Alpine and subalpine ecosystems are vulnerable to impacts from acidic 

deposition and changes in climate because they have a short growing 

season, and large amounts of snowmelt are released from deep seasonal 

snowpacks in the spring (Campbell and others, 1995). Research at the 

Loch Vale Watershed provides process-level information that augments 

synoptic and long-term monitoring of atmospheric deposition and water 

quality in other sensitive Rocky Mountain ecosystems. Together, these 

research and monitoring studies support policy decisions by other 

government agencies regarding natural resource management issues.

The Loch Vale Watershed drains 

661 hectares of alpine and subalpine terrain in Rocky Mountain 

National Park, about 100 kilometers 

northwest of Denver, Colorado. The 

western boundary of the watershed is the 

Continental Divide; streams drain to the 

U.S. Department of the Interior 

U.S. Geological Survey

Fact Sheet-164-99

June 2000

northeast. The basin ranges in elevation 

from 3,110 meters (10,200 feet) at the 

outlet to 4,192 meters (13,153 feet) at 

Taylor Peak. The two main tributaries 

in Loch Vale—Andrews Creek and Icy 

Brook—join above the Loch, which is 

the lowest of three lakes in the watershed. Streamflow gages are operated on 

Andrews Creek, Icy Brook, and at the 

outlet of The Loch. 

Geomorphology in Loch Vale is typical 

of recently glaciated terrain in the southern Rockies. The lower, forested part of 

the watershed is a U-shaped valley overlain by poorly developed soils formed 

on thin, patchy till. The steep valley 

sidewalls are mostly unvegetated and are 

characterized by bedrock cliffs that are 

mantled with talus near the base. Small 

glaciers, permanent snowfields, and rock 

glaciers occupy cirques in the headwater 

areas. Tundra is found along the ridge 

tops. Climate in Loch Vale is characterized by long, cold winters with short 

growing seasons (3–4 months). Precipitation averages 123 centimeters per year 

(1983–97) of which about 75 percent 

accumulates in a seasonal snowpack 

from November through April. Melting 

of the snowpack is the major hydrologic 

event of the year; over 60 percent of the 

annual streamflow typically occurs in June 

and July. Average annual runoff (1992–97) 

from Andrews Creek and Icy Brook was 

100 and 78 centimeters, respectively. 

Hydrologic Processes

One of the objectives of the WEBB study 

has been to improve the understanding of 

hydrologic processes in snowmelt-dominated watersheds. Isotopic and chemical 

tracers were used as tools to evaluate 

sources and flowpaths of water in Andrews Creek during the 1994 snowmelt 

season (Mast and others, 1995). Bulk 

snowpack, snowmelt, rain, and streamwater samples were analyzed for δ18O 

and dissolved silica. Hydrograph separation calculations based on δ18O were 

used to estimate premelt versus snowmelt 

(old versus new) sources of water to 

the stream. Hydrograph separations based 

on silica indicated how precipitation was 

transported to the stream—through the 

subsurface or overland. 

The Loch Vale watershed is located in a roadless area of Rocky Mountain National Park. 

Loch Vale basin, looking upstream from the 

Loch outlet

The Water, Energy, and Biogeochemical Budgets (WEBB) Program was started 

in 1991 at five small watersheds in the United States to examine water, energy, 

and biogeochemical fluxes and to determine the effects of atmospheric 

deposition, climatic variables, and human influences on watershed processes. 

The five sites are at Loch Vale, Colorado; Luquillo Experimental Forest, Puerto 

Rico; Panola Mountain, Georgia; Sleepers River, Vermont; and Trout Lake, 

Wisconsin. These sites are supported, in part, by other programs in the 

USGS, other Federal and State Agencies, and Universities.

THE WEBB PROGRAM

Loch Vale, Colorado

A Water, Energy, and Biogeochemical Budgets Program Site

The Loch

(el 3110 m)

Rocky

Mountain

National

Park

C O L O R A D O

Subalpine

Sky Pond

Glass Lake

Andrews

Glacier

Andrews Cr.

Icy Brook

Taylor Glacier Continental

Divide

Contour interval 70 meters

Alpine

Weather station

Streamflow-gaging station

Denver

3080

3220

3360

3360

3500

3500

3640

3640

3780

3780

3920

N

0 0.5 km

0 0.5 mi
Most streamflow is derived from 

melting snow, although much of it 

passes through soil before entering 

the stream.

N-cycling in plants and soils, as 

well as direct flushing of nitrogen 

from atmospheric deposition, controls watershed export of N.

Calculations based on δ18O indicate that 

premelt water was a significant component of streamflow only at the beginning 

of the melt cycle, accounting for 33 percent of flow before June 1 but only 17 

percent of flow for the entire season. 

This suggests that 

streamflow at the 

beginning of melt 

is derived primarily from premelt 

water that is displaced into the stream by infiltrating 

snowmelt. As melt continued, premelt 

water was eventually flushed from soils 

and shallow subsurface reservoirs. Afterwards streamflow was almost entirely 

generated by recent snowmelt. The apparent increase in the premelt component in late summer probably is an artifact of the model and represents recharge 

of subsurface reservoirs by isotopically 

heavy summer rains. 

Calculations based on silica indicate that 

flow through the subsurface accounted 

for more than 75 percent of streamflow 

during the initial stages of snowmelt and 

about 45 percent during the peak-flow 

period (overland flow accounted for the 

rest). This result indicates that even at 

peak discharge, as much as half of snowmelt moves through soil and shallow 

subsurface reservoirs before discharging 

into the stream.

Nitrogen Deposition and Cycling

Atmospheric deposition of two forms 

of nitrogen—nitrate and ammonium—is 

higher in the Front Range of Colorado 

than in many other parts of the Rocky 

Mountains, although it is moderate 

compared to other regions, such as 

northern Europe and the northeastern 

United States. Elevated nitrogen deposition can result from atmospheric emissions of nitrogen compounds from many 

human activities, such as agriculture 

(crop or livestock production) and fossil-fuel combustion (automobiles, electric power generation, or industrial operations). In most undisturbed watersheds, 

nitrogen is used to meet nutrient requirements of the 

ecosystem, and little nitrogen is exported in streamflow. In the Front 

Range, however, 

nitrogen in atmospheric deposition exceeds nutrient requirements. Consequently, there is export of nitrate in 

streams even during the growing season 

when biological demand for nitrogen 

is greatest. Excess 

stream-water nitrogen can result in 

changes in natural 

biogeochemical cycling that lead to acidification or eutrophication of lakes and 

streams, diminished ecosystem health, 

and changes in terrestrial or aquatic species composition. 

For water years 1992–97, annual nitrogen export from two tributary streams 

in the Loch Vale Watershed was not 

directly related to annual nitrogen 

deposition, indicating that ecosystem 

biogeochemical processes (nitrogencycling) controlled the release of nitrogen (Campbell and others, 2000). There 

was a positive relation between annual 

nitrogen retention and annual nitrogen 

deposition, indicating that parts of the 

ecosystem are nitrogen-limited. However, a large percentage of these 

watersheds are covered by bare rock 

and talus deposits that have little capacity for nitrogen assimilation and 

large potential for 

mineralization and 

nitrification. Even 

in the areas with 

sparse vegetation 

and soil, the nitrate 

in streams and ground water has an isotopic signature (δ15N and δ18O) that is 

distinct from that of nitrate in atmospheric deposition, as expected when nitrogen from deposition is cycled through 

biological processes (Kendall and others, 

1995). Because of the importance of biological processes, climatic factors also 

play a role in controlling export of atmospherically deposited nitrogen from 

alpine/subalpine watersheds. 

Sources of Alkalinity and Base Cations

Geochemical processes, including mineral weathering, cation exchange, and 

adsorption/desorption, are major sources 

of solutes to stream water in alpine/

subalpine environments. Of these processes, mineral weathering is the main 

long-term source of alkalinity, silica, and 

base cations, and also is an important 

sink for CO2 in the atmosphere. The 

amount of alkalinity produced or CO2 

consumed by mineral weathering dePremelt component of 

streamflow from hydrograph separations using 

δ18O measurements; 

subsurface contribution 

to streamflow from 

hydrograph separations 

using silica; and total 

stream discharge.

Nitrate export in Andrews Creek peaks 

during snowmelt, as nitrogen is released 

from both soil and the snowpack. Peak 

nitrate concentrations sometimes exceed 50 

microequivalents per liter (μeq/L). Biological 

demand for nitrogen causes nitrate concentrations in the stream to fall to a minimum 

of around 10 μeq/L during summer before 

rising to 20–30 μeq/L during fall and winter.

0

20

40

60

1992 1994 1996

Nitrate

(microequivalents per liter)

0.0

0.1

0.2

0.3

Apr May June July Aug Sept Oct

Discharge

(cubic meters per second)

Stream discharge

Subsurface

Premelt
About one-quarter of the calcium 

in streams is derived from silicate 

minerals, one-half is from bedrock 

calcite, and the remainder is from 

eolian calcite.

pends on the types of minerals involved. 

Weathering of carbonate minerals generally produces more alkalinity and consumes less CO2 than weathering of silicate minerals. 

Isotopes of Sr have been used in Loch 

Vale to identify mineral weathering reactions and to evaluate sources of Ca in 

natural water. The Sr-isotope technique 

takes advantage of the fact that Sr substitutes for Ca in mineral lattices, and 

each type of mineral tends to have 

a different 87Sr/86Sr 

ratio. When different types of minerals weather, they 

release Sr with a 

unique 87Sr/86Sr ratio, imparting a signature on the interacting water. Results 

indicate that about one-quarter of the annual flux of calcium in surface water 

in Loch Vale was derived from silicate 

weathering, one-half came from weathering of bedrock calcite, and the remaining one-quarter was from eolian carbonates (Clow and others, 1997). 

The relative importance of mineral 

weathering reactions differs according to 

landscape because of differences in the 

mineralogic and hydrologic characteristics of those landscapes. These differences in weathering reactions are exemplified in a plot of 87Sr/86Sr against 

Ca/Na ratios. Low 87Sr/86Sr and high 

Ca/Na ratios in precipitation and bedrock runoff indicate that these water 

types are strongly influenced by dissolution of eolian carbonates. Soil water has 

a somewhat higher 87Sr/86Sr and lower 

Ca/Na ratio, indicating that it is influenced more by weathering of silicate 

minerals than carbonates. Stream and 

CO2 are found in aerobic soils that typically have low moisture content, whereas CH4 producers live in anaerobic soils 

that are saturated. CH4 also is consumed 

by microbes that live in aerobic soils. 

The rates of CO2 and CH4 exchange 

between soils and the atmosphere were 

measured along a moisture gradient that 

included a wetland with standing water, 

soils on the wetland perimeter with intermediate moisture content, and dry soils. 

These different areas are representative 

of some of the soil and vegetation types 

found in the Loch Vale Watershed. The 

goals of the study were (1) to estimate 

CO2 and CH4 exchange on an annual 

scale, (2) to determine the importance 

of exchange during snow-covered periods, and (3) determine the influence of 

environmental factors, such as soil temperature and moisture, on CO2 and CH4

exchange rates. 

On an annual basis, the wetland and 

moist soils emitted CH4 and the dry 

soils consumed CH4. Exchange during 

spring water have even higher 87Sr/86Sr 

ratios, indicating that they are influenced 

by weathering of biotite, microcline, and 

bedrock calcite. 

Basin Characteristics and StreamWater Chemistry

Relations between stream-water chemistry and topographic, vegetative, and geologic characteristics of basins were evaluated for nine alpine/subalpine watersheds 

in Rocky Mountain National Park 

(Clow and Sueker, 

2000) to improve 

the understanding 

of processes regulating stream-water chemistry and solute fluxes in alpine/

subalpine watersheds. 

Fractional amounts of steep slopes 

(>30 ̊), unvegetated terrain, and young 

geologic deposits within each watershed 

were positively correlated to each other. 

These terrain features, which commonly 

are found on steep valley sidewalls that 

are covered by talus, were inversely correlated with concentrations of base cations, silica, and alkalinity in steam water 

and were positively correlated with nitrate, pH, and runoff. These relations 

probably are due to the short residence 

times and limited soil development in the 

talus environment, which limit chemical 

weathering and nitrogen uptake. Steep, 

unvegetated terrains also tend to promote 

high Ca/Na ratios in stream water, probably because high physical weathering 

rates in those areas expose fresh bedrock 

that contains interstitial calcite, which 

weathers relatively quickly. The fractional amounts of subalpine meadow and 

old geologic debris in the watersheds 

were positively correlated to concentrations of Ca, Na, and alkalinity, and inversely correlated to nitrate and pH. This 

may reflect more opportunities for silicate weathering and nitrogen uptake in 

the lower-energy environments of the valley floor, where soils are better developed 

than those on the steep valley walls and 

where slopes are relatively flat. 

Carbon Gas Exchange

Carbon dioxide (CO2) and methane 

(CH4) are gases produced by soil microbial organisms during decomposition of 

organic materials. Microbes that produce 

Relation between 87Sr/86Sr and Ca/Na for 

various water types in Loch Vale.

Relationships between fraction of steep slopes 

(>30 ̊) and concentrations of (a) alkalinity, (b) 

NO3, and (c) Ca/Na ratios in stream water.

Precipitation

0 4 8 12

Bedrock calcite

 Eolian carbonates

Biotite, Microcline

0.705

0.715

0.735

0.745

0.725

Soil water

Springs

Andrews Cr.

Icy Brook

Plagioclase

87Sr/86Sr

Ca:Na ratio

(microequivalents per liter)

40

0

80

120

160

R2 = 0.85

R2 = 0.89

R2 = 0.91

NO3

(microequivalents per liter)

Alkalinity

(microequivalents per liter)

Ca:Na ratio

(microequivalents per liter)

5

0

10

15

20

25

1

0

2

3

4

5

0.0 0.2 0.4 0.6 0.8

a

b

c

Fraction steep slope
CO2 and CH4 fluxes in winter are 

a substantial part of the annual 

carbon gas budget in seasonally 

snow-covered basins.

For more information about the

Loch Vale WEBB study visit:

http://co.water.usgs.gov/lochvale/

For more information or for reprints, please contact:

WEBB Site Coordinator

U.S. Geological Survey

MS 415, Denver Federal Center

Denver, CO 80225

REFERENCES

Campbell, D.H., Clow, D.W., Ingersoll, G.P., Mast, 

M.A., Spahr, N.E., and Turk, J.T., 1995, Processes 

controlling the chemistry of two snowmelt-dominated 

streams in the Rocky Mountains: Water Resources 

Research, v. 31, no. 11, p. 2811–2821.

Campbell, D.H., Baron, J.S., Tonnessen, K.A., Brooks, 

P.D., and Schuster, P.F., 2000, Controls on nitrogen 

flux in alpine/subalpine watersheds of Colorado: 

Water Resources Research, v. 36, no. 1, p. 37–47.

Clow, D.W., Mast, M.A., Bullen, T.D., and Turk, 

J.T., 1997, 87Sr/86Sr as a tracer of mineral weathering 

reactions and calcium sources in an alpine/subalpine 

watershed, Loch Vale, Colorado: Water Resources 

Research, v. 33, no. 6, p. 1335–1351.

Clow, D.W., and Sueker, J.K., 2000, Relations 

between basin characteristics and stream-water chemistry in alpine/subalpine basins in Rocky Mountain 

National Park, Colorado: Water Resources Research, 

v. 36, no. 1, p. 49–61.

Kendall, C., Silva, S.R., Chang, C.C.Y., Burns, D.A., 

Campbell, D.H., and Shanley, J.B., 1995, Use of the 

δ18O and δ15N of nitrate to determine sources of 

nitrate in early spring runoff in forested catchments, 

in International Atomic Energy Agency Symposium, 

Vienna, Austria, IAEA, p. 167–176.

Mast, M.A., Kendall, C., Campbell, D.H., Clow, D.W., 

and Back, J., 1995, Determination of hydrologic pathways in an alpine-subalpine basin using isotopic and 

chemical tracers, in Tonnessen, K., Williams, M., and 

Tranter, M., eds., Biogeochemistry of seasonally snowcovered catchments: Boulder, Colorado, International 

Association of Hydrological Sciences, p. 263–270.

Mast, M.A., Wickland, K.P., Striegl, R.T., and Clow, 

D.W., 1998, Winter fluxes of CO2 and CH4 from subalpine soils in Rocky Mountain National Park, Colorado: Global Biogeochemical Cycles, v. 12, no. 4, p. 

607–620.

Wickland, K.P., Striegl, R.G., Schmidt, S.K., and 

Mast, M.A., 1999, Methane flux in subalpine wetland 

and unsaturated soils in the southern Rocky Mountains: Global Biogeochemical Cycles, v. 13, no. 1, p. 

101–113.

—D.W. Clow, D.H. Campbell, 

M.A. Mast, R.G. Striegl, 

K.P. Wickland, and G.P. Ingersoll

snow-covered periods was significant, 

accounting for 25 to 73 percent of the 

total annual CH4

exchange (Wickland and others, 

1999). Microbial 

activity continues 

through the winter because the deep snowpack provides 

insulation from cold air temperatures 

and prevents the soil from freezing 

(Mast and others, 1998). The spatial pattern of CH4 exchange was related to soilmoisture content during snow-covered and 

snow-free periods. CO2

emissions exhibited an 

opposite spatial pattern; CO2 emissions increased as soil moisture decreased. CO2

emissions during snow-covered periods 

accounted for 8 percent, 12 percent, and 

COLLABORATORS

The USGS conducts research at the Loch Vale site 

with support from National Park Service, USDA Forest Service, US Environmental Protection Agency, 

National Science Foundation, and Colorado Department of Public Health and Environment.

Collaborators include the National Weather Service, 

National Center for Atmospheric Research, University 

of Colorado, Colorado State University, University of Arizona, University of Wyoming, University of California, 

University of British Columbia, University of Southern 

Mississippi, and E&S Consulting.

Annual, snow-covered, and snow-free 

period methane exchange from four soil 

types. Fluxes are expressed as moles of 

methane per square meter of land surface. 

Positive values are CH4 emission from the 

soil to the atmosphere and negative values 

are fluxes from the atmosphere to the soils. 

SUMMARY

• The response of high-elevation ecosystems to perturbations such as 

climatic extremes and atmospheric deposition is dependent on landscape characteristics such as topography, geology and vegetation.

• Most streamflow is derived from melting snow, although much of it 

is transported along shallow soil and groundwater flowpaths before 

entering the stream.

• Soil and vegetation are sparse in alpine landscapes, but they are 

important in controlling the seasonal pattern and chemical composition of streamflow.

• Soil microbial activity continues during the winter months under deep 

snowpacks and is important in controlling cycling of carbon and nitrogen.

• Naturally occurring isotopic and chemical tracers have proven useful 

for determining sources of water and solutes in remote areas where 

conventional hydrologic measurements are impractical.

23 percent of annual emissions from 

wetland, moist, and dry soils, respectively (Mast and others, 1998).

0

1

2

Wetland Moist

(grass)

Moist

(moss)

Dry

Total annual

Snow-covered (255 days)

Snow-free (111 days)

mol CH4 m-2

0.0

0.1

Moist

(grass)

Moist

(moss) Dry

mol CH4 m-2