Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem

Journal of Hydrology (2006) 329, 500– 513

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Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem Christopher D. Arp a,*, Michael N. Gooseff b, Michelle A. Baker a, Wayne Wurtsbaugh c a b c

Department of Biology, The Ecology Center, Utah State University, Logan, UT 84322, USA Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA Department of Aquatic, Watershed and Earth Resources, The Ecology Center, Utah State University, Logan, UT 84322, USA

Received 19 September 2005; received in revised form 4 March 2006; accepted 8 March 2006

KEYWORDS

Summary The hydrology of streams and lakes has been well studied as separate ecosystems; however, the behavior and implications of hydrologic linkages between these ecosystems have been little considered. We analyzed the surface-water hydrodynamics of a stream–lake ecosystem for 3 years in the Sawtooth Mountains, Idaho, USA to understand how this coupled aquatic ecosystem behaved hydrologically. This analysis included quantifying streamflow regimes above and below the lake, the expansion and contraction of the lake, stream, and floodplain, and downstream responses to snowmelt and rainstorm events. Our results showed that flow regime metrics from both hydrology and ecology were similar between above and below-lake reaches, but analysis that considered channel capacity and overbank flooding showed 2· more frequent and >5· longer duration floods below the lake compared to upstream reaches. The lake surface area expanded by as much as 19% during snowmelt runoff because of a 0.5 m rise in the lake level, and the littoral zone expanded by an even greater proportion. The lake had little influence on peakflows during spring snowmelt, though the inlet stream in the delta had a reduced flood magnitude compared to upstream stations. However, during large summer rainstorms when potential storage capacity was maximized, the lake strongly attenuated peakflows downstream from the lake. Water level changes at seven other stream–lake ecosystems in the region showed a similar range of variation. Our results provide a case study of how coupled aquatic ecosystems behave hydrodynamically and underscore the need to consider the hydrologic connections and interactions that drive aquatic ecosystems. c 2006 Elsevier B.V. All rights reserved.

Streams; Lakes; Flooding; Flow regimes; Littoral zone; Sawtooth Mountains; Idaho; River regulation; Lake morphometry



* Corresponding author. Tel.: +1 435 797 8963. E-mail addresses: [email protected], chrisarp_70@hotmail. com (C.D. Arp).



0022-1694/$ - see front matter c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.03.006

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem

Introduction The dynamic hydrologic behavior of aquatic environments is a principle driver of ecosystem processes (Baker et al., 2000; Brinson, 1993; Fisher et al., 1998) that creates habitat for aquatic and riparian organisms (Andersen and Cooper, 2000; Merritt and Cooper, 2000; Poff et al., 1997; Richter et al., 1997), provides human services (e.g., supply of quality water) and causes hazards (e.g., drought and flooding) (Black, 1997; Naiman and Turner, 2000). The ways in which hydrologic regimes are conceptualized, studied, and managed, however, often vary considerably among ecosystem types (e.g., streams, lakes, and wetlands) and disciplinary interests (e.g., engineering, geology, hydrology, and ecology), even though different aquatic ecosystems are usually hydrologically connected (Winter, 1999, 2001a). In lotic environments, hydrodynamics are described by annual or seasonal flow regimes (Poff, 1996; Post and Jones, 2001) and how these flows interact with the stream channel and floodplain (Leopold, 1994; Wohl, 2000). Stream ecologists have quantified flow regimes by statistically summarizing annual hydrograph records to describe aspects such as streamflow magnitude, frequency, duration, timing, and rate of change (Poff, 1996; Poff et al., 1997; Richter et al., 1996; Sanz and Jalon del Garcı´a, 2005), while hydrologists and engineers often use probabilistic analyses of flow regimes such as flow duration and flood frequency curves (Mosley and McKerchar, 1993); both of which provide similar characterization and understanding of flow regimes in many respects. Geomorphologists and riparian ecologists often place particular emphasis on sediment transporting flows (i.e., effective discharge) and flows related to channel and floodplain dimensions (i.e., bankfull discharge) (Knighton, 1998; Mosley and McKerchar, 1993; Nakamura et al., 2000). In lentic environments, system hydrodynamics are more commonly considered and quantified in terms of mixing, storage, and hydraulic residence time (Hostetler, 1995; Monsen et al., 2002). For example, a lake may be classified as dimictic because of spring and fall turnover and have a mean residence time calculated from stream inflow and lake volume. Many limnologists are also beginning to consider hydroperiods and water sources as important ecological drivers in lakes (Kling et al., 2000; Magnuson and Kratz, 2000). Both lotic and lentic ecosystems are commonly separated and further classified by hydrogeomorphology according to such factors as landscape position, landform, water sources, and seasonal hydrodynamic regimes (Brinson, 1993; Cowardin et al., 1979; Magnuson and Kratz, 2000; Poff, 1996; Whiting and Bradley, 1993; Winter, 2001a). For example, a mountain snowmelt-driven stream mainly occupies a channel maintained by annually consistent peakflows and sediment flux (Poff, 1996; Whiting et al., 1999; Wohl, 2000), while a drainage lake is an open-water basin fed mainly by stream water, and it stores and mixes water according to rates of inflow and outflow combined with vertical stratification regimes (Monsen et al., 2002). These examples of a snowmelt-driven stream and a drainage lake illustrate that the amount and timing of streamflow to a lake will influence lake mixing, residence time, and size,

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while lake storage dynamics control the hydrologic behavior of the effluent stream. Connections among adjacent aquatic ecosystems, such as streams and lakes set in a surface drainage network, warrant consideration when analyzing and evaluating their collective hydrologic behavior and consequential ecological significance. Few studies have considered how natural stream–lake systems mutually operate and interact (but see: Cole and Pace, 1998; Essington and Carpenter, 2000; Magnuson and Kratz, 2000), even though drainage lakes are common from the lowland tropics to arctic and alpine regions (Meybeck, 1995). Understanding how connected streams and lakes interact and mutually operate seasonally and interannually follows the need of increasing our scale of study in the hydrological sciences (Kling et al., 2000; Magnuson and Kratz, 2000; Winter, 2001a) and may provide insight into how streams and lakes behave individually and as coupled watershed components that influence watershed hydrology and ecology. Our objective is to analyze and describe the hydrodynamics and hydrologic regimes of a coupled stream–lake ecosystem located in the Sawtooth Mountains, Idaho, USA. This is a small moraine-dammed lake located in a secondorder drainage network that is representative of many headwater stream–lake ecosystems, particularly those in glaciated mountain and boreal environments. We used 3years (spring to fall) of streamflow, lake and stream water levels, and climate data, as well as detailed topographic maps to quantify and compare flow regimes, stream and lake morphometric changes, and responses to snowmelt and rainstorm events. Fortuitously, the year 2003 corresponded to a high runoff year, while 2002 and 2004 were near-average water years, allowing us to compare interannual responses to climate variation. We also measured seasonal water-level changes in seven other stream–lake ecosystems in the Sawtooth Mountains to provide some comparison of intersite variation. Our analysis uses hydrologic methods and descriptions used by lake and stream ecologists, hydrologists, and geomorphologists to compare approaches and the insights they provide for understanding coupled hydrologic ecosystems.

Study area Our primary study site, Bull Trout Lake (BTL), is located at the northern edge of the Sawtooth Mountains in south-central Idaho in the headwaters of the South Fork Payette River (Fig. 1). Springs Creek, which feeds BTL, is a second-order, low-gradient (0.003–0.007 m/m), gravel-bedded stream that flows through a wide, glaciated valley. A small, active sand-gravel delta is formed at the lake inlet, and a delta plain extends at least 200 m above the lake as evidenced by flat valley bottom topography, sediment deposits (Kiilsgaard et al., 2003), and divergent secondary channels (Fig. 1). BTL was formed by a moraine dam and is mostly fed by Springs Creek. The lake has an average surface area of 30 ha and mean and maximum depths of 4.3 and 15.0 m, respectively. The deepest portion of the lake is near the stream inlet and much of the west and north edges have expansive littoral shelves; 17% of the lake’s surface area is <1 m deep during average lake levels. The lake outlet forms

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Figure 1 A map of the principle study area at Bull Trout Lake showing the location of streamflow gauging stations (A represents above lake stations and B represents below lake stations) and a map of the drainage network and lake study sites of the Sawtooth Mountain lake district area (AL = Alturus Lake, BTL = Bull Trout Lake, CP = Curiosity Pond, MDL = MacDonald Lake, PL = Pettit Lake, SDP = Sundew Pond, SL = Stanley Lake, and YBL = Yellowbelly Lake).

Warm Spring Creek, which flows down a wide straight channel, through a small fluvial marsh that is partially formed by a road dam and culvert 450 m below the lake. The stream continues to flow over a slightly steeper moraine, 0.005 m/ m, before entering a flatter, 0.002 m/m, glacial outwash valley 1000 m downstream from the lake. The entire stream–lake–stream segment used for this study was delineated at 1.9 stream-km above the lake from where two first-order streams merge, through BTL (0.4 km long), and 2 km below the lake to where the next major tributary enters Warm Springs Creek (Fig. 1). The drainage area increases from 9 km2 at the upstream head to 14 km2 at the downstream tail of this study segment, and drops 15 m in elevation over this distance of 4.3 km. The watershed is mountainous, but less so than the central portion of the Sawtooth Mountains that has extensive alpine zones. The maximum watershed elevation is 2550 m and lake outlet is circa 2118 m. Watershed lithology is biotite granodiorite from the Idaho Batholith (Kiilsgaard et al., 2003) and was most recently shaped by late Pleistocene glaciers that reached a last glacial maximum 17 ka with some glaciers still advancing at 13 ka (Thackray

et al., 2004). The valley bottom above the lake is composed of mixed Pleistocene till and Holocene alluvium and colluvium, while below the lake is primarily glacial till and proglacial outwash (Kiilsgaard et al., 2003). Landcover is mostly lodgepole pine (Picea contorta) forest. The valley bottom riparian areas are dominated by willows (Salix spp.), sedges (Carex spp.), and various grasses with more diverse herbaceous and bryophyte vegetation associated with springbrooks, fens, and other seep wetlands. Winter snowfall and spring snowmelt strongly influence regional hydrology of the Sawtooth Mountains. Average water-year precipitation for this area is 107 cm, with 64% as snowfall (Banner Summit NRCS SNOTEL #312, 2140 m elevation located <2 km from BTL). For the study years 2002– 04 maximum snow–water equivalents (SWE) were 102, 122, and 90 cm, respectively, and summer rainfall totals were 10.2, 11.4, 16.5 cm, respectively. During these study years, the majority of snowpack had ablated by mid-June, and rainfall came in a mix of both intense thunderstorms and low intensity multi-day storms. Ice-out of BTL occurred approximately on May-20 in 2002, May-31 in 2003, and May-1 in 2004.

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem The locations of the other 7 stream–lake ecosystems surveyed in 2004 are shown in Fig. 1. These lakes are all moraine-dammed and range in size from <0.1 to >5 km2 and are fed by first- to fourth-order streams (Fig. 1). Five of these stream–lake sites occur farther south of BTL where the climate is slightly drier, 72 cm mean annual precipitation (Galena Pass NRCS SNOTEL #489, 2270 m elevation).

Methods The Bull Trout Lake study area was instrumented hydrometrically beginning in the spring of 2002 with the primary goal of measuring water fluxes through the lake and its upstream and downstream channel segments (Fig. 1). Streamflow was gauged at stations 1895 and 336 m stream-distance above (henceforth referred to as A-1895m and A-336m) and 477 and 2023 m below (henceforth referred to as B-477m and B-2023m) BTL in 2002–2004 using pressure transducers to record hourly water levels. An additional 4 gauging stations were installed in 2004 at 995 and 30 m above (A-995m, A30m) and 90 and 1400 m below (B-90m, B-1400m) the lake using capacitance rods to record hourly water levels. Stations remained at the same locations each year, but the pressure transducers were reinstalled in April of 2003 and 2004 to avoid ice and snow damage during the extreme winter. All gauging stations were located in pools as natural control points, and instruments were anchored with steel fence posts, which also served as staff gauges and survey benchmarks. Hourly flow records were developed for each station and year using a rating curve from 8 to 17 area–velocity discharge measurements using a top-setting wading rod and electromagnetic velocity meter throughout the full range of flows while stage was being recorded following methods in Buchanan and Somers (1976). The annual peakflow was almost exactly measured at most stations in 2002 and 2004, however, in 2003 runoff was larger and exact peakflow measurements were not made for safety and logistical reasons. Area–velocity measurements using a wading rod and velocity meter are often subject to ±5% error (Mosley and McKerchar, 1993), which potentially added even greater uncertainty to our rating curves and thus overall discharge records. Although we recorded stage hourly, many of the data used for analyses in this study are based on daily means. Two water-level recording capacitance rods were installed in the lake near the inlet and outlet during 2003 and 2004 before full ice out and each datum was incorporated into topographic surveys. Lake staff gauge measurements were also taken manually in 2002, when no stage recorder was installed. The lake perimeter and littoral zone, stream channels, floodplains, and valley bottom surfaces were mapped using an electronic total station during mid-June of 2004 when the regional water table was high. We recorded >4000 points for this survey with emphasis on land-water interfaces (i.e., lake shore and stream banks) and 16 of these points were geo-referenced benchmarks. The fixed datum for each staff gauge was included in this survey. Deeper lake bottom surfaces were measured from a boat with an acoustic depth sounder and concurrent GPS readings. A 4-m digi-

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tal elevation model (DEM) was developed from this topographic dataset and more detailed 1-m DEMs were developed for the lake and 4 individual stream reaches (200 m length) near gauging stations where more detailed surveying was completed. Volumes and surface areas for the lake were calculated for each daily mean surface elevation, along with the 4 stream reaches, using 1-m DEMs and geometric analysis tools (Surfer, Golden Software 2003). Exact stream reach distances were measured using a 100-m tape held along the channel thalweg during baseflow. Bankfull depth (DB) and discharge (Q B) were determined for each gauging station from longitudinal plots of the channel bed, water-surface, 2004 high-water marks, and the bank; each was fit with a least-squares line, where DB is the elevation distance between the average bed and average bank elevations (Emmett, 1975). Rain and snow precipitation data were obtained from the Banner Summit SNOTEL (located at 2140 m elevation <2 km from BTL), which records daily precipitation totals to ±0.025 cm water equivalent. Hourly precipitation data were obtained from the USFS weather station at the Stanley Ranger Station, which is located 20 km SE of BTL. To compare our analysis of stream–lake hydrodynamics at BTL to similar ecosystems in the Sawtooth Mountains, we measured approximate annual minimum and maximum water levels at 7 other stream–lake ecosystems (Fig. 1). During snowmelt runoff in May 2004, we flagged the water surface at 5 points along the channel of each stream (inlets and outlets) and 6 points around each lake. These sites were revisited in late August and we measured the relative elevation of flagged points compared to current water levels using an engineer’s level and stadia rod. In our analysis of hydrologic-event responses, rainstorm lags were measured using hourly stage data from the point of streamflow rise (Q prestorm) to the preceding stream discharge maximum (Q stormpk) (Dingman, 2002). Snowmelt peakflow lags were measured using daily stream stage data from the day of maximum snowmelt, determined from the rate of change in SWE, to the preceding maximum daily stream discharge (Q pk). Flow regime metrics follow the standard methods and equations presented in Clausen and Biggs (2000), including mean annual flow (MF), annual coefficient of variation (CV), number of high flow events exceeding 5· the median flow (FRE-5), and average duration of high flows exceeding 5· the median flow (DUR-5). The development of flow duration curves follow methods in Dingman (2002) for multiyear records, however we used minimum discharge estimates for the winter months when flow was not measured (these data were not directly included in the analysis).

Results Runoff event responses The stream stations above BTL showed a relatively consistent pattern of decreasing magnitude of snowmelt peakflow approaching the lake in all 3 years (Table 1). These abovelake stations were separated by less than 2 km with no major tributary contributions; yet the peakflow magnitude (Q pk) was 5% higher on average at A-995m compared to

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Table 1 Summary of peakflow responses at study area gauging stations during spring snowmelt and summer rainstorms for the years 2002–2004 (stations are labeled A for above lake and B for below lake and distances are stream lengths from the lake; Qpk = maximum hourly mean discharge, MF = mean annual discharge, Qstormpk = maximum hourly discharge following a stormevent, Qprestorm = discharge prior to the stormevent; snowmelt response lag is in days from maximum snow–water equivalent loss and rainstorm lag response is in hours from the start of the rising limb) Year/station

Snowmelt responses 3

Rainstorm responses

Qpk (m /s)

Qpk/MF

Date of peak flow

Lag to peak (days)

Qstormpk (m3/s)

Qstormpk/Qprestorm

Date of storm

2002 A-1895m A-336m B-477m B-2023m

1.35 1.30 1.87 –

7.2 5.3 7.8 –

Jun-1 Jun-1 Jun-1 –

12 12 12 –

0.73 0.81 0.59 0.77

2.5 2.2 1.4 1.8

Jul-8 Jul-8 Jul-8 Jul-8

4 4 9 5

2003 A-1895m A-336m B-477m B-2023m

1.89 1.78 3.10 3.73

7.8 5.3 7.4 8.4

May-29 May-31 May-31 Jun-3

4 6 6 9

0.50 0.42 0.30 0.31

4.8 4.1 1.9 1.7

Aug-22 Aug-22 Aug-22 Aug-22

3 4 1 3

2004 A-1895m A-995m A-336m A-30m B-90m B-477m B-1400m B-2023m

0.89 1.58 1.05 1.00 1.13 1.53 1.23 1.31

5.5 8.2 5.7 5.0 5.7 6.6 5.1 4.7

Jun-5 Jun-5 May-5 Jun-6 May-29 Jun-6 Jun-6 May-28

34 34 3 35 27 35 35 26

0.23 0.25 0.29 0.20 0.23 0.25 0.25 0.31

1.4 1.3 1.3 1.1 1.1 1.2 1.1 1.3

Jul-18 Jul-18 Jul-18 Jul-18 Jul-18 Jul-18 Jul-18 Jul-18

3 4 4 3 20 20 33 21

A-336m downstream nearer to the lake inlet. The relative flood magnitudes were also higher on average at A-1895m, 6.8· greater than the mean annual flow (MF), than at A336m, 5.4 · MF (Table 1). This variable pattern of relative snowmelt response upstream from the lake was best seen in 2004 when Q pk events ranged from 5.0 to 8.2· MF and were separated in timing by a month; although a nearly identical magnitude flow to the May-5 Q pk also occurred on Jun-6 at A-336m (Fig. 2C). Stream snowmelt responses downstream from the lake were in the same range of variation in timing and magnitude as upstream from the lake, and also showed considerable variation among relatively close stations (Table 1). In the high runoff year 2003, below-lake Q pk were nearly twice the magnitude as at upstream stations, while in the lower runoff year 2004, Q pk were similar in magnitude between stations upstream and downstream from the lake, with the exception of A-995m that peaked much higher (Table 1 and Fig. 2). The storm events we selected for analysis had daily rainfall totals of 2.0 cm in 2002, 3.6 cm in 2003, and 1.5 cm in 2004, each with approximate durations of 2–3 h. For each of these runoff events, Q stormpk consistently lagged the initial streamflow response by 3–4 h at all stations above the lake with magnitudes that varied by storm-event size (Table 1). However, the streamflow responses at stations below the lake were less consistent among these 3 storms. On July 7th of 2002, an intermediate-sized storm produced a peak-

Lag to peak (h)

Figure 2 Stream hydrographs and cumulative water yields for Bull Trout Lake inlet (A-336m) and outlet (B-477m) for 2002 (A), 2003 (B), and 2004 (C).

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem flow 9 h later at B-447m compared to a 4 h response-lag in upstream reaches. During this same event, the station B2023m farther downstream from the lake lagged the storm by 5 h (Table 1). The largest rainstorm recorded during this 3-year period is detailed in Fig. 3. The influence of lake storage was apparent during this event by comparing Q stormpk above the lake, 0.42 m3/s at A-336m, to below the lake, 0.30 m3/s at B477m. However, our analysis showed only a 1-h lag at B477m compared to a 3–4 h lag at stations upstream and far downstream from the lake (Fig. 3 and Table 1). This seeming discrepancy was due to rainfall directly onto the lake surface, which we estimated to be 10,800 m3 and closely approximated the additional water flux at the outlet of 8150 m3 during the 2-h storm. Besides this rapid initial response to rainfall on the lake surface, no streamflow response was observed at the lake outlet (i.e., Q stormpk was completely attenuated; Fig. 3). At B-2023m we observed a similar lag response as in above-lake stations, but with a much smaller relative peakflow of 1.7 · Q prestorm compared to upstream stations that averaged 4.5 · Q prestorm (Table 1). The smaller Q stormpk at the below-lake station B-2023m had a much longer duration than above-lake stations. The smallest storm we analyzed was in 2004 and showed the longest below-lake response-lag of >20 h with a correspondingly small streamflow-peak response of 1.1–1.3 · Q prestorm. During this storm, above-lake stations had lag

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responses of 3–4 h and relative magnitudes of 1.1–1.4 · Q prestorm; the 1.1 · Q prestorm occurred at the station nearest the lake delta (Fig. 1 and Table 1).

Flow regime comparisons Mean annual flows (MF) increased progressively downstream among stations with increasing drainage area. Mean annual runoff (MF/drainage area) averaged over the 3 years of record by water year were similar among most stations with runoff of 0.60 m/year at A-1895m, 0.61 m/year at B-477m, and 0.62 m/year at B-2023m, though station A-336m, located in the delta plain, had somewhat higher runoff of 0.65 m/year. Average annual precipitation for the study period was 1.05 m, 58% of this amount was runoff. Coefficients of variation (CV) for the daily mean discharge of the 3-year (non-winter) record were relatively similar among the 4 stations ranging from 1.42 to 1.55, with no differences noted between above and below lake stations (Table 2). Similarly, standard metrics of streamflow frequency and duration at 5·-MF showed minimal differences among the stations relative to the lake. Flow duration curves plotted for these same 4 stations also were similar in shape and only offset to higher flows in the downstream direction (Fig. 4 and Table 2). These statistical and graphical analyses indicated similar flow regimes between stations above and below the lake for daily mean flow observations.

Figure 3 Stream hydrographs and local hyetograph during the largest stormevent on record (Aug-22 2003) for the 3-year study period at Bull Trout Lake study area.

Table 2 A summary of the 2002–2004 flow regime of stream reaches above and below Bull Trout Lake describing mean annual flow (MF), coefficient of variation (CV), frequency of flows above 5· median annual flow (FRE-5), frequency of flows exceeding bankfull discharge (FRE-QB), duration of flows exceeding 5· median annual flow (DUR-5), and duration of flows exceeding bankfull discharge (DUR-QB) (study reaches are labeled A for above lake and B for below lake and distances are stream lengths from the lake) Study reach

Qmean (m3/s)

CV

FRE-5 (days)

FRE-QB (days)

DUR-5 (days)

DUR-QB (days)

A-1895m A-336m B-477m B-2023m

0.17 0.20 0.23 0.27

1.51 1.42 1.55 1.47

1.33 1.33 1.33 1.33

0.33 1 2 2

31 33 27 27

4 2 11 16

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Figure 4 Flow duration curves for water years 2002–2004 for stream gauging stations at the Bull Trout Lake study area (winter and early spring periods were not measured, but estimated for computation purposes and these data not shown in this graph; QB = bankfull discharge and days/year are flood durations).

However, when making comparisons of flow regimes relative to channel size and bankfull discharge (QB), a different pattern was very evident between stations above and below the lake. Our gauging record coupled with geomorphic maps of channels and floodplains showed that annual frequency of overbank flooding occurred 1-in-3 years at A-1895m and once a year at A-336m, while below-lake streams topped their banks twice a year on average during the 3 years of mean daily flow record (Table 3). Differences in overbank flood duration among stations was even more pronounced; on average streams were in flood 16 days/year below the lake compared to 2–4 days/year above the lake. When flow duration curves were compared relative to QB, a similar pattern of longer flood durations was shown in below-lake stations compared to above-lake stations (Fig. 4).

Lake and stream morphometry dynamics The mean BTL elevation during ice-free months of 2003 and 2004 was 2117.85 m, which corresponded to a surface area of 30.1 ha and volume of 1.29 · 106 m3. However, the lake stage varied by 0.76 m in 2003, peaking on May-31 and reaching a minimum level on Jul-27,and 0.36 m in 2004, peaking on May-6 and reaching a minimum level on Septem-

ber-20. Maximum lake levels in these 2 years occurred on the same date as Q pk at the delta plain station A-336m. In 2002, we documented a lake stage change of 0.28 m from weekly measurements compared to hourly measurements in the subsequent years. For reference, an adjacent groundwater fed lake, Martin Lake, varied by 0.26 m in 2003. The mean lake surface elevation of 2117.85 m may correspond to the flood stage of the lake, according to analysis of changes in mean lake depth (volume/surface area), which reached its maximum of 4.3 m at this elevation, but declined to 4.2 m at both lower and higher lake surface elevations. These changing water levels in BTL expanded the lake surface area by 18% in 2003 and 5% in 2004 relative to the mean surface area (Fig. 5A) with similar, proportional expansion of lake volume during the same years (Fig. 5B). Additionally, the lake surface area shrank to 28.5 ha during the lowest surface elevation recorded, 2117.59 m, during the dry summer of 2003. Lake water-levels were more stable during the wet summer of 2004, varying by less than 0.13 m in July and August. By arbitrarily delineating the lake’s littoral zone as areas <1 m depth, we examined how this ecologically important limnetic zone changed in size with water level. At mean lake surface elevation, the littoral zone accounted for 18%

Table 3 Summary of water level (WL) changes from May to August 2004 at eight stream–lake ecosystems located in the Sawtooth Mountains, Idaho and some of their geographic characteristics Lake

Lake area (km2)

Drainage area (km2)

Area of lakes upstream (km2)

Lake WL change (m)

Inlet stream WL change (m)

Outlet stream WL change (m)

Alturus Bull Trout Curiosity MacDonald Redfish Sundew Stanley Yellowbelly

3.06 0.29 0.12 0.11 5.67 0.19 0.65 0.37

84.9 11.7 1.0 26.1 108.9 4.03 40.6 31.3

0.06 0 0 0.47 0.09 0 0.02 0.87

0.40 0.27 0.20 0.21 0.27 0.22 0.41 0.12

0.45 0.31 0.11 0.26 0.12 0.05 0.12 –

0.29 0.15 0.20 – 0.26 0.18 0.26 0.12

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem

Figure 5

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Temporal changes in lake surface-area (A), littoral zone (B), and lake volume (C) of Bull Trout Lake during 2003 and 2004.

of the total lake surface-area. The littoral zone expanded to 21% of the total area at the maximum lake elevation of 2118.4 m and contracted to 17% of the total area at the minimum lake elevation of 2117.6 m. The littoral zone was 55% larger during the maximum lake elevation compared to the lowest lake elevation in 2003, but only 15% larger in 2004 (Fig. 5C). We performed a similar water level response analysis on 4 stream reaches, each 200 m long, located above and below the lake to describe how surface-water extents changed over time among these reaches (Fig. 6). This analysis showed a number of important differences among stream reaches primarily related to interactions between channel and floodplain morphology. The most upstream reach, 1.7 km above the lake, showed the smallest temporal variation in wetted width (wetted surface area/stream length) among reaches ranging from 1.5 to 10.1 m with a median width of 3 m (Fig. 6). The wetted width of the reach 0.2 km above the lake, in the delta, ranged from 3.6 to >100 m over this 2-year period, with a median width of 5.5 m. Stream bankfull width in this reach was 2.8 m, which only represented active channel dimensions, whereas our analysis of wetted width also accounted for numerous secondary channels and backwaters. Even at stream stages lower than bankfull, we observed substantial perirheic

flooding throughout the delta likely related to its convex surface, shallow water table, and interactions with the lake. The stream reach directly below the lake had a median wetted width of 14.2 m that ranged from 11.8 to 28.6 m with overbank flooding occurring for 43 days in 2003 and 53 days in 2004, though the spatial flood extent was less than in other reaches (Fig. 6). At the study reach 1.9 km below the lake, wetted width ranged from 2.9 to 44.6 m with a median width of 3.7 m (Fig. 6).

Intersite comparisons To examine how representative our results from BTL were to other stream–lake ecosystems, we compared water level and hydrogeographic data from several other stream–lake ecosystems throughout the Sawtooth Mountains. The mean annual stage variation of these lakes was 0.27 m and ranged from 0.12 m at Yellowbelly Lake to 0.41 m at Stanley Lake. These two lakes and their watersheds are of similar size, yet Yellowbelly Lake has 3% of its drainage area covered by upstream lakes, while Stanley has only a tiny fraction, <0.01%, which are all headwater tarns. Alturus Lake also had a similar stage change as Stanley Lake and a similarly small area of headwater lakes upstream (Table 3). Otherwise, lake size and drainage area did not appear to be

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Figure 6 Temporal changes in wetted width (wetted surface-area/channel length; includes the stream channel, backwaters, secondary channels, and overbank and perirheic flood water) in our stream reaches near Bull Trout Lake during 2003 (A) and 2004 (B). Dashed lines independently measured bankfull depth (DB) for each reach (depth is in parenthesis).

related to observed patterns in annual stage change for this set of lakes. Though these paired inlet and outlet streams occur in similar landscape positions and with similar drainage areas, their annual stage changes were not correlated (r = 0.48, p = 0.28), nor were lake stage changes correlated to those of influent streams (r = 0.45, p = 0.26). However, stage changes between lakes and their effluent streams were more strongly related (r = 0.84, p < 0.01), which generally followed the more detailed patterns observed between BTL and its effluent stream.

Discussion Stream flow regimes Characteristic changes in discharge over relevant time periods (i.e., a day, season, or year) describe a stream’s flow regime (Poff, 1996; Post and Jones, 2001) and represent an important aspect of watershed and river behavior (Leopold, 1994) that strongly influences stream and riparian ecosystems (Poff et al., 1997; Richter et al., 1997) and human water supplies and potential flood hazards (Black, 1997; Naiman et al., 2002). Climate, coupled with watershed size, topography, and runoff characteristics, are

the principle drivers affecting flow regimes (Poff, 1996; Black, 1997; Mosley and McKerchar, 1993; Wolock et al., 2004). Streams in the Sawtooth Mountains have snowmelt-dominated flow regimes characterized by predictable spring floodpeaks that decline by an order of magnitude to baseflow conditions by the late summer and fall with relatively little interannual variation (Emmett, 1975; Whiting et al., 1999). A primary interest in this study was to compare flow regimes upstream and downstream of lakes to understand the effects of natural river regulation. Using standard methods from hydrology (i.e., hydrograph analysis and flow duration curves) and stream ecology (i.e., statistical metrics of flow records) we found little difference between reaches above and below the lake. However, when we analyzed flow regimes relative to channel size and flow conditions (i.e., overbank flow or flood stage), a very different comparison of flow regimes emerged. Our results show more frequent overbank flooding of much longer duration in reaches downstream from the lake compared to upstream stations. The flood frequencies associated with stations above BTL, albeit a very short record, corresponded to average bankfull return intervals reported for other streams throughout the Sawtooth Mountains of 1.5–2 years based on long-term records (Emmett, 1975; Whiting et al.,

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem 1999). Our data suggested that streams below lakes experience overbank floods every year giving a return interval of <1.1 years; but a longer flow record or intensive hydrologic modeling would be required to verify this. An analysis of stream geomorphology in the Sawtooth Mountains showed that lake outlet streams consistently have smaller bankfull discharge compared to paired lake inlets, yet have similar peakflow magnitudes (Arp et al., unpublished data). This may be partially related to buffering of flood from rainstorms that was observed in this study, but most likely is a function of reduced sediment delivery below lakes for construction of floodplains that are sized to bankfull flows. Similar variation in flow regimes has been observed in larger stream–lake ecosystems on the Kenai River in Alaska (Dorava and Milner, 2000), though these flow regimes were not directly placed in the context of channel capacity. These findings, coupled with our results, suggest notable differences in streamflow regimes above and below lakes that occur due to variation in channel and floodplain size rather than major seasonal lake-modifications to streamflow quantity and timing. The discrepancy we found using different approaches to flow regime analysis points to the need to consider flow regimes relative to their channels and floodplains, which place the varying fluxes of water into a context relevant to lotic and riparian ecosystem processes, habitat, and organism requirements. Poff et al. (1997) and Richter et al. (1996) suggest that flow regimes are constituted by magnitude, frequency, timing, duration, and rate of change, which can be quantified and compared using a redundantly large variety of statistical metrics (>171) (Olden and Poff, 2003), and the number of metrics continues growing (e.g., Sanz and Jalon del Garcı´a, 2005). The physical and ecological significance of these metrics have only been demonstrated using correlation approaches (i.e., Clausen and Biggs, 1997), but without a necessarily strong mechanistic basis. Conversely, the frequency, timing, and duration of overbank flooding have been convincingly demonstrated to have physical and ecological significance to the interrelated processes of channel structuring and maintenance (Andrews, 1984; Knighton, 1998; Leopold and Wolman, 1957; Whiting et al., 1999), riverine sediment and nutrient cycling (Arp and Cooper, 2004; Valett et al., 2005), and habitat maintenance and connectivity (Andersen and Cooper, 2000; Cooper et al., 2003; Junk et al., 1989; Robertson et al., 2001). Results from our study provide a case where statistical hydrograph analysis provides no clear indication of major differences between streamflow regimes, which were otherwise very apparent. How much water a river needs (i.e., Richter et al., 1997), depends on the size of its channel and how often and to what extent this water exceeds the channel capacity to inundate the floodplain and other fluvial habitats. Furthermore, with increasingly common and coupled modifications to both channels (e.g., straightening and diking) and streamflow (e.g., river regulation from dams and urban and tiledagriculture runoff), analyzing flow regimes according to channel capacity and flooding should be a necessity for all types of environmental assessment, not just public safety and design of structures.

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Lake expansion and contraction Much of physical limnology has focused on large lakes where stratification, vertical mixing patterns, and regional climate are the dominant drivers of lake hydrodynamics (Hostetler, 1995). In large lakes, changes in lake water levels may have negligible effects on surface area and volume; though examples of large shallow lakes profoundly affected by water level change certainly exist (e.g., Great Salt Lake and Lake Chad). In small lakes, which are abundant and common in glaciated mountain and boreal regions (Meybeck, 1995), relatively small variation in water levels may cause large changes in lake surface-area and volume, and thus other lake functions (Juutinen et al., 2001; Smith et al., 2005; Winter, 1995). For example, susceptibility of lakes to eutrophication is often predicted using mean hydraulic residence time or flushing time, which is a function of lake volume. Also because small lakes vary greatly in morphology, small changes in water level may exert large proportional changes in functional zones, such as the littoral relative to pelagic zones, which can be an important determinant of lake productivity and food web structure (Turner et al., 2005; Vander Zanden and Vadeboncoeur, 2002). In the Sawtooth Mountains, we found considerable variation in lake level fluctuations from 0.28 to 0.76 m interannually at one site and variation among 7 lakes from 0.12 to 0.41 m in the same year. Reported water level fluctuations for two intensively monitored small boreal lakes in Finland were 0.30 and 1.70 m (Juutinen et al., 2001) and a general survey of shallow Turkish lakes suggested annual ranges of <1 to >3 m annual variation (Coops et al., 2003). In our review of the primary lake literature, we found few published data on lake level fluctuations, but active calls for such reporting (Coops et al., 2003). Results from BTL and other Sawtooth Mountain lakes suggest water levels fluctuate within the lower range of other small lakes reported, however this is based on a very limited comparison. Water level fluctuations at our study lake corresponded to notable changes in both lake volume and surface area, which likely impact the analysis of other lake functions. For example, if seasonal changes in lake volume were not considered in measurements of flushing time at BTL, this would result in 13% error during peakflow and 3% error during baseflow in 2003. Similarly, the surface area of Bull Trout Lake expanded by up to 18% and contracted >5% in 2003 compared to the mean lake area; such variation would potentially produce corresponding change in other lake ecosystem calculations, such as evaporation, gas exchange, and ecosystem metabolism, if the surface area were treated as a constant, which is typically the case in such analyses. During this cycle of lake expansion and contraction we also showed changes in the littoral zone extent and proportion. Given the importance of the littoral zone in a variety of lake ecosystem processes (Turner et al., 2005; Vander Zanden and Vadeboncoeur, 2002), particularly in smaller lakes, these seasonal littoral zone dynamics warrant further consideration and quantification in other lakes. Of particular interest to lake ecologists may be the phenomenon of lake flooding, which we documented at BTL. Though likely a common hydrologic event in many lakes

510 fed by surface waters, particularly those associated with large rivers (e.g., oxbows and floodplain depressions) (Hillbricht-Ilkowska, 1999; Melack and Forsberg, 2001), this is rarely treated in studies of boreal and mountain lakes (Coops et al., 2003; Juutinen et al., 2001). We suggest that the flood stage of Bull Trout Lake is just above the mean lake elevation of 2117.85 m based on observed elevational zonation of aquatic, wetland, and upland vegetation and a maximum volume to surface area ratio (mean depth) occurring at this lake elevation. Whether this estimate has hydrologic or geomorphic significance is uncertain and should be further considered and studied. It seems intuitive, however, that lake flooding may play an important role in biogeochemical cycles (Juutinen et al., 2001), the abundance and composition of plankton and macrophyte communities, and as habitat for fishes and other organisms (Turner et al., 2005; Vander Zanden and Vadeboncoeur, 2002). We think the extent and timing at which flooding occurs interannually within and among lakes, how it should be physically defined, and how flooding might drive other lake functions warrant further investigation.

Hydrology of stream–lake ecosystems A potentially important area for hydrologic research is studying linked or coupled ecosystems (Kling et al., 2000; Winter, 2001b), such as where a stream merges with a lake. Ecosystems can be defined at any relevant spatial unit, but most studies and concepts of aquatic ecosystems are consistently bounded by surface-water extents, so that lakes, streams, and wetlands are studied independently and as discrete units (Magnuson and Kratz, 2000), while in hydrology and biogeochemistry, the watershed is often the relevant ecosystem unit of study (Black, 1997; Bormann et al., 1967). Commonly, watershed processes are further spatially distributed within these units (Beven and Kirby, 1979; Robson et al., 1992) and these analyses should make considerations beyond hillslopes and channels to hydrologic linkages among various terrestrial and aquatic ecosystems (Band et al., 2001), such as lakes, which are common in many watersheds. An important objective of this research was to provide a case study of how one coupled aquatic ecosystem functions hydrologically. In watershed hydrology, lakes are treated as depression storage on the landscape and are considered to affect runoff and streamflow processes (Pilgrim and Cordery, 1993). The construction of reservoirs is often, in part, for the purposes of flood control and their hydrologic functioning has been quantified intensively (e.g., Kessler and Diskin, 1991; Montaldo et al., 2004). However, few studies appear to have directly considered how natural lakes influence runoff and streamflow processes (but see: Dorava and Milner, 2000); though some work has quantified the influence of wetland extent and landscape position on flood attenuation (Mitsch and Gosselink, 2000). We analyzed the flood attenuation function of stream– lake ecosystems by quantifying peakflow timing and magnitudes at stations above and below a lake during both snowmelt and rainstorm events. Interestingly, during spring snowmelt runoff, when peakflows are largest, no clear flood attenuation response was observed in peakflow magnitude

C.D. Arp et al. or timing below the lake compared to upstream. This was likely related to slow snowmelt over a period of weeks coupled with gradual lake water-level rise and corresponding loss of storage capacity. Even in 2003 when rapid snowmelt occurred over several days, a sharper peak occurred downstream of the lake due to quick runoff from the expansive lower elevation snowpack. However, during summer rainstorms, lake effects on runoff responses downstream were notable and relatively consistent. In fact, during the largest rainstorm we recorded, 3.6 cm in 2 h, the only rise in streamflow below the lake occurred during the storm with only 90% increase in discharge above pre-storm baseflow, while the upstream response was 380% higher than prestorm baseflow. These differing responses in flood attenuation by a lake during snowmelt and rainstorm events correspond well to pre-event lake levels and corresponding storage capacity, representing an important and seasonally variable stream–lake hydrologic interaction. We also observed differences in runoff response at the station directly above the lake compared to stations 1– 2 km upstream. This station is located in a wide, flat to convex, valley that is part of the lake delta plain, and here floodpeaks were often smaller in magnitude during both snowmelt and rainstorm events compared to upstream stations. This varying response in the delta plain likely occurs due to divergent flow from the main stream channel into side channels, backwaters, and the floodplain during the rising limb of the hydrograph. Perirheic floodplain inundation often precedes overbank flooding (Mertes, 1997) and in this case may be driven by this deltaic topography and interactions with the lake surface elevation. Comparison of surface topography and flow-net maps confirm these gradients in the delta region above BTL suggesting the extension of stream–lake interactions above the lake into the delta plain, which was part of the historic lake basin and has since filled with sediments.

Implications to artificial stream–lake ecosystems As hydrologic systems, small reservoirs are very similar to drainage lakes, but have been analyzed more extensively (Kessler and Diskin, 1991; Montaldo et al., 2004) because they are specifically designed and built for water storage, hydroelectric power, and flood control. The primary difference in these human-created aquatic ecosystems is that in many cases water levels can be actively controlled by regulating outflow and dam landscape position. Management of reservoir water levels and storage, however, is highly dependent on seasonal and interannual variation in climate and subject to societal demands (i.e., flood control and water and power supply) and environmental regulations (i.e., in-stream flows and riparian maintenance). In light of the continuing need to manage regulated rivers, the hydrologic behavior of naturally regulated river ecosystems warrant further consideration and analysis. The major differences in hydrologic behavior we quantified between reaches upstream and downstream from a lake, also corresponded to other differences including sediment size, channel shape (Arp et al., unpublished data), water chemistry, and temperature (Arp and Baker, unpublished

Surface-water hydrodynamics and regimes of a small mountain stream–lake ecosystem data). Because the regulation of rivers by dams and diversions is increasing and will likely persist into the foreseeable future (Graf, 1999; Nilsson et al., 2005), understanding the hydrologic and other behavior of naturally regulated rivers may provide insight into the long-term effects of artificial river regulation and how to best guide the management of regulated rivers more effectively long-term.

Conclusions This research provides a case study of how disparately studied aquatic ecosystems – streams and lakes – when studied together as coupled, interacting systems show interesting and potentially insightful hydrologic behavior. Though the hydrologic interactions between a stream and a lake seem simple and obvious, much of the surface-water hydrodynamics and flow regimes we observed are not directly addressed in the general sciences of streams, lakes, and mountain watersheds. Our results show that streamflow regimes and interactions with floodplains were strongly modified by a small mountain lake, both downstream from the lakes and also in the upstream delta plain. These responses were often not expressed by metrics commonly used for analyzing and classifying streamflow regimes. We suggest that analyses of flow regimes should evaluate flows relative to channel morphometry to better characterize fluvial processes and habitats. Similarly, our results show that lake morphometry can be dynamic, but may vary among lakes in a similar region, necessitating the consideration of these site-specific dynamics when evaluating other whole lake functions. Lakes also interact with stream networks to modify the largest rainstorm floodpeaks, while having little effect on the typically dominant snowmelt peaks – possibly having significance to the structuring and organization of stream channels in mountain lake districts. Whether these interactions occur in most other stream– lake ecosystems is uncertain, but seems likely. Our work here suggests the need to analyze and evaluate strongly coupled aquatic ecosystems, such as streams and lakes, which may provide insight into unexpected hydrologic behavior, controls on lotic and lentic ecosystem processes, and guidance toward more appropriately managing watershed ecosystems.

Acknowledgements We thank P. Brown, L. Symms, A. Myers, B. Koch, R. Hall, L. Jeffs, C. Craemer, K. Nydick, and M. Bozeman who helped with fieldwork for this project. J. Schmidt, K. Nydick, and R. Hall provided important technical and intellectual support. We thank R. Metz and K. Grover-Wier with Boise National Forest and L. Dean with the Sawtooth National Recreation Area for allowing us access to study areas where we did this work. This research was funded by the National Science Foundation (DEB 01-32983) and C. Arp’s graduate education was partially supported by a Subsurface Science Fellowship from the Inland Northwest Research Alliance and a STAR Fellowship from the US Environmental Protection Agency.

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