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Bellino, Jason C.
Effects of geomorphic setting on shallow-groundwater exchange and water temperature of salmon-bearing headwater streams of the Lower Kenai Peninsula, Alaska
h [electronic resource] /
by Jason C. Bellino.
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b University of South Florida,
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Thesis (M.S.)--University of South Florida, 2009.
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ABSTRACT: Temperature is an important physical characteristic of headwater streams that controls the presence and health of juvenile salmonids. Surface water temperature is controlled by many factors including exchanges with groundwater. A study of the hydrology of wetlands associated with headwater streams on the Lower Kenai Peninsula was conducted to determine the effect of geomorphic setting on groundwater discharge to streams and ultimately on in-stream water temperature. Attention was focused on drainage-way and discharge slope wetlands as two end-members of the geomorphic settings of the study area. Data were collected at 18 study reaches spanning four major watersheds in the study area. Surface water temperature and geochemical data were collected at all sites, while water levels were recorded at two heavily instrumented. Data showed discharge slopes had lower summer temperatures and more diffuse groundwater discharge than drainage-ways, though geochemical data showed the proportion of groundwater flowing through stream reaches was the same in both geomorphic settings. Thus, surface water temperature is influenced by groundwater discharge at the local scale, but not at the basin scale. Once groundwater emerges and becomes part of the surface water reservoir, it exchanges heat with the new environment and loses its temperature moderating properties, though it retains its geochemical signature.
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Advisor: Mark Cable Rains, Ph.D.
t USF Electronic Theses and Dissertations.
Effects of Geomorphic Setting on Shallow-Groundwater Exchange and Water Temperature of Salmon-Bearing Headwater Streams of the Lower Kenai Peninsula, Alaska By Jason C. Bellino A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Mark Cable Rains, Ph.D. Mark T. Stewart, Ph.D. Thomas L. Crisman, Ph.D. Date of Approval: November 2, 2009 Keywords: Hydrogeology, groundwater, te mperature, anadromous, land use Copyright 2009, Jason C. Bellino
i TABLE OF CONTENTS LIST OF TABLES ii LIST OF FIGURES iii ABSTRACT iv INTRODUCTION 1 SITE DESCRIPTION 3 METHODS 7 Physical Hydrology 7 Chemical Hydrology 9 Evapoconcentration and Ma ss-Balance Mixing Modeling 10 RESULTS 13 Physical Hydrology 13 Chemical Hydrology 18 Evapoconcentration Modeling 18 Mass-Balance Mixing Modeling 21 DISCUSSION 24 CONCLUSIONS 29 REFERENCES 30
ii LIST OF TABLES Table 1. Summary of continuous su rface water temperature data from lightly instrumented study reaches, 2007 through 2008 14 Table 2. Summary of instantane ous groundwater (GW) and surface water (SW) temperature data from heavily instrumented study reaches, 2007 through 2008 15 Table 3. Water chemistry data ( standard deviation), spring (May) through summer (August) 2008 20 Table 4. Proportion of groundwater c ontribution to surface water flow, spring (May) and summer (August) 2008 23
iii LIST OF FIGURES Figure 1. Location of the study area on the Kenai Peninsula, Alaska 4 Figure 2. Location of study reaches on the Lower Kenai Peninsula, Alaska 6 Figure 3. Site diagrams for (a) NANC44 upper (drainage-way) and (b) SANC1203 middle (discharge slope) 8 Figure 4. Water temperature data collected at (a) lightly instrumented and (b) heavily instrumented study reaches 14 Figure 5. Comparison of daily average surface water and groundwater temperature at (a) NANC44 and (b) SANC1203 16 Figure 6. Evapoconcentration model showi ng calculated evapoconcentration trend line 21 Figure 7. Conceptualization of shallow-groundwater flow in drainage-way wetlands in the (a) summer and (b) winter 25 Figure 8. Conceptualization of shallow-groundwater flow in discharge slope wetlands in the (a) summer and (b) winter 26
iv EFFECTS OF GEOMORPHIC SETTI NG ON SHALLOW-GROUNDWATER EXCHANGE AND WATER TEMPERATURE OF SALMON-BEARING HEADWATER STREAMS OF THE LOWER KENAI PENINSULA, ALASKA JASON C. BELLINO ABSTRACT Temperature is an important physical char acteristic of headwater streams that controls the presence and health of juve nile salmonids. Surface water temperature is controlled by many factors including exch anges with groundwater. A study of the hydrology of wetlands associated with headwa ter streams on the Lower Kenai Peninsula was conducted to determine the effect of geomorphic setting on groundwater discharge to streams and ultimately on in-stream water temperature. Attention was focused on drainageway and discharge slope wetlands as two endmembers of the geomorphic settings of the study area. Data were collected at 18 study reaches spanning four major watersheds in the study area. Surface water temperature and geoche mical data were coll ected at all sites, while water levels were recorded at two h eavily instrumented. Data showed discharge slopes had lower summer temperatures and more diffuse groundwater discharge than drainage-ways, though geochemical data s howed the proportion of groundwater flowing through stream reaches was the same in both geomorphic settings. Thus, surface water temperature is influenced by groundwater discharge at the local scale, but not at the basin scale. Once groundwater emerges and becomes part of the surface water reservoir, it exchanges heat with the new environment and loses its temperature moderating properties, though it retains its geochemical signature.
1 INTRODUCTION First-order headwater streams are abunda nt landscape features and comprise approximately 53 percent of to tal stream length in the United States (Nadeau and Rains, 2007). On the Kenai Peninsula of Alaska, they host juvenile and a dult salmonids during a key phase in their anadromous life-history cycle. One of the main factors controlling salmon presence and health is in-stream water temperature (Bjorn and Reiser, 1991; Matthews and Berg, 1997; Quinn, 1997; Mellina et al., 2002, Mauger, 2008; Sullivan et al., 2000; Craig et al., 1996), which is controlled in turn by ma ny factors (Constantz, 1998; Johnson, 2003) which are grouped into atmospheric and geologic controls. Atmospheric controls include magnitude of incident so lar radiation, shading, and air temperature. Geologic controls include stream width-depth ratio, ground temperat ure, and groundwater discharge. The focus of this study is geol ogic control, and groundw ater discharge in particular. Streams occupying two primary geomorphic se ttings were addressed in this study: drainage-ways and discharge slopes. These were previously defined by Walker et al. (2007) and are considered two end-members of a c ontinuum of geomorphic settings found within the study area. Drainage-ways are typically broa d, flat fens with lit tle vegetative cover; low-gradient streams meander slowly through channels cut deep in to peaty substrates. Beaver dams are common and form large, deep pools. Discharge slope s are typically steep valleys; shallow, high-gradient streams move quickly over gr avelly substrates. Streams commonly occur along forested valley floors with abundant vegetative cover. The two
2 geomorphic settings are not mutually exclusiv e within any given watershed and often grade from one to the other and back again along the course of a stream channel. It is hypothesized that geomorphology exerts a strong control on shallow groundwater discharge to streams that in turn influences in-stream surface water temperature. Because groundwater is insulated from diurnal and seasonal fluctuations in atmospheric temperature, any exchanges be tween groundwater and su rface water should have a moderating effect, whereby groundwater discharge cools surface water in summer and warms it in winter. The high topographic relief of discharge slope settings produces larger head gradients capable of driving la rger quantities of groundwater through shallow surficial sediments adjacent to streams, re sulting in a more pronounced moderating effect on surface water temperature at discharge slop e sites than at drainage-way sites. Furthermore, while shallow groundwater di scharge affects in-stream surface water temperature at the local scale, it likely has no overall effect at the basin scale. At the point of discharge, groundwater has di stinct physical and chemical properties that it imparts on receiving surface water. While the chemical si gnature is retained downstream, heat is exchanged between surface water and groundwater and the moderating effect is lost within some finite distance of the discharge point.
3 SITE DESCRIPTION The Kenai Lowlands physiographic province occupies 3,300 square kilometers of a broad, low-lying platform on the Kenai Peninsula, bounded to the south and east by Kachemak Bay and the Kenai Mountain range, respectivel y, and to the west by Cook Inlet (Karlstrom, 1964; Walker et al., 2007) (fi g.1). Elevation ranges from s ea level to 950 meters atop the Caribou Hills sub-unit, with most of the re gion lying below 120 meters above sea level (Karlstrom, 1964; Walker et al., 2007). A general hydrologic report on the Lower Kenai Peninsula was completed by Nelson and Johnson (1981) and concluded that between 60 to 70 percent of streamflow in the Anchor and Ninilchik rivers was derived from groundwater. This study focuses on headwater streams that are part of four major river drainage networks on the Lower Kenai Peninsula: St ariski Creek, Ninilchik River, North Fork Anchor River, and South Fork Anchor River (fig.2). Six headwater st reams were selected from these drainage networks and divided into upper, middle, and lower study reaches resulting in a total of 18 st udy reaches. Each study reach was identified either as a discharge slope or drainage-way based on previous wetland mapping efforts (Reeve and Gracz, 2008).
Figure 1. Location of the study area on the Kenai Peninsula, Alaska A two-tiered approach was employed for this study in which the 18 study reaches were divided into two groups. The first group consisted of 16 study reaches that were lightly instrumented with temperature sens ors only. The second group consisted of 2 study 4
5 reaches that were heavily instrumented a nd studied in greater detail. The lightly instrumented sites encompassed 7 drainage-way and 9 discharge slope stream reaches. The heavily instrumented sites were chosen as representatives, one from each geomorphologic setting. The representative dr ainage-way stream reach was NANC44 upper (abbreviated NANC44), and the discharge slope stream reach was SANC1203 middle (abbreviated SANC1203).
Figure 2. Location of study reaches on the Lower Kenai Pe ninsula, Alaska 6
7 METHODS Physical Hydrology Lightly instrumented sites we re outfitted with sensors that measured surface water temperature. Sensors were deployed in Ju ly and August 2007 at each of the 16 lightly instrumented study reaches and both heavily instrumented study reaches. A pair of temperature sensors was deployed at each reach approximately 150 m upstream or downstream from the midpoint, for a total of 36 temperature monitoring locations. Temperature was measured with model TBI 32 StowAway TidbiT temperature sensors with built-in data loggers (Onset Computer Cor poration, Cape Cod, MA). Each sensor was secured to the bottom of the ch annel using stainless steel wi re attached to rebar pounded into the channel. In addition to surface water temperature sensors, heavily instrumented study reaches were outfitted with piezometers, ground water temperature sensors, and water level sensors that measured both stream stage and piezometric head. A total of seven piezometers were installed in the peat substrate at the drainage-way site. Three transects running perpendicular to the stream channel were established with two piezometers installed along both the upper and lower transects and three piezometers along the middle transect (fig.3). Piezometers we re installed within 2 m of th e stream channel at all three transects with subsequent piezometers installed in 60 m increments away from the channel.
Two water level sensors were installed at this study reach, one in the piezometer closest to the channel and one in the stream ch annel adjacent to the piezometer. Figure 3. Site diagrams for (a) NANC44 upper (dra inage-way) and (b) SANC1203 middle (discharge slope) The substrate at the discharge slope site wa s composed of poorly -sorted glacial till that made the installation of piezometers difficult. Consequently, only one piezometer was installed and was located 2 m from the channel. Two water level sensors were also installed at this study reach in the same manner as at the drainage-way site. A benchmark was installed at a small spring located approxima tely 100 m from the channel where periodic water level measurements were used as a pr oxy for hydraulic head in underlying sediments. 8
9 Stages (i.e., surface-water levels) we re measured hourly with model 3001 Levelogger Gold pressure trans ducers with built-in data logger s (Solinst, Inc., Georgetown, Ontario). Hydraulic heads (i.e., groundwater levels) were measured either hourly with model 3001 Levelogger Gold pressure trans ducers and data loggers (Solinst, Inc., Georgetown, Ontario) or peri odically with a model 101 Wate r Level Indicator (Solinst, Georgetown, Ontario). Hydraulic heads were m easured at piezometers, with an inside diameter of approximately 5 cm and a 0.3 m sc reened interval from 0.9 to 1.2 m below the soil surface. Time-lag errors can arise in piezometers screened in low-conductivity formations (Hanschke and Baird, 2001). The pot ential for time-lag errors was minimized by using small-diameter standpipes so small ex changes of water were sufficient to allow water in the standpipes to reach equilibri um with water in surrounding formations. Hydraulic conductivity of sedime nts located at the highly-in strumented study reaches was calculated using the Hvorslev (1951) slug test method. Temper ature was also recorded by the Levellogger Gold pressure transducer with built-in data logger at one hour intervals. Chemical Hydrology Water quality samples were collected during spring (May) and summer (August) 2008 at each of the 18 study reaches. Samples were taken from the channel, piezometers (where available), and small groundwater se eps and springs found near the monitoring locations. All samples were collected using a peristaltic pump and filtered with an inline Whatman Polycap HD 0.45 m capsule filter (Whatman, Ltd., Maidstone, Kent, UK). Samples were then refrigerated at the laborator y facility at the end of each day. Rain water and snow samples were also collected duri ng the study period but were not filtered.
10 Temperature and specific conductance were measured in the field using a YSI 650 multi-parameter probe (YSI, Inc., Yellow Springs, OH). Dissolved major (Na, Mg, K, Ca) and trace (Si, Fe, Ba, Sr, B) cations were analyzed with a Perkin-Elmer Elan II DRC Quadrupole ICP-MS in the Mass Spectrometry Lab at the University of South Florida Geology Department. Detection limits were better than 1.0 g/L for major elements and 0.1 g/L for trace elements except B, which was not detected. Each sample was acquired by 5 separate measurements, and relative standa rd deviation of the five acquisitions was generally 6 percent or better. Accuracy was checked by repeated measurement of NIST 1640 inserted every 20 samples; an unknown external standard was better than 7 percent for all elements except B, which was better than 14 percent. Fe (mass 56) was measured separately using the dynamic reaction cell (DRC) with NH3 reaction gas to eliminate Ar-O interference at mass 56. Error on Fe using the DRC was < 3.9 percent. Duplicate samples were inserted every 15th sample, and results agreed with in 3.6 percent or better for all elements. Chloride concentration was analyzed at Advanced Environmental Laboratories, Inc. of Tampa, FL, with ion chromatogr aphy using EPA method 325.2 and a detection limit of 0.20 mg/L (Clesceri et al. 1998). All concentr ations were reported in milligrams per liter (mg/L). Evapoconcentration and Mass-Balance Mixing Modeling Evapoconcentration is the process by which solute concentrations increase as water evaporates and solutes are retained in the remaining solution. A model was developed to determine whether solutes in surface water samples were primarily derived from evapoconcentration of surface runoff or water-ro ck interactions (i.e. groundwater). The
11 surficial geology of the study area is composed of unsorted glacial drift, proglacial lake, and other fluvial deposits of Pleistocene age (Karlstrom 1964, Freethey and Scully 1980). The rocks from which these sediments origin ated are igneous and sedimentary and are relatively enriched in sodium (Na), magnesium (Mg), and ca lcium (Ca), but not Cl. Thus, Na and Cl were used as cons ervative natural tracers to determine whether the primary mechanism controlling in-stream water chemis try is evapoconcentration or water-rock interaction. The ratio of these two ions was calculated for the only precipitation sample that had detectable quantities of Cl. An evapoc oncentration trend line was then calculated using: Cresidual = Cinitial / fresidual (M/L3)r = (M/L3)i / ( L3/L3)r (1) where Cresidual is the concentration of the residual solution in mg/L, Cinitial is the concentration of the original solution in mg/L, and fresidual is the fraction of the original solution remaining. The Na:Cl ratio was then pl otted and fit with a tr end line for all other non-precipitation surface water samples (spri ng and summer) having a Cl concentration above the laboratory prac tical quantitation limit. A two-end-member, mass-balance mixing mode l was then created to calculate the relative contribution of preci pitation and groundwater for ea ch sample using specific conductance, Na, Mg, and Ca as conservative tracers. Precipitation and groundwater endmember values for each tracer were calculated as the average value for that tracer in all samples of each end-member type. Only gr oundwater samples collect ed during the summer were used for the analysis because of the effects of surface runoff during spring break-up. The concentration of the theoretical mixtures was calculated using:
12 fsw = (Cgw Cmixture) / (Cgw Csw), (2) where Cmixture is the concentration of the mixed solution in mg/L, fsw is the fraction of the mixture contributed by surface water, Csw is the concentration of surface water in mg/L, and Cgw is the concentration of groundwater in mg/L. The final value for the proportional groundwater contribution is expressed as the average value computed from all tracers combined. Application of th e mixing model assumes both that all samples were instantaneous mixtures of the two end members and that evapoconcentration was negligible.
13 RESULTS Physical Hydrology A two-sample, equal variance t-test was a pplied to surface-water temperature data from the lightly instrumented sites and reve aled a small, but statistically significant difference in average annual temper ature between geomorphic settings ( << 0.001). Figure 4a and table 1 show that the average annual temperature was higher at drainage-way sites (3.4C) versus discharge slope sites (2.2C) Maximum instantaneous temperature at the drainage-way sites (24.6C) was also higher than discharge sl ope sites (10.7C) (fig. 4a, table 1). Similar patterns in surface-water temperature were observed at the highly instrumented sites (fig. 4b, table 2), wh ere comparisons between surface water and groundwater temperature were also made. Maximum instantaneous surface water and groundwater temperatures at the discharge slope site were very similar (11.8C for surface water and 10.9C for groundwater). Figure 4b shows that maximum instantaneous temperatures at the drainage-way site were very different (22.3C for surface water and 4.6C for groundwater). The maximum differe nce between surface water and groundwater temperatures in summer was also higher at the drainage-way site where daily average surface-water temperatures were as much as 5.3C warmer than the groundwater while the maximum difference at the discharge slope site was 1.6C.
Figure 4. Water temperature data collected at (a) lightly instrumented and (b) heavily instrumented study reaches 14
Figure 5 compares daily average surface water and groundwater temperatures at both heavily instrumented sites. Temperatures are coupled at the discharge slope site, where groundwater and surface water temperatures are very similar throughout the year, and especially so from roughly January through J une. This is not the case at the drainageway site, where groundwater and surface water te mperatures are similar only during brief periods in May and October. A two-sample, equa l variance t-test showed that differences between surface water and groundwater temper atures at the drainage-way site are significant ( < 0.001), but were not at the discharge slope site ( = 0.07). In addition, the correlation coefficient computed between data at the drainage-way site was low (r = 0.50), while it was high (r = 0.87) for the discharge slope data, further indicating close coupling between surface water and groundwater at that site. Peak correlati ons were achieved by shifting data to align the su mmer 2007 maxima for surface wate r and groundwater at each site. The shift for the drainage-way site is 44 days (r = 0.93) and the shift for the discharge slope site is 5 days (r = 0.91), which suggest s that surface water and groundwater mix very 15
slowly at drainage-way sites where the shif t, or lag time, was 44 days and much more quickly at discharge slope sites where the shift was only 5 days. The amplitude between the summer 2007 peak groundwater temperature and minimum winter temperature is visibly different between the two sites. It coincided with the annual range at both sites and was 10.9C at the drainage-way site and 3.2C at the discharge slope site (table 2). Figure 5. Comparison of daily average surface water a nd groundwater temperature at (a) NANC44 and (b) SANC1203 16
17 Continuous stage and piezometric head da ta collected at SANC1203 indicate that there was net discharge of groundwater into the stream channel from August 2007 through December 1, 2007, with a maximum gradient of 0.02 and an average of 0.01. On December 2, 2007, the head gradient indicates that flow di rection reversed and wa ter leaked out of the stream channel into the shallow surficial aquife r. Water generally leak ed out of the channel during winter, with a maximum outflow-gradi ent of -0.01 and an average of -0.01. On April 20, 2008, the head gradient became neutral and then on April 22, 2008 the gradient reversed and groundwater flowed into the stream channel until instruments were removed on August 5, 2008. The maximum gradient during this period was 0.03, and the average was 0.01. The volume of diffuse groundwater discharge through the sediments to the stream was calculated to be approximately 1x10-7 m3/s per stream meter using instantaneous head data measured in A ugust 2007 and the computed hydraulic conductivity of local sediments. Continuous stage and piezometric head da ta were also collected at NANC44, but extremely cold weather during winter dama ged the pressure transducer used for barometric-compensation, which render ed the data un-useable. Based on hand measurements made during field visits, th e volume of diffuse groundwater discharge through the sediments to the str eam was calculated to be 2x10-8 m3/s per stream-meter, roughly an order of magnitude smaller than th e calculated discharge from the discharge slope sediments.
18 Chemical Hydrology Selected analytes (Na, Ca, and Mg) were compared to determine if there were statistical differences in concentration between groundwater samples collected during summer at drainage-ways and discharge slopes. Groundwater samples are those taken from seeps and springs from adjacent hillslopes as well as those taken from piezometers at heavily instrumented sites. A two-sample t-te st (unequal variances) was first run on analyte concentrations for all groundwater types a nd showed there was a difference between drainage-way and discharg e slope populations (for = 0.05; -valueNa << 0.01, -valueCa << 0.01, -valueMg = 0.02). The t-test was then r un only on groundwater samples taken from seeps or springs to compare groundwater from similar landscape features. The second t-test showed there was no stat istical difference between drainage-way and discharge slope populations (for = 0.05; -valueNa = 0.06, -valueCa = 0.14, -valueMg = 0.44). Evapoconcentration Modeling Table 3 is a summary of water chemistr y data used in the evapoconcentration model. All analyte concentrations were higher in both surface water and groundwater samples collected at the study sites than in precipitation. The evapoconcentration model (figure 6) compares the Na:Cl solute ratio of the site samples versus evapoconcentrated precipitation. The trend of the Na:Cl ratio of the site samples does not follow that of evapoconcentrated precipitation. It is also much higher in the site samples, which ranged from 0.56 to 2.52 with an average of 1.11, than evapoconcentrated precipitation, which was a constant 0.27. The stark difference between the ratios of the tw o groups suggests that evapoconcentration is not the main control for solute concentration of the field samples.
19 Elimination of evapoconcentr ation as the controlling m echanism leaves water-rock interactions as the most probable mechanism by which solutes were introduced to sampled water.
Figure 6. Evapoconcentration model showing calculated evapoconcentration trend line Mass-Balance Mixing Modeling Thirty five water quality samples collected at 18 different locations across 16 study reaches were compared to determine differences both in the proportion of groundwater contribution from spring to summer and between geomorphic se ttings. The average proportion of groundwater contribution for all si te types in spring wa s 42 percent and rose to 58 percent in summer (table 4). The va lues ranged from 12 (NINI545 upper) to 68 percent (STAR69 middle) in spring and fr om 2 (NINI545 upper) to 101 percent (NINI545 lower) in summer. The average difference from spring to summer was +22 percent and ranged from -10 to +72 percent. A two-sample t-test indicated that the higher proportion of groundwater contribution in summer was statistically significant ( /2 = 0.005) though the 21
22 calculated percent groundwater contributi on was approximately the same between geomorphic settings regardless of season (table 4).
24 DISCUSSION Drainage-way wetlands are low-gradient geomorphic settings characterized by lowpermeability sediments composed of peat. Slow moving streams flow through deeply incised channels and groundwater exchange between sediments and streams is much weaker than at the discharge slope wetlands, primarily due to the lower head-gradient (fig. 7). Despite lack of continuous head data, it is likely that the directi on, if not magnitude, of the head gradients at drainage-way sites fo llow a similar pattern as was observed at discharge slope wetland sites. However, the deeply incised channels allow pools to be maintained throughout winter which protects these reaches from freezing solid. Discharge slope wetlands, on the other ha nd, are high-gradient landscape features and are characterized by a low-permeability s ubstrate composed of glacial till and other poorly-sorted sediments. Head data indicat e that these systems provide moderate groundwater discharge to shallow, fast-moving stream reaches in summer (fig. 8). During winter, surface water leaks out of the stream into the shallow surficial aquifer, leaving these shallow stream reaches more vulnerable to freezing over.
Figure 7. Conceptualization of shallow-groundwater flow in drainage-way wetlands in the (a) summer and (b) winter25
Figure 8. Conceptualization of shallow-groundwater flow in discharge slope wetlands in the (a) summer and (b) winter 26
27 Temperature data presented showed maximum instantaneous surface water temperatures and differences between surface water and groundwater temperatures were greater at drainage-way site s where calculated diffuse groundw ater discharge is less. At discharge slope sites, surface water and ground water temperatures are closely coupled and highly correlated with one anot her suggesting a higher degree of connection between the reservoirs. Geochemical data, however, s howed the proportion of groundwater flowing through stream reaches was the same in bot h geomorphic settings which implies that geomorphic setting does influence surface water temperature through groundwater discharge, but only at the local-scale. On ce groundwater emerges and flows downstream, it exchanges heat with the rece iving surface water and atmosphere and becomes part of the surface water reservoir, though it re tains its geochemical signature. In addition, statistical tests on geochemi cal data show that groundwater flowing from hillslopes adjacent to both drainage-ways and discharge slopes is the same, but suggest that groundwater collect ed from drainage-way piezometers is different from the discharge slope piezometer. Because the groundwater flowing from hillslopes toward piezometers is the same at both geomorphic se ttings, differences in residence time between the hillslope and stream channel must account for differences in analyte concentration at piezometers. Higher mean concentration of the se lected analytes at drainage-ways suggests longer residence times, while lower mean c oncentrations at discharge slopes suggests shorter residence times. This was expected based on the differences in gradient between geomorphic settings and also helps explain differences in coupling between surface water and groundwater, as well as groundwate r temperature range and lag time. There are certainly other important factors that control surface water temperature, of which the most pertinent for this study is the prevalence of dens e vegetative cover at
28 discharge slope sites, where a thick canopy of willow ( Salix sp.) alder (Alnus sp.), black spruce ( Picea mariana) and Lutz spruce ( Picea x lutzii ) often shades streams. Drainageway sites, conversely, were much more exposed with little vegetati ve cover aside from grasses, primarily Calamagrostis canadensis and sedges (Cyperaceae). A study by Johnson (2004) concluded that shade does not provide cooling for streams, though it significantly decreases maximum water temperat ure by reducing the amount of heat energy from incident solar radiation. Furthermore, th e reduced input of heat energy allows other components of the stream heat budget to dominate water temperature. In summer, the shading provided by vegetati on at discharge slope sites reduces heat input from solar radiation and allows the cooling effects of gr oundwater discharge to moderate surface water temperature. Conversel y, lack of shading at drainage-way sites results in excessive heating from solar ra diation for which the cooling effect of groundwater discharge cannot compensate. Thus, differences in shading between geomorphic settings results in differences in the effectiveness of groundwater discharge to moderate stream temperature.
29 CONCLUSIONS Temperature is an important physical char acteristic of headwater streams that controls the presence and hea lth of juvenile salmonids. Signi ficant differences in surface water temperature were observed between di scharge slope and drainage-way stream reaches. Discharge slopes contributed larger qua ntities of groundwater to their associated stream reaches, which had more vegetative cove r and were statistically cooler in summer than were drainage-way stream reaches. This indicates that geomorphic setting plays a role in moderation of surface water temperature th rough groundwater discharge and degree of shading. A detailed heat-budget should be de veloped to determine the degree to which groundwater discharge plays a role in maintaining reduced summer temperatures.
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