Temporal and spatial variations in the solute content of an alpine stream, Colorado Front Range

Geomorphology, 4 (1990) 55-72

55

Elsevier Science Publishers B.V., Amsterdam

Temporal and spatial variations in the solute content of an alpine stream, Colorado Front Range N. Caine a and E.M. Thurmanb aDepartment of Geography and Institute of Arctic & Alpine Research, University of Colorado, Boulder, CO 80309 (U.S.A.) bU.S. Geological Survey, 4821 Quail Crest Place, Lawrence, KS 66049 (U.S.A.) (Received May 20, 1989; accepted after revision January 10, 1990)

ABSTRACT Caine, N. and Thurman, E.M., 1990. Temporal and spatial variations in the solute content of an alpine stream, Colorado Front Range. Geomorphology, 4: 55-72. Seven years of discharge and water quality records define temporal and spatial patterns of solute movement in a Colorado alpine stream system. Dissolved solids concentrations are low, generally less than 30 mg 1- ~ and occasionally less than 3 mg 1- ~ at the highest elevations. Calcium is the dominant cation and bicarbonate and sulfate are the main anions. Temporal changes in solute concentrations are dominated by an annual cycle with high values in late winter and spring that decrease rapidly during early summer and then return more slowly through fall. This pattern corresponds to the seasonal streamflow regime and reflects differential elution of the snowpack by meltwater and changing proportions of surface and subsurface water in the streamflow. The amplitude of the annual cycle of solute concentration is reduced with increasing catchment area and where the groundwater contribution to flow is relatively high. In general, solute concentrations increase down valley but this trend is reversed in the case of biologically important solutes, such as nitrate and potassium. Rates of geochemical denudation are dominated by the volume of water discharge and thus are highest in the parts of the basin that accumulate the greatest depths of winter snow. They vary between 5 and 26 g m-2 yr- ~for different parts of the catchment and average less than 9 g m -2 yr-i. These rates are low compared to those from high-elevation catchments elsewhere but are an order of magnitude higher than rates of sediment removal from the basin.

Introduction

Except for studies in karstic (Bogli, 1980) and glacial systems (Reynolds and Johnson, 1972; Hallet, 1976; Raiswell, 1984; Souchez and Lemmens, 1987 ), chemical denudation by the fluvial systems of alpine areas has rarely been evaluated. In the Southern Rockies, preliminary studies in a variety of field areas suggest that contemporary removal of rock material in solution constitutes an important part of the erosional work performed on the landscape today (Miller, 1961; Caine, 1976; Thorn, 1976; Vitek et al., 1981; Baron, 1983). This pattern corresponds to the results found in other high mountain areas (Barsch and Caine, 1984).

The processes that contribute to geochemical denudation are also important in nutrient and material cycling in the ecosystem (Likens et al., 1977; Swank, 1986; Trudgill, 1986) and are likely to be impacted by inadvertant, maninduced environmental changes such as acid deposition (Lewis and Grant, 1979; Lewis, 1982; Kling and Grant, 1984 ). They may even be early indicators of such an impact. For these reasons, the geochemical cycles of high mountain terrain should be of wider scientific interest than that of their geomorphic significance alone. In this paper, we report on a seven-year study of the geochemistry of stream water draining an alpine catchment in the Colorado Front Range. This work has been part of an ongoing

56

N. CAINE AND E,M. THURMAN

more than 30 years (Ives, 1980). The Green Lake Valley is part of the municipal water supply of Boulder and has been closed to public access for about 60 years. This means that its hydrologic and geochemical systems are as close to natural as any in the Colorado Front Range. Apart from the summer of 1985, when the lower three Green Lakes were drained, the hydrologic system of the valley has not been actively managed during the period of study. The catchment includes two contrasting sections: the upper basin (2.1 km 2 area) comprising the area draining through Green Lake 4; and the lower valley ( 5.0 km 2 ) between Green Lake 4 and the former settlement of Albion. The upper Green Lakes Valley is typical of high alpine environments in the range, with steep rock walls and talus slopes, a valley floor of glaciated bedrock, many permanent snowbanks (including the Arikaree Glacier below the Continential Divide) and relatively little vegetation (Fig. 2). The lower Green Lakes Valley has less area of exposed bedrock, fewer cliff-talus slopes, more debris-mantled valley sides with lower gradients, rounded interfluves and a more extensive soil and vegetation cover on the valley floor (Fig. 3 ). A glaciated valleystep of about 75 m height between Green Lakes

program of Long Term Ecological Research (LTER) and has three objectives. First, by sampling throughout the year, we have been able to estimate the seasonal variations in water quality. Second, we have evaluated the nature of spatial variations in the stream solute load within the basin to indicate its sources. Third, we use these results to evaluate the magnitude and significance of chemical denudation in the parts of the basin which provide most of the streamflow, as well as the system as a whole. Field area

During the 1981-1987 period, observations of water discharge and solute chemistry have been made in the 7.1 km 2 catchment of the Green Lakes Valley above 3200 m elevation in the Colorado Front Range (Fig. 1). This stream basin is essentially alpine in nature, although 40 ha of forest occurs on the north valley wall at the lowest elevations and spruce krummholz of tree line may be found on sheltered sites up to 3600 m. The catchment appears typical of the high-elevation environment of the Colorado Front Range and includes the south slope of Niwot Ridge, where work on alpine environments has been conducted for

_ _ ~

• ~

~',

~_/

f~/~ ~---~

~

t

4

'9 I

4-

0

GI-I- -

~

c ~ l

COLORADO

"-

b> 105°37'30"

Fig. 1. Location map of the Green Lakes Valley. Study sites are shown: ( • ) sites with a continuous discharge record; ( • ) stream sampling sites; ( + ) climate stations. Site identification: AI= Albion; S = Spillway at Lake A l b i o n ; / = i n f l o w to Lake Albion; G1 = Green Lake 1; G3 = Green Lake 3; M = Martinelli Snowpatch; G4 = Green Lake 4; G5 = Green Lake 5; N = Navajo; Ak = Arikaree.

SOLUTE CONTENT OF AN ALPINE STREAM, COLORADO FRONT RANGE

57

Fig. 2. The upper Green Lakes Valley. Green Lake 5 is in the foregroundwith the Arikaree Glacier and the Continental Divide at the top of the photo (July 4, 1981). 3 and 4 separates the two sections of the valley. Hydrographically, the catchment is a linear cascade of 5 lakes with only Green Lake 1, on the north valley wall, subsidiary to this sequence (Fig. 1). The channels between and above the lakes are invariably steep and rocky, with the shallow flow diverging and converning around large boulders. They are generally floored with cobbles and gravel and appear to be sufficiently well armored to prevent sediment m o v e m e n t even at peak flows. During spring snowmelt, flows are occasionally forced out of the channels by ice blockages and superimposed from the snow cover onto the valley floor by processes similar to those reported

from high arctic environments (Woo, 1982). It is only at these times that coarse sediment ( > 2 m m size) transport occurs, and that is usually not in the main stream channels. At other times, sand and silt may be observed moving through channels and rills draining from snowbanks but this sediment flux does not continue to the main drainage (Caine, 1986). The Green Lakes Valley has a continental high mountain climate. A 30 year record of conditions on Niwot Ridge, which forms the northern drainage divide of the basin, has been summarised by Losleben (1983). The early period of this record was evaluated by Barry

58

N. CAINE AND E.M. THURMAN

Fig. 3. The lower Green Lakes Valley. Lake Albion and Green Lakes 1, 2 and 3 are in the center of the photo with the Martinelli Snowpatchon the right (July 4, 1981). (1973). Shorter periods of observations are available for a variety of stations on the valley floor. These have not been treated comprehensively but they combine to suggest a less exposed and windy environment with greater snow accumulation than on the surrounding ridges. Within the valley, south-facing slopes are warmer than north-facing ones, which are underlain by permafrost (Ives, 1973 ). The bedrock underlying the lower Green Lakes Valley is dominated by granites and quartz monzonites of two different intrusions: the Silver Plume monzonite of Precambrian age and the Audubon-Albion stock of Miocene age. The Silver Plume monzonite extends into the upper Green Lakes Valley where the metasediments of Precambrian age into which it was intruded are c o m m o n on the northern slopes of Kiowa Peak and on Niwot Ridge above Green Lake 5. The alpine tundra vegetation of Niwot Ridge has been m a p p e d and described by Komarkova and Webber (1978). Above the treeline at 3400 m elevation, a complex mosaic of turf

and shrub communities reflects local contrasts in soil moisture, wind exposure and snow cover. In Green Lakes Valley, wetter communities are more extensive than on Niwot Ridge and reflect the same controls and the effects of cirainage on the valley floor. Komarkova and Webber (1978) show the upper Green Lakes "Valley as part of a subnival belt of bare rock, lalus and boulder slopes with little vegetation, other than mosses and lichens. The soils and surficial materials of the field area match the patterns of vegetation. They consist of a heterogeneous cover of cryic inceptisols and entisols with histosols in the wetter parts of the valley floor (Burns, 1980). The valley floor appears to have been deglaciated about 12,000 years ago (Harbor, 1984). The soils and superficial deposits are all of Holocene age. The interfluves like Niwot Ridge were not glaciated in the Late Pleistocene (Madole, 1982 ) and are mantled with older deposits in which relict patterned ground and mass wasting features are extensively developed (Benedict, it970).

SOLUTE CONTENT OF AN ALPINE STREAM, COLORADO FRONT RANGE

Procedures

Stream discharges have been continuously monitored during the May-October flow season at Albion and the outlet from Green Lake 4. Water sampling and field observations have been made at these and six other sites along the channel of North Boulder Creek and at the streams draining the Martinelli Snowpatch and Green Lake 1 on the north side of the valley (Fig. 1 ). During the May to October period, sampling and field measurements have been conducted on a weekly interval at all sites and on an approximately monthly interval at the Albion and Green Lake 4 sites during the rest of the year. In the field, routine observations have been confined to the measurement of water temperature (mercury in glass thermometer: _+0.5 ° C), specific conductance (Lectro-mho meter: _+4% ), and pH (digital meter: _+0.1 pH units). At the same time that field measurements were made, water samples were collected and kept refrigerated until analysis. Analysis has usually been completed within 5 days of sample collection, and sample preservation, other than refrigeration, has not been necessary. Further, samples have not been filtered prior to analysis, because suspended particulate concentrations in them are low ( < 3 mg 1- ~). Occasional comparisons to fractions passed through a 0.45 ~tm millipore filter show no difference to the unfiltered samples. Only data on the main inorganic constituents in solution are reported here. In the laboratory, major cation (calcium, magnesium, sodium, and potassium) and silicon concentrations have been determined by atomic absorption spectrophotometry (Perkin-Elmer Model 2280) using standard procedures (Skougstad et al., 1979). In the first two years of the program, silica was analyzed by the molybdenum-silicate method (Skougstad et al., 1979). Concentrations of sulfate, fluoride, chloride, orthophosphate, nitrate, and nitrite have been estimated by ion chromato-

59

graphy (Dionex 2110i interfaced to a SpectraPhysics SP4270 integrator) using three replicates of each sample. Bicarbonate was determined by titration to a pH 4.5 endpoint. In general, analytical results show errors in the ionic balance of less than 10%, usually as an excess of anions, which is satisfactory for waters with such low solute concentrations. However, occasional samples, especially those from the higher sites in the basin, have a difference of more than 30% in the cation and anion equivalents. This seems to result from an overestimation of bicarbonate but we have not attempted any empirical correction in the results reported here. Nor have we removed sample results from consideration because of a poor ion balance. Estimates of precipitation quality in the Green Lakes Valley are based upon a set of monthly bulk samples from a collector (plastic lined) at the Martinelli Snowpatch and are summarized in Table 1 (Reddy and Caine, 1990). They are appreciably lower than those from Como Creek, 5 km east of and 500 m TABLEI Precipitation chemistry

Ca (lag 1-' ) Mg (lag 1-~ ) Na (lagl -I ) K ( l a g l -~) SO4 (lagl -~ )

NO3 (lagl - l ) C1 (lagl -~ ) F(~tgl -j) SiO2 (lag1-1 ) Cond. (IxS cm -~ ) H + (~tgl -I )

N"

Xb

Sb

Load c

41 41 41 39 40 40 40 40 40 41 41

187 9 41 110 622 342 152 <1 < 1 8.2 9.4

0.63 1.18 1.28 0.46 0.33 0.84 0.73 1.63 1.88 0.24 9.3

0.31 0.02 0.07 0.18 1.03 0.57 0.23 <0.01 0.01 0.015

Estimates are based on bulk precipitation samples collected on an approximately montly interval at the Martinelli Snowdrift in the period October 1982-March 1986 (Reddy and Caine, 1990). a N = n u m b e r of samples. b X = g e o m e t r i c m e a n concentration (p.g l ~, except conductance which is ~tS c m - ~). S = standard deviation (lag l - t, except conductance which is laS c m - t ). CLoading (g m -2 yr-1 ).

60

lower than the Martinelli site (Grant and Lewis, 1982 ). These estimates have been augmented by occasional collections of rainwater from individual storms, by samples of the seasonal snow cover and by data from a National Atmospheric Deposition Program ( N A D P ) collector at the Saddle on Niwot Ridge (Fig. 1 ). When seasonally averaged, the estimates of precipitation concentrations from the Martinelli collector are close to those from the wetprecipitation fractions of the NADP collector and show the same seasonal pattern of higher concentrations (especially of sulfate and nitrate) in the summer. Specific yields of dissolved solids are best estimated for the Albion, Green Lake 4 and Martinelli sites, where a continuous record of water discharge is available. For this purpose, total dissolved solids (TDS) has been approximated as the sum of the major ions (calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, fluoride, nitrate) and silica. The regression Of TDS (y) on specific conductance (x) allows the estimation of TDS for the times when only conductance was measured (Table 2). Similar empirical relations between conductance and individual dissolved species allow estimation of their concentrations when samples are missing. The yield of any single constituent or TDS on any day has been estimated as the product of water discharge and its concentration. This allows use of the more comprehensive set of data on specific conductance in 1982 and estimation of yields for 1981, for which only flow and conductance records are available. For periods between sample dates, a linear interpolation of TDS has been used since the empirical regression between discharge and TDS at any site is not simple. At sites other than those for which a continuous flow record is maintained, dissolved-material yields have been derived from stream discharges estimated as a fraction of the flow at the next downstream site with a continuous record. For much of the summer, the flow proportions are readily defined by occasional dis-

N. CAINEAND E.M. THURMAN TABLE 2 Dissolved solids concentrations in streams of Green Lakes Valley Site

N

x

S

r

a

b

SEest

Albion Spillway Inlet GreenL 1 Green L 3 GreenL4 Green L 5 Navajo Arikaree Martinelli

151 72 73 60 23 131 61 51 50 69

16.7 13.8 12,9 30.6 11,3 11.1 8.4 6.6 4.9 10,3

3.6 1.8 3.8 2.3 2.6 3.2 1.9 2.0 2.0 2.3

0.88 0.66 0.82 0.58 0.94 0.85 0.80 0.88 0.89 0.75

2.43 4.04 4.59 8.26 2.36 4.25 3.45 1.22 2.50 5.00

0.65 0.60 0.53 0.57 0.66 0.51 0.56 0.76 0.42 0.49

1.45 1.02 1.24 1.78 0.91 1.48 0.74 0.90 0.88 1.16

The period of study varies between sites: 1982-1987 at Albion and Green Lake 4; 1984-1987 at all other sites except Green Lake 1 (1985-1987) and Green Lake 3 ( 1985 only). Dissolved solids concentrations (mg 1-~ ) is estimated as the sum of the major cations and SiO2. N is the number of samples. x is the arithmetic mean TDS (rag 1-~ ). S is the standard deviation. r is the correlation coefficient between TDS and specific conductance. a and b are parameters of the linear regression model Of TDS (y) on specific conductance (x) with SEestthe standard error of the estimate.

charge measurements but they are subject to wider error on the rising limb of the seasonal hydrograph, when the area of snowcover contributing to flow is changing rapidly.

Stream water quality In general, the stream waters of Green Lakes Valley are of high quality in the first sense of Averett and Marzolf (1987 ), that is with low concentration of dissolved and particulate material. Dissolved solids concentrations in them are low (Table 2 ), amounting to no more than 30 mg 1- ~ and falling to 3 mg 1-1 on occasion in the small headwater streams. Suspended solids concentrations are generally an order of magnitude lower: they rarely exceed 3 mg 1-1 and are commonly no more than 0.1 mg 1-1 at Albion. The low yield of clastic sediments resuits from low rates of geomorphic activity in

SOLUTE CONTENT OF AN ALPINE STREAM, COLORADO FRONT RANGE

the catchment (Caine, 1982, 1986) and is also evident in low sedimentation rates in the lakes (Harbor, 1984). The concentrations of major ions and silica in the stream water of the Green Lakes Valley are summarized in Table 3. The same set of constituents, and the same relative concentrations, are evident at all sites. At all sites, the relative importance of the major cations is calcium > sodium > potassium = magnesium in molar proportions of 6: 4: 1. Bicarbonate and sulfate are normally the dominant anions. At

61

the highest sites, nitrate concentrations may exceed those of bicarbonate and sulfate early in the flow season when meltwater flushes the snowpack (Brimblecombe et al., 1985) and moves rapidly to the stream with little opportunity to interact with soil and vegetation. At these high sites, this is the only time when trace concentrations of orthophosphate and nitrite are detected in the streamwater. High nitrate concentrations (equivalent to those reported for the Arikaree and Navajo sites in Table 3 ) have been reported in alpine surface waters

TABLE 3 Solute concentrations Ca

Mg

Na

K

HCO3

504

NO 3

C1

SiO2

Cond.

pH

Albion ( N = 151; 1982-1987) X 2.62 0.31 0.67 S 0.64 0.08 0.18

0.34 0.16

8.09 1.96

2.91 0.70

0.34 0.18

0.23 0.21

1.41 0.82

21.4 4.9

6.51 0.28

Spillway ( N = 72; 1984-1987 ) X 2.27 0.24 0.48 S 0.47 0.03 0.09

0.31 0.14

6.81 0.82

2.67 0.48

0.22 0.21

0.12 0.05

0.84 0.53

16.1 2.0

6.63 0.28

Inlet ( N = 7 3 ; 1984-1987) X 1.92 0.25 S 0.49 0.10

0.48 0.24

0.33 0.16

5.21 1.25

2.96 1.22

0.74 0.35

0.14 0.06

1.11 0.92

15.4 5.7

6.50 0.29

Green Lake 1 ( N = 5 1 ; 1985-1987) X 5.17 0.34 0.86 S 0.60 0.05 0.09

0.44 0.16

5.31 0.89

0.54 0.33

0.17 0.06

2.03 0.80

39.3 3.1

6.64 0.33

Green Lake 3 ( N = 2 6 ; 1985) X 1.44 0.18 0.32 S 0.39 0.04 0.08

0.23 0.05

4.60 0.89

3.12 1.26

0.54 0.34

0.14 0.05

0.76 0.49

13.6 3.6

6.21 0.36

Green Lake 4 ( N = 131; 1982-1987 ) X 1.49 0.19 0.37 S 0.55 0.07 0.24

0.33 0.21

4.71 1.46

2.28 0.88

0.74 0.50

0.23 0.24

1.18 0.67

12.6 5.4

6.40 0.29

Green Lake 5 ( N = 6 1 ; 1984-1987) X 1.16 0.12 0.24 S 0.41 0.05 0.09

0.25 0.20

3.32 0.57

1.61 0.62

0.81 0.52

0.12 0.07

1.01 0.59

9.0 2.7

6.24 0.28

Navajo ( N = 5 1 ; 1984-1987) X 0.74 0.08 0.24 S 0.38 0.04 0.11

0.22 0.19

2.07 0.54

1.07 0.65

1.41 0.70

0.11 0.07

0.95 0.59

7.1 3.0

5.70 0.27

Arikaree ( N = 50; 1984-1987 ) X 0.44 0.03 0.10 S 0.36 0.03 0.07

0.14 0.21

2.32 1.44

0.60 0.47

0.70 0.54

0.10 0.05

0.55 0.40

5.5 3.6

5.41 0.37

Martinelli ( N = 6 9 ; 1984-1987) X 1.14 0.10 0.39 S 0.58 0.04 0.19

0.21 0.12

4.63 2.02

0.84 0.25

0.89 0.76

0.14 0.09

1.71 0.92

10.2 3.1

6.24 0.34

16.4 1.3

In parentheses after the site identification is the total number of samples analysed and the period of sampling. Values are arithmetic means (X) and standard deviations (S). Concentrations are in mg l-~ and conductance in/aS cm -~.

62

elsewhere in the Colorado Front Range (Baron, 1983; Baron and Bricker, 1987). The fact that they exceed concentrations in precipitation (Table 1 ) may indicate the significance of dry deposition onto the catchment. Chloride and fluoride concentrations are one and two orders of magnitude lower respectively than that of bicarbonate. The pH of the surface water of the Green Lakes Valley is generally above 6.0 (Table 3 ). This is appreciably higher than the pH of the precipitation here (Table 1 ) and elsewhere in Colorado during the period of study (Reddy and Claasen, 1985). However, it is lower than the surface water pH values reported for the Front Range alpine zone by Kling and Grant ( 1984 ) and is associated with low alkalinities: less than 10 mg 1-1 (as bicarbonate ) at all sites along the main stream and below 2 mg 1-1 at the highest sites during the main period of stream flow. Although low buffering allows the stream pH to vary widely at a site, there is a general tendency for it to increase, along with alkalinity, in the downstream direction (Table 3~. Factor analysis of the concentration data, by identifying surrogate variables, might allow the identification of sources and flowpaths of the water (e.g. Baron, 1983). Postulating two sources for the dissolved material in the stream water (an atmospheric one and a soil and bedrock one) suggests that two factors be sought. For most sites and years, two factors are sufficient to account for 70% of the variability in the correlation matrix and show no differences between the five years of study nor between the range of sites within the catchment. The first factor, usually identified with the main cations, bicarbonate, sulfate and specific conductance often accounts for more than 50% of the total variance. Chloride and nitrate are the important constituents associated with the second factor which appears to reflect atmospheric sources. When a third factor is extracted, it is frequently aligned with silica and so may be identified with a groundwater source, al-

N. CAINE AND E.M. THURMAN

though it is usually weak (eigenvalue of less than 1.0). The consistency of these results, de,;pite changes in absolute concentrations that amount to an order of magnitude according to the site and time of year, suggests a conservative and consistent set of sources for the materials in solution in the stream waters of the Green Lakes Valley, with the catchment soil and bedrock as a dominant one. An equivalent factor analysis of precipitation data contrasts these results in that two factors account for only 55% of the total variance, with different factor identification (Reddy and Caine, 1990). These results are consistent with those derived from other studies of alpine waters in the southern Rocky Mountains (Miller, 1961; Vitek et al., 1981; Baron, 1983; Baron and Bricker, 1987). They also match occasional earlier estimates from within the same catch~ e n t (e.g. Thorn, 1976). On the other hand, they contrast with data from catchments in oceanic and more arid regions, where sodium and chloride (presumably cycled through the atmosphere) are more important in the ionic balance than they are in the Green Lakes Valley (e.g. Reynolds and Johnson, 1972; Reid et al., 1981 ). Nor are they differentiated by life zone or elevation as those of Dethier ( 1988 ) from the Cascade Range, Washington.

Temporal variations in water quality Sampling for water quality has been conclucted at the Albion and Green Lake 4 sites on a regular basis since 1982. These data are augmented by field observations from 1981 at both sites and by data collected between 1968 and 1971 at Green Lake 4. Results of water analyses from the other sites in the valley are available for the 1984-1987 period, with specific conductance, pH and temperature for the period since 1981. A six-year record of specific conductance at Albion and Green Lake 4 (Fig. 4) shows no evidence of consistent change during the pe-

SOLUTECONTENTOF AN ALPINESTREAM,COLORADOFRONT RANGE

63

TABLE 4 Annual mean concentrations (mg 1- ~) at Albion and Green Lake 4 Year

Green Lake 4

Albion

1982 1983 1984 1985 1986 1987

40

Ca

Mg

HCO3

SO4

pH

Ca

Mg

HCO3

SO4

pH

2.78 2.66 2.51 2.14 2.94 2.78

0.33 0.32 0.28 0.27 0.34 0.32

6.36 5.24 8.19 8.69 9.66 9.27

3.02 2.56 2.59 3.10 3.17 2.92

6.68 5.83 6.76 6.46 6.63 6.73

1.86 1.58 1.49 1.14 1.56 1.56

0.24 0.23 0.19 0.16 0.17 0.20

5.32 5.06 4.82 4.27 4.53 4.53

2.88 2.54 2.10 2.32 1.98 2.08

6.38 6.11 7.02 6.26 6.34 6.29

GREEN LAKE 4

]

25 q 1

7 ~30 --.~S

/ Y '~/

e-,

'\U

'ts

5

~e~ A 1981

o 1981

("

1982

1983

1984

1985

1982

1983

19'84

1985

1986

1987

1988

1987

Fig. 5. Annual variations in specific conductance, 19811988. Mean values for the J u n e - O c t o b e r period each year are shown. Site identification: Al= Albion; S = Spillway at Lake Albion; I = inflow to Lake Albion; G4 = Green Lake 4; M = M a r t i n e l l i Snowpatch; G 5 = G r e e n Lake 5; N = Navajo; Ak= Arikaree.

ALBION

40 -~

A I

3_30

=

1986

,,' ~j~

i

I

iI,

TABLE 5

.'~ 20-

Power spectrum analysis of conductance records

0 ~ I0

ge~ o' 1981

1962

1983

1984

1985

1986

1987

Fig. 4. Specific conductance records at Albion (bottom) and Green Lake 4 (top).

riod of study (correlations of 0.078 a n d - 0.128, respectively). Other characteristics for the same two sites confirm this conclu-

Green L 4

Albion

Mean Variance

15.67 24.70

23.85 27.65

Annual cycle Phase Power

10 May 56.3%

23 April 58.0%

Semi-annual cycle Phase Power

12 June 6.4%

21 June 2.0%

The analysis is based on the data of Fig. 4. The power is the percentage of the total variance in the series explained by the annual or semi-annual cycle respectively.

64

N. CAINEAND E.M. THURMAN (a)

45-

1985

.

.

.

.

.

x

40 ¸

(b)

45

GI

t986

40 2~ 35

i

ca 30-50 2t_ ~25

~ 202 ca

-AI

(/3 30 ::L

:' S

:

"

"~25

;.

/

AI

:'

"1

/

~ 2O

/

tz- ~

.z 15~

~15 010 r.)

5June

July

(c)

20-

Aug

Sept

June

Oct

:Ak

1985

(d)

20

July

Aug

Sept

Oet

1986 rAk

: :

G4 ~

-~

i : : s

15

9 7 /

,/l/G4

~o

\1

/ "

s

~ 5 0

()

June

July

Aug

Sept

Oct

June

July

Aug

Sept

0ct

Fig. 6. Seasonal patterns in specific conductance. (a) Lower Green Lakes Valley 1985. ( b ) Lower Green Lakes Valley 1986. ( c ) Upper Green Lakes Valley 1985. ( d ) Upper Green Lakes Valley 1986. Site identification is the same as in Fig.

5. In August and September 1985, the effects of dam closure and lake drainage at Green Lake 2 are especially evident at the Lake Albion inlet record. sion (Table 4), which is also supported by the consistent results of basin-wide factor analysis. Other sites in the valley show the same lack of time trends (Fig. 5 ). Observations made at Green Lake 4 during 1969-1971, a period with lower seasonal discharges than 1982-1986, allow some comparison across a longer interval. These observations show no difference in specific conductance for the June-September seasons of 1969-1971 and 1982-1987 ( t = l . 3 with dr-- 85 ) and suggest that solute concentrations have not changed over 20 years. A reduction in pH ( t = 5.5 with df= 85 ) and a corresponding decrease in alkalinity (t = 2.8 with df= 85 )

over the same period may be indicated. However, such differences need careful interpretation, because of inhomogeneity in the variances used in testing. They are not supported by an equivalent trend in the 1981-1988 record (Fig. 5 ). On a shorter temporal scale, Fig. 4 also defines the dominant pattern for shorter intervals: a strong seasonal signal that reflects snowmelt-produced flows. These introduce dilution effects to the temporal pattern of water quality at the time of highest streamflow. At Albion and Green Lake 4, an annual cycle of specific conductance explains more than 50% of the variance in the 6 year record shown in

SOLUTE CONTENT

OF AN ALPINE STREAM, COLORADO

20

MARTINELLI

65

FRONT RANGE

20-

1983

MARTINELLI

1984

t

X O3

16 ¸

16

::t_

"\ t~+_

_

+--.._

12

_

~.~, *.

c9

',

O 8

8

,+/

1'0 20 3~0 D i s c h a r g e (l s -~)

4

4'0

0

10 2'0 3~0 D i s c h a r g e (1 s -1)

4'0

t 20 -

;

20

:

MARTINELLI

1985

I

MARTINELLI

1986

f

16

10

=

=12-

"'.,

:,,

ii

~ 12

',

"-+~_

O

fl

-

0

8

.

~"" x,.. -.g

4

xb Discharge

z'o

3'0

4'0

(1 s -t)

40

lb Discharge

z'o

3'0

4'0

(1 s -t)

Fig. 7. Specific conductance and discharge at Martinelli snowpatch. Data points shown are for observations on a weekly interval in four seasons ( 1983-1986). In all four years, a clockwise hysteresis is evident.

Fig. 4 (Table 5 ). The seasonal pattern is characterized by high conductance associated with the low flows of winter and a reduction of these (by a factor of 2 or 3 ) during the higher flows of summer. The cycle shows an asymmetry which yields a half-year periodicity in the power spectrum of the record (Table 5). The amplitude and timing of the seasonal cycle is approximately the same for both sites, although examination of the annual minima in conductance suggests that Albion leads Green

Lake 4 by about a week on average. This is a shorter lead than that in the peak flows at the same two sites. The record of specific conductance at the other sites in the Green Lakes Valley has not been maintained through the winters, when most of them have no flow. For the May-October season, they show a partial cycle equivalent to that of Fig. 4 (Fig. 6). This includes a decline in conductance during the spring and summer with a more gradual increase as flows

66

N. C A I N E A N D E . M T H U R M A N

90

GI + GI All •

80-

+

ta~70+ J

L.)

\

/ I

60-

\÷ /I //

I 50 •

40

io

+

//

÷÷/

2'0

3~o

4o

5'0

6'0

7'0

B'0

9'0

16o

S04+CI+NOa

Fig. 8. Changes in surface water quality in Green Lakes Valley on two days in 1987. The evolution of water quality along the main drainages is shown for June 6, 1987 ( • with solid arrows) and August 6, 1987 ( + w i t h broken arrows). Plotted are the percentages of SO4 + C1 + NO3 in the total anion equivalents and C a + M g in the cation equivalents. Site identification: Ak= Arikaree; G4 = Green Lake 4; G l = G r e e n Lake 1; M=Martinelli; and Al=Albion.

recede into the winter, i.e., an assymetry as at Albion and Green Lake 4. Occasional measurements in winter show much higher values than those in summer and so confirm the suggestion that the pattern of Fig. 4 is c o m m o n to the entire catchment. The May-October conductance pattern for each year (Fig. 6) has been fitted to a cubic equation. This usually defines a concave-upward curve involving a decline in conductance during early summer followed by a rise into fall. It is best defined in the upper valley and is not evident at all in some years at sites on the main channel in the lower valley. During the flow season, its amplitude decreases and the minim u m conductance occurs earlier in the downvalley direction. The m i n i m a usually occur in late July and early August, i.e., more than a m o n t h after the peak streamflows from snowmelt, which suggests that the temporal pattern of water quality does not result from simple mixing of water from snowmelt and groundwater. Instead, the solute concentrations in meltwater alone decline through the summer: the Arikaree site, which receives drainage from

a permanent ice and snow mass, clearly shows tile early season elution of the snow cover (Fig. 6). Green Lake 1, with consistently high concentrations for this environment through four years of record, is an exception to this (Table 3). No data are available for this site for Nowember-May but those for the flow season do not show the cyclic pattern of other sites (Fig. 6a). Green Lake 1 is predominantly groundwater fed and the stream from it (where samples are taken) derives from seepage through the dam impounding the lake. Assymetry in the seasonal cycle of solute concentrations and the seasonal hydrograph leads to a clockwise hysteresis which is clearly evident in the conductance record from the Martinelli site (Fig. 7). This, too, suggests a decrease in solute concentrations in snowmelt through the summer. While this occurs, the area of snow declines and the proportion of streamflow routed through groundwater storage increases. The product is a mixed flow from sources that vary in both quantity and quality with time. Variations on a shorter time scale than the seasonal one are not well defined by data with a weekly or longer sampling interval, though they undoubtedly contribute to the noise in the longer term record. A diurnal cycle (which, like the seasonal one, is modulated by snowmelt effects) is clearly evident at the Martinelli site (Caine, 1989). Because most sites in the valley have a similar diurnal flow cycle, they too should show a 24 h cycle in solute concentrations. The further effect of rainstorms would require an even finer sampling resolution and one that has not yet been attempted in the Green Lakes Valley. The intense but short duration rainstorms characteristic of the Colorado alpine environment in summer could introduce water from sources other than the snowpatches which give most of the streamflow, e.g., from large areas of exposed bedrock. Rainstorm influences on streamflow are usually only evident in the smallest channels,

SOLUTE CONTENT OF AN ALPINE STREAM, COLORADO

•I O

67

FRONT RANGE

G 1

7.0

L

~- 3i)

GI

y = 4.846 x °°**~

02 t .v = 2 4 6 7 x °~°.4

c~

i/÷

1 oi---

1o()

6

200

300 ('al('liinelil

4()0

500 Area (hal

600

L-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.0 ] 0 10(1 200 300 ('alchmenl

700

1.0

6,0

4I)0 500 Area (hal

600

700

i

GI

5.0

0.8

4.0

~

t

GI

---i

y = 0.168 + 0.0007 x / ~ 4

~D ~3.0

y ~ 0.165 x °*js~ i

_

2.0

0.4

$

J

.M

4

/ ~ ' J

J~

r~ 8.2 t

1.0

./~j

¥ o.o o

'

,~o

200

300 Catchment

400 500 Area (ha)

600

700

0

1O0

200

300

400

500

Catchment Area (ha)

600

700

Fig. 9. Water quality and drainage area. Mean values for the 9 sites with more than three years of record are shown. Only sites on the main stream ( + ) have been used in fitting the regressions shown. Other sites (not used regression) are: G! = Green Lake 1; M = Martinelli Snowpatch. Broken line denotes mean concentration in precipitation (1982-1986) at the Martinelli site.

above the influence of lake and reservoir storage. Spatial variations in water quality A common spatial pattern is evident in Tables 2 and 3 and Figs. 4, 5, and 6 and has been repeated consistently during the seven years of this study. Regardless of the time scale, whether it be a long-term average or the observations of a single day, specific conductance and dissolved solids concentration increase downstream along the main drainage. Exceptions to

this occur at the highest sampling sites at the start of the runoff season when high solute concentrations are derived from the initial flushing of a thick snowpack. At this time, the relative proportion of strong acid anions in the streamwater at the highest sites is much higher than that at later dates (e.g. Fig. 8 ). The down-valley pattern has been reported earlier with regard to specific conductance (Caine, 1984 ) but it is also true of most of the dissolved constituents (Fig. 9 ). This pattern is the spatial equivalent of the seasonal cycle, in that it reflects the mixing of water from differ-

68

N, CAINE AND E.M. THURMAN

G[

50

4.0

y

=

0.261

x °'3~

3.0 ~(78

.t

1

B

4-

~+0 . 6

GI

;2

-"

"' =

"

'

-

~ 2.0

"

•

÷

Z0.4 I.O 0,2

-

~"

~4~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.0

0

I00

200

300

400

500

600

700

Fig. 9

0

100

200

300

400

500

600

700

C a t c h m e n t Area {ha)

Catehmet~t Area (ha)

(continued).

E

~'\x

R

:

18.05

A~ s

o

2, r.)

~-

E

......

fo

,oo

C a t e h m e n t Area (ha)

,0;o

Fig. 10. Attenuation of the seasonal cycle in specific conductance with drainage area. The amplitude of the seasonal cycle in specific conductance (pS c m - l ) is estimated by the ratio (Cj-Cm)/Cn~ where Cj is the conductance at June 1 in any year and Cmis the minimum conductance for that season. Points shown are averages for 3 to 7 years of record, accordingto site. The empirical regression fitted to sites along the main drainage (solid line) has a correlation of 0.964 with N= 7. The broken line joins the data points for Green Lake 1 and Martinelli. ent sources within the catchment. Hydrologic lags in the basin are generally less than 12 h, including transmission of meltwater through the snow cover. For this reason, we suggest that spatial changes in water quality reflect two influences: ( 1 ) the increased relative contributions of soil and groundwater, and (2) the

hJigher biologic activity at lower elevations. This conclusion is supported by the data from Green Lake 1 and Martinelli, two exceptions to the catchment area - concentration relationship of the main drainage (Fig. 9 ). Both catchments include a higher proportion of tundra vegetation, soil and colluvium than the area above Arikaree and Navajo where exposed bedrock, talus, and ice substrates predominate. Flows at both sites appear to include a larger than usual groundwater component, including relatively high SiO2 concentrations (?Fable 3 ), which would account for their failure to fit the pattern of the main stream (Figs. 9 and 10). Other exceptions to the simple pattern o f increasing solute concentrations in the downstream direction are evident when individual solute species are considered. Nitrate and potassium, taken up by organisms or involved in ion exchange, both show a general decline in mean concentration down-valley (Table 4, Fig. 9). Variations in silica concentrations, e.g. across Lake Albion, probably reflects a similar biologic uptake. Geochemical denudation These data allow the estimation o f average denudation rates for the entire Green Lakes

69

SOLUTECONTENT OF AN ALPINESTREAM,COLORADOFRONT RANGE TABLE 6

Denudation rates in Green Lakes Valley Site

Dissolved sum of solids

Sum of cations

SiO2

Cl

Water

Suspended sediment

Albion Spillway Inlet Green L 1

11.24 10.00 10.23 4.75

2.66 2.49 2.58 1.02

1.32 0.70 0.89 0.34

0.25 0.11 0.12 0.03

736 835 1060 154

0.85

Green L 4 Navajo Arikaree Martinelli

9.71 11.71 26.12 14.64

1.90 2.30 4.62 2.68

1.21 1.44 3.63 2.41

0.20 0.25 0.54 0.22

1010 1956 3271 1492

0.46

5.63

0.58

0.01

0.23

1262

2.5 ~

Precipitation

0.49

All yields are gross, uncorrected for atmospheric inputs, and estimated as the average of between 4 and 7 years of observation. Values are g m - 2 y r - 1 except for water yield which is mm. aDustfall estimate from Litaor ( 1987 ).

30

GREEN LAKES VALLEY

(" \ \

"-~20

k X x, \

Q) >.r.,

~

.

.

.

.

o (/3

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

÷ ÷

/

J

l;o

2;0

a;o Catchment

4do Area

sdo

860

7ao

(ha)

Fig. 1 I. Dissolved material yields and drainage area. The solute yield for each site and year o f observation are shown and enveloping curves fitted by inspection.

catchment and for partial areas within it (Table 6, Fig. 11 ). These are consistently low and average about 6 g m - 2 yr- 1 for the entire basin over seven years, corrected for the solute input in precipitation. Not all of this is derived from soil and bedrock because dry atmospheric deposition and the partial dissolution of dust are included. These contributions cannot yet be estimated for the Green Lakes Valley. Treating

6 g m-2 yr- ~as geologic denudation suggests a surface lowering rate of about 0.003 mm yrfor the basin as a whole. As a cationic denudation rate, this is about 100 meq m - 2 yr -1, appreciably less than those estimated for other high mountain catchments (e.g. Reynolds and Johnson, 1972; Eyles et al., 1982; Collins, 1983; Souchez and Lemmens, 1987). When dissolved material yields are consid-

70

ered, the simple down-valley trends in conductance and solute concentrations tend to be inverted (Table 6). Changes in concentration with drainage area are balanced by the opposite change in the specific discharge of water with the result that, for a catchment area greater than 1 km 2, specific solute yields tend to be about the same (though varying by a factor of 2 or 3 on a year to year basis: Fig. 11 ). On average, yields from smaller sub-basins are higher by about twofold, reflecting the accumulation of snow to depths of more than 10 m by wind drifting in the Martinelli, Arikaree and Navajo catchments. The high annual variability of solute yields at the smallest catchment areas (less than 1 km 2 ) is especially great (Fig. I I ) in response to the variability in snow distribution at this scale in alpine environments. It includes the effects noted earlier with regard to the Green Lake 1 catchment. This spatial variability is probably retained within these partial areas of less than 1 km 2 for most of the discharge there derives from only 40 to 70% of their catchment area. Within these partial areas of the Green Lakes Valley, geochemical denudation rates may be as high as 40 to 50 g m -2 yr -~ but even that is still a low rate when compared to estimates from elsewhere. Within other partial areas of the valley, with a soil and mesic tundra vegetation cover, the disparity between the chemistry of soil-interstitial waters and stream water is such as to suggest an almost complete decoupling of the two systems (Litaor and Thurman, 1988). In these relatively dry areas, geochemical denudation associated with water drainage may be negligible. Although these rates of geochemical denudation are low on a global scale (Meybeck, 1976, 1979), they are nevertheless a very important part of the total material budget of the catchment. Clastic sediment removal through the stream channels of the Green Lakes system is more than an order of magnitude less effective in terms of total yield (Table 6) and such low estimates do not result from sedimenta-

N. CAINE AND E.M. THURMAN

tion in the lakes of the valley (Caine, 1986). Mass wasting processes within the basin involve a much greater mass of material (Benedict, 1970; Caine, 1986) but little of that is converted ~into sediment removal through the streams. Even rare catastrophic storms do not appear to change this because little evidence exists of their impact on lake sedimentation in the last 4000 yr (Harbor, 1984). Hence, solute removal, especially from areas of snow accumulation, seems to be the most significant influence on material exports from the alpine of tlhe Colorado Front Range. Conclusion With regard to three objectives stated in the Introduction to this paper, this study has demonstrated that the solute concentrations in waters draining from the alpine zone of the Colorado Front Range are consistently low. They are, nevertheless, subject to marked temporal fluctuations, particularly ones related to hydrologic events involving the mixing of water from different sources. The chemical characteristics of surface waters of the Green Lakes Valley also vary spatially and, in that, reflect the mosaic of the alpine environment. This spatial variability resuits from some of the same controls that are effective temporally. In particular, it reflects the fact that, in summer, stream water is derived from a number of distinct sources with different characteristics. Surface flow is generated on the relatively small parts of the catchment that are snow covered and which contribute water to the streams at rates which vary with energy inputs for snowmelt and the changing area of snow cover in the catchment. Groundwater and soil water contributions to ,;treamflow derive from some of the same source areas but appear to be more uniform in their distribution. The low solute concentrations in the surface waters of the alpine zone gives a low rate of geochemical denudation. This is not to suggest l~hat solute denudation may be neglected, how-

SOLUTE CONTENT OFAN ALPINE STREAM, COLORADO FRONT RANGE

ever, for its rate remains much greater than that of other processes of sediment removal. The patchy nature of the source areas for both stream flow and the solutes which it contains ensures that its influence on the landscape is not uniform. In this sense, our observations suggest that geochemical denudation is most effective in areas which accumulate the greatest thickness of winter snow cover and is an important contributor to the processes of snow patch erosion in the Colorado Front Range today.

Acknowledgements As part of the Long-Term Ecological Research Program, our work in the Green Lakes Valley has been supported by the National Science Foundation through grants BSR 8012095 and BSR 8514329. Our colleagues at the Niwot Ridge/Green Lakes Valley LTER site have contributed much through assistance in the field and laboratory and through discussions over the life of this project. In particular, we have benefitted from the help of Dave Furbish and Mark Losleben in the field, Jennifer Caine for laboratory analyses, and Iggy Litaor, Mike Reddy and Pat Webber for reviews and comments.

References Averett, R.C. and Marzolf, G.R., 1987. Water quality. Environm. Sci. Technol., 21: 827. Baron, J., 1983. Comparative water chemistry of four lakes in Rocky Mountain National Park. Water Resour. Bull., 19: 897-902. Baron, J. and Bricker, O.P., 1987. Hydrologic and chemical flux in Loch Vale watershed, Rocky Mountain National Park. In: R.C. Averett and D.M. McKnight (Editors), Chemical Quality of Water and the Hydrologic Cycle. Lewis, Chelsea, Mich., pp. 141-155. Barry, R.G., 1973. A climatological transect on the east slope of the Front Range, Colorado. Arct. Alp. Res., 5" 89-110. Barsch, D. and Caine, N., 1984. The nature of mountain geomorphology. Mountain Res. Developm., 4: 287298. Benedict, J.B., 1970. Downslope soil movement in a

71

Colorado alpine region: rates, processes and climatic significance. Arct. Alp. Res., 2: 165-226. Bogli, A., 1980. Karst Hydrology and Physical Speleology. Springer, Berlin, 284 pp. Brimblecombe, P., Tranter, M., Abrahams, P.W., Blackwood, I., Davies, T.D. and Vincent, C.E., 1985. Relocation and preferential elution of acidic solutes through the snowpack of a small, high-altitude Scottish catchment. Ann. Glaciology, 7:141-147. Burns, S.F., 1980. Alpine soil distribution and development, Indian Peaks, Colorado Front Range. Thesis, University of Colorado, Boulder, Colo., 360 pp. Caine, N., 1976. A uniform measure ofsubaerial erosion. Geol. Soc. Am. Bull., 87: 137-140. Caine, N., 1982. Water and sediment flows in the Green Lakes Valley, Colorado Front Range. In: J.C. Halfpenny (Editor), Ecological Studies in the Colorado Alpine: A Festschrifl for John W. Marr. Institute of Arctic & Alpine Research, University of Colorado, Boulder, Colo., Occ. Pap., 37:13-22. Caine, N., 1984. Elevational contrasts in contemporary geomorphic activity in the Colorado Front Range. Stud. Geomor. Carpatho-Balcanica, 18:5-31. Caine, N., 1986. Sediment movement and storage on alpine hillslopes in the Colorado Rocky Mountains. In: A.D. Abrahams (Editor), Hillslope Processes. Allen and Unwin, London, pp. 115-137. Caine, N., 1989. Diurnal variations in the quality of water draining from an alpine snowpatch. Catena, 16:153162. Collins, D.N., 1983. Solute yield from a glacierized high mountain basin. In: Dissolved Load of Rivers and Surface Water Quantity/Quality Relationships. Int. Ass. Sci. Hydrol. Publ., 141: 41-49. Dethier, D., 1988. A hydrogeochemical model for stream chemistry, Cascade Range, Washington, U.S.A. Earth Surf. Proc. Landforms, 13: 321-333. Eyles, N., Sassaville, D.R., Slatt, R.M. and Rogerson, R.J., 1982. Geochemical denudation rates and solute transport mechanisms in a maritime temperate glacier basin. Can. J. Earth Sci., 18: 41-49. Grant, M.C. and Lewis Jr., W.M., 1982. Chemical loading rates from precipitation in the Colorado Rockies. Tellus, 34: 74-88. Hallet, B., 1976. Deposits formed by subglacial precipitation of CaCO3. Geol. Soc. Am. Bull., 85: 1003-1015. Harbor, J.M., 1984. Terrestrial and lacustrine evidence for Holocene climatic/geomorphic change in the Blue Lake and Green Lakes Valleys of the Colorado Front Range. Thesis, University of Colorado, Boulder, Colo., 205 pp. Ives, J.D., 1973. Permafrost and its relationship to other environmental parameters in a mid-latitude, high-altitude setting, Front Range, Colorado Rocky Mountains. In: Permafrost - the North American Contributions to the 2nd Int. Permafrost Conf. National Academy of Sciences, Washington, pp. 13-28.

72 Ives, J.D., 1973. Permafrost and its relationship to other environmental parameters in a mid-latitude, high-altitude setting, Front Range, Colorado Rocky Mountains. In: Permafrost - the North American Contributions to the 2nd Int. Permafrost Conf. National Academy of Sciences, Washington, pp. 13-28. Ives, J.D. (Editor), 1980). Geoecology of the Colorado Front Range: A Study of Alpine and Subalpine Environment. Westview Press, Boulder, Colo., 484 pp. Kling, G.W. and Grant, M.C., 1984. Acid precipitation in the Colorado Front Range: an overview with time predictions for significant effects. Arct. Alp. Res., 16: 321329. Komarkova, V. and Webber, P.J., 1978. An alpine vegetation map of Niwot Ridge, Colorado. Arct. Alp. Res., 10: 1-29. Lewis Jr., W.M., 1982. Changes in pH and buffering capacity of lakes in the Colorado Rockies. Limnol. Oceanogr., 27: 167-172. Lewis Jr., W.M. and Grant, M.C., 1979. Relationships between stream discharge and yield of dissolved substances from a Colorado mountain watershed. Soil Sci., 128: 353-363. Likens, G.E., Borman, F.H., Pierce, R.S., Eaton, J.S. and Johnson, N.M., 1977. Biogeochemistry of a Forested Ecosystem. Springer, New York, 146 pp. Litaor, M.I., 1987. The influence of eolian dust on the genesis of alpine soils in the Front Range, Colorado. Soil Sci. Soc. Am. J., 151: 142-147. Litaor, M.I. and Thurman, E.M., 1988. Acid neutralizing capacity in an alpine watershed, Front Range, Colorado. I. The chemistry of dissolved organic carbon. Appl. Geochem., 3: 645-652. Losleben, M., 1983. Climatological data from Niwot Ridge, East Slope, Front Range, Colorado 1970-1982. University of Colorado LTER Data Report 83/10, 193 PP. Madole, R.F., 1982. Possible origins of till-like deposits near the summits of the Front Range in north-central Colorado. U.S. Geol. Surv. Prof. Pap., 1243:31 pp. Meybeck, M., 1976. Total mineral dissolved transport by world major rivers. Hydrol. Sci. Bull., 21: 265-281. Meybeck, M., 1979. Concentrations des eaux fluviales en elements majeurs et apports en solution aux oceans. Rev. Geol. Dynam. Geogr. Phys., 21:215-246. Miller, J.P., 1961. Solutes in small streams draining single rock types, Sangre de Cristo Range, New Mexico. U.S.

N, CAINE AND E.M. THURMAN

Geol. Surv. Water Supply Pap., 1535F: 23 pp. Raiswell, R., 1984. Chemical models of solute acquisition in glacial meltwaters. J. Glaciol., 30: 49-57. Reddy, M.M. and Caine, N., 1990. Dissolved solutes budget of a small alpine basin, Colorado. Proc. Int. Mountain Watershed Conf., South Lake Tahoe, July 1988, in press. Reddy, M.M. and Claasen, H.C., 1985. Estimates of average major ion concentrations in bulk precipitation at two high-altitude sites near the Continental Divide in southwestern Colorado. Atmos. Environm., 19:11991203. Reid, J.M., Macleod, D.A. and Cresser, M.S., 1981. The assessment of chemical weathering rates within an upland catchment in northeast Scotland. Earth Surf. Proc. Landforms, 6: 447-457. Reynolds Jr., R.C. and Johnson, N.M., 1972. Chemical weathering rates in the temperate glacial environment of the Northern Cascade Mountains. Geochim. Cosmochim. Acta, 36: 537-554. Skougstad, M.W., Fishman, M.J., Friedman, L.C., Erdman, D.E. and Duncan, S.S., 1979. Methods for analysis of inorganic substances in water and fluvial sediments. U.S. Geol. Surv, Open File Rep., 78-679: 1006 pp. Souchez, R.A. and Lemmens, M.M., 1987. Solutes. In: A.M. Gurnell and M.J. Clark (Editors), Glacio-Fluvial Sediment Transfer: An Alpine Perspective. Wiley, Chichester, pp. 285-303. Swank, W.T., 1986. Biological control of solute losses from forest ecosystems. In: S.T. Trudgill (Editor), Solute Processes. Wiley, Chichester, pp. 85-139. Thorn, C.E., 1976. Quantitative evaluation ofnivation in the Colorado Front Range. Geol. Soc. Am. Bull., 87: 1169-1178.

Trudgill, S.T., 1986. Solute processes and landforms: an assessment. In: S.T. Trudgill (Editor), Solute Processes. Wiley, Chichester, pp. 497-509. Vitek, J.D., Deutch, A.L. and Parson, C.G., 1981. Summer measurements of dissolved ion concentrations in alpine streams, Blanca Peak Region, Colorado. Prof. Geogr., 33: 436-444. Woo, M.-K., 1982. Snow hydrology of the high arctic. Proc. 50th Western Show Conf., Reno, Nev. Colorado State Univ., Fort Collins, pp. 63-74.