams may be more abundant in preda-tor-free intermittent streams. For example,because of the lack of large predatory fish, sala-manders and crayfish are sometimes more abun-dant in fishless intermittent streams rather thanthose with permanent flow. In contrast, for ani-mals such as brook trout that require steady watertemperatures and constant water flow, perennialstreams provide better habitat.

A water shrew (Sorex

palustris) in the water’s of

Oregon’s Mt. Hood. Photo

courtesy of RB Forbes,

Mammal Images Library

A westslope cutthroat trout

from Deep Creek, a

headwater of the Kettle

River. Cutthroat trout

spawn in headwaters

where the young trout seek

shelter amid piles of debris,

moving on to larger waters

for their adult lives. Photo

courtesy of Bill McMillan,

Washington Trout

A coho salmon migrating up

a spring-fed tributary of the

Snoqualmie River watershed

in Washington’s Puget Sound

region. Many anadromous

fish species spawn in head-

water streams that are so

small as to be omitted from

standard USGS topographi-

cal maps. Photo courtesy of

Washington Trout.

18

LINKAGES BETWEEN HEADWATER ANDSTREAMSIDE ECOSYSTEMS BOOSTBIOLOGICAL DIVERSITYThe movement of plants and animals betweenheadwater and streamside ecosystems boosts biodi-versity in both areas. Headwater streams are tightlylinked to adjacent riparian ecosystems, the zonesalong a stream bank. Riparian ecosystems havehigh species diversity, particularly in arid environ-ments where the stream provides a unique micro-climate. Typical riparian vegetation depends uponmoist streamside soils. Some plants must have “wetfeet,” meaning their roots have to stretch into por-tions of soil that are saturated with water. Seeds ofsome riparian plants, such as those of cottonwoodtrees found along rivers in the Southwest, requireperiodic floods to germinate and take root.

Another link between stream and land is oftenprovided by insects, such as mayflies, that emergefrom streams and provide a vital food resource foranimals, including birds, spiders, lizards and bats.For example, insect-eating birds living by a prairiestream in Kansas consume as much as 87 percentof the adult aquatic insects that emerged from thestream each day. Such exchanges between landand water help maintain animal populationsacross landscapes. In many landscapes, the net-work of headwater streams is so dense that itoffers a nearly continuous system of intercon-nected habitat for the movement of mobilespecies that rely on streams and riparian areas.

BIOLOGICAL DIVERSITY OF HEADWATERSYSTEMS IS THREATENED BY HABITATDESTRUCTIONBecause of their small size and intimate connec-tions with surrounding landscape, headwatersand their inhabitants are easily influenced byhuman activities in watersheds and riparianzones. Changes to riparian vegetation or hydrol-ogy, water pollution, or the introduction ofexotic species can have profound effects on biotaliving in headwaters.

Specialized headwater species can be particularlysensitive to habitat destruction because of theirsmall geographic ranges, sometimes as small as asingle headwater stream or spring. Thus, humanactivities have driven some headwater specialists,like the whiteline topminnow, to extinction, andimperiled many others. Furthermore, as the nat-ural disjunction of headwater systems isincreased by human activities such as pollution,impoundment, and destruction of riparian vege-tation, more populations of headwater specialistsmay be extirpated.

Many headwater species, including fish, snails,crayfish, insects and salamanders, are now indanger of extinction as a result of humanactions. A few dozen headwater species arealready listed under the U.S. Endangered SpeciesAct; hundreds of others are rare enough to beconsidered for listing. Given the diversity andsensitivity of headwater biota, it seems likely thatcontinued degradation of headwater habitatswill put more species at risk of extinction.

Canelo Hills ladies' tresses

[Sprianthes delitescens] in a

southwestern freshwater

marsh known as a cienega.

The cienegas of Arizona and

New Mexico and Mexico, are

the exclusive habitat for this

member of the orchid family.

Photo courtesy of Jim

Rorabaugh, USFWS

The Cleistes, a member of the

orchid family, is found in

pocosin wetlands of North

Carolina. Photo courtesy of

Vince Bellis

19

WETLANDS MAKE KEY CONTRIBUTIONS TOBIOLOGICAL DIVERSITYThe presence of wetlands adds another aspect ofhabitat diversity to headwater systems and there-fore increases the variety of species a headwatersystem may support. Most headwater wetlandsare depressions in the ground that hold waterpermanently or seasonally. Wetlands providecritical habitat for a variety of plants and ani-mals. Scientists usually distinguish betweenephemeral and perennial wetlands.

BIODIVERSITY IN EPHEMERAL WETLANDSSome species of plants and animals prefer orrequire ephemeral wetlands. Certain zooplank-ton, amphibians, and aquatic plants need thewet phase of an ephemeral wetland to completeall or part of their life cycles. Other species thatrely on ephemeral wetlands wait out the aquaticphase, flourishing only when pools shrink or dis-appear. For example, although adult spottedsalamanders are generally terrestrial, during thespringtime they trek to vernal pools to breed andreproduce. So-called amphibious plants, includ-

ing button celery, meadowfoam, wooly marblesand many others do the opposite; although theylive in water, they cannot reproduce until waterlevels drop. Some plants and crustaceans moststrongly identified with ephemeral wetlandsworldwide, including quillworts, fairy shrimp,and tadpole shrimp, are ancient groups thatprobably originated at least 140 million yearsago. The disappearance of ephemeral wetlandswould mean the loss of these highly specializedand ancient groups of plants and animals.

One type of ephemeral wetland found in bothCalifornia and the Northeast is known as a ver-nal pool because it generally fills with water inthe spring. In California, blooming flowers ringthe edges and fill depressions of such pools. Ofthe 450 species, subspecies, or varieties ofplants found in California’s vernal pools, 44 arevernal pool specialists. Several such plants arealready on the Endangered Species list. IfCalifornia’s vernal pool habitats were com-pletely destroyed, at least 44 species would dis-appear. Although vernal pool animals are lesswell known, there appear to be at least as many

Pitcher plants, such as this

white top (Sarracenia leuco-

phylla), pictured top left; and

sundews, such as this

Drosera brevifolia, pictured

bottom right; are among the

carnivorous plants found in

the Carolina Bay wetlands of

the Southeastern U.S. Photo

courtesy of David Scott/SREL

20

specialized animals as plants. New species ofspecialists such as fairy shrimp and clam shrimpcontinue to be discovered.

Other ephemeral wetlands also make significantcontributions to biodiversity. A study of wetlandsin the Southeast including cypress-gum swamps,cypress savannas, and grass-sedge marshes, foundthat plants from one wetland are often very dif-ferent from those in others nearby. Such differ-ences in nearby habitats increase overallbiodiversity in a region. In some cases, differencesin periods of wetting and drying appear to beimportant for the persistence of many species.Different wetting and drying patterns explainsome differences between Gromme Marsh and

Stedman Marsh, two prairie pothole wetlands inWisconsin. Although the two marshes are onlyabout 450 yards apart, they have different speciesof dragonflies; also, Stedman Marsh has dam-selflies and caddis flies that Gromme Marsh lacks.

Amphibians are key parts of the food web insmall wetlands. Some wetlands are hot spots foramphibian biodiversity; twenty-seven amphib-ian species, one of the highest numbers ofamphibian species known from such a smallarea, inhabited a 1.2-acre ephemeral wetland inSouth Carolina. Other small wetlands in theregion have been found to have similar numbersof amphibian species, demonstrating how smallwetlands are especially important for maintain-ing the regional biodiversity of amphibians.Larger, more permanent wetlands may be lessdiverse because they may also be home to preda-tors-such as crayfish and dragonfly larvae-thateat amphibian larvae.

BIODIVERSITY IN FENS (A TYPE OF PEREN-NIAL WETLAND)Plant biodiversity peaks in fens, unique peren-nial wetlands that occur where groundwaterflows to the surface. Fens also provide cleanwater that supports downstream ecosystems;outflows from such wetlands are critical to theformation of the cold, low-nutrient streams thatare ideal for trout. Although fens are rarely inun-dated, water seeps continuously into root zones.

Similar to other wetlands, the small land areacovered by fens belies the high biodiversityfound within them. For example, in northeast-ern Iowa, fens contain 18 percent of the state’splant species but cover only 0.01 percent of theland surface. Fens are probably the wetlandswith the greatest numbers of plant species.Because groundwater that comes to the surface istypically low in available nutrients, fen plants areoften dwarfed and the total mass of vegetation istypically low. As a result, no one species canbecome dominant and exclude other species.

In the Upper Midwest, more than 1,169 speciesof plants have been identified in fens, with morethan half needing wet conditions. Fens also have

Although spotted salaman-

ders are generally terrestrial

animals, they only breed and

reproduce in vernal pools.

Photo courtesy of Vernal

Pool Association

A female fairy shrimp from

the Ipswich River Basin in

Massachusetts. Fairy shrimp

spend their entire life cycles

in vernal pools. Photo cour-

tesy of Vernal Pool

Association

21

a high proportion of plant species known tooccur primarily in pristine sites. Often, suchspecies are listed as rare, threatened or endan-gered. Of 320 vascular plant species foundwithin fens in northeastern Iowa, 44 percent areconsidered rare. Fens themselves are imperiled:160 fens that one researcher sampled in north-eastern Iowa were all that remained from 2,333historic fens.

Because diversity in fens stems from low nutrientavailability, overfertilization can harm fens and,in turn, downstream ecosystems. Examining onefen in New York, researchers found the lowestdiversity of plants where nitrogen and phospho-rus inflows were greatest. Both nutrients camefrom agricultural activities: phosphorus wasentering the fen primarily through surface waterflows, while the nitrogen-containing compoundnitrate was flowing with the groundwater. Thus,a loss of plant diversity in fens is a clear indica-tion they are receiving excess nutrients, such ascan occur when fertilizer runs off a field or urbanlawn or water carries animal waste from farm-

yards. Allowing excess nutrients to enter fenscan also damage downstream trout streamsbecause trout prefer cold, low-nutrient streams.Therefore, the low-nutrient conditions of fensrequire protection from nutrient contamination.

A wood frog (Rana sylvatica)

in an autumnal vernal pool

in central Pennsylvania.

Photo courtesy of

Gene Wingert

Fens are unique perennial

wetlands that occur where

groundwater flows to the

surface. Plant biodiversity

peaks in fens: Among the

320 vascular plant species

found in northeastern Iowa

fens, 44% are considered

rare. However, fens them-

selves are imperiled. Pictured

is a fen wetland in Illinois.

Photo courtesy of Steve

Byers, Bluff Spring Fen

Nature Preserve

22

Conclusion

Headwater streams and wetlands aboundon the American landscape, providingkey linkages between stream networks and sur-rounding land. Although often unnamed,unrecorded, and underappreciated, small headwa-ter streams and wetlands-including those that aredry for parts of the year-are an integral part of ournation’s river networks. Small wetlands, eventhose without visible surface connections, arejoined to stream systems by ground-water, subsurface flows of water, andperiodic surface flows. Currentdatabases and maps do not ade-quately reflect the extent of headwa-ter streams and associated wetlands.The resulting underestimate of theoccurrence of such ecosystems ham-pers our ability to measure the keyroles headwater systems play inmaintaining quality of surfacewaters and diversity of life.

Essential ecosystem services providedby headwater systems include attenu-ating floods, maintaining water sup-plies, preventing siltation of downstream streams

and rivers, maintaining water quality, and support-ing biodiversity. These small ecosystems also providea steady supply of food resources to downstreamecosystems by recycling organic matter.

Small streams and wetlands provide a rich diversityof habitats that supports unique, diverse, andincreasingly endangered plants and animals.Headwater systems, used by many animal species atdifferent stages in their life history, provide shelter,

food, protection from predators,spawning sites and nursery areas, andtravel corridors between terrestrialand aquatic habitats.

Since the 1970s, the federal CleanWater Act has played a key role inprotecting streams and wetlandsfrom destruction and pollution. Wehave made progress toward cleanerwater, in part because the law hashistorically recognized the need toprotect all waters of the UnitedStates. The health of downstreamwaters depends on continuing pro-

tection for even seemingly isolated wetlands andsmall streams that flow only part of the year.

These small streams and wetlands are beingdegraded and even eliminated by ongoinghuman activities. Among the earliest and mostvisible indicators of degradation is the loss ofplant diversity in headwater wetlands. The phys-ical, chemical, and biotic integrity of our nation’swaters is sustained by services provided by wet-lands and headwater streams.

Today’s scientists understand the importance ofsmall streams and wetlands even better than theydid when Congress passed the Clean Water Act.If we are to continue to make progress towardclean water goals, we must continue to protectthese small but crucial waters. The goal of pro-tecting water quality, plant and animal habitat,navigable waterways, and other downstreamresources is not achievable without careful pro-tection of headwater stream systems.

Photo courtesy of

Raymond Eubanks.

“THE PHYSICAL,

CHEMICAL, AND

BIOTIC INTEGRITY OF

OUR NATION’S

WATERS IS SUSTAINED

BY SERVICES PRO-

VIDED BY WETLANDS

AND HEADWATER

STREAMS.”

23

R E F E R E N C E S :

The authors used more than 235 scientific publications inpreparing this paper. This brief list is intended to provide thereader with an introduction to the relevant scientific literature.

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Amon, J.P., C.A. Thompson, Q.J. Carpenter, and J. Miner. 2002.Temperate zone fens of the glaciated Midwestern USA. Wetlands22(2): 301-317.

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Erman, D.C., and V.M. Hawthorne. 1976. The quantitativeimportance of an intermittent stream in the spawning of rainbowtrout. Transactions of the American Fisheries Society 105: 675-681.

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Labbe, T.R., and K.D. Fausch. 2000. Dynamics of intermittentstream habitat regulate persistence of a threatened fish at multiplescales. Ecological Applications 10: 1774-1791.

Leopold, L. B. 1994. A View of the River. Cambridge, Mass:Harvard University Press.

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Ohio Environmental Protection Agency. 2002a. Clean riversspring from their source: the importance and management of head-waters streams. Columbus, Ohio: State of Ohio EnvironmentalProtection Agency, Division of Surface Water.

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Tiner, R.W., H.C. Bergquist, G.P. DeAlessio, and M.J. Starr. 2002.Geographically isolated wetlands: A preliminary assessment of theircharacteristics and status in selected areas of the United States.USDA Fish and Wildlife Service, Northeast Region, Hadley, MA.http://wetlands.fws.gov/Pubs_Reports/isolated/report.htm

Waterhouse, F.L., A.S. Harestad, and P.K. Ott. 2002. Use of smallstreams and forest gaps for breeding habitats by winter wrens incoastal British Columbia. Northwest Science 76: 335-346.

Wipfli, M. S., and D. P. Gregovich. 2002. Export of invertebratesand detritus from fishless headwater streams in southeastern Alaska:implications for downstream salmonid production. FreshwaterBiology 47:957-969.

Zedler, J.B. 2003. Wetlands at your service: reducing impacts ofagriculture at the watershed scale. Frontiers in Ecology 1 (2): 65-72.

FOR ADDITIONAL INFORMATION ON TOPICS DISCUSSED INTHIS REPORT, READERS MAY CONSULT THE FOLLOWINGWEBSITES:http://www.nwrc.usgs.gov/

http://www.cwp.org/pubs_download.htm

http://www.epa.gov/OWOW/NPS/urbanize/report.html