Mark W Williams
INSTAAR and Dept. of Geography University of Colorado, Boulder CB 450 Boulder, CO 80309 United States
Department of Geography and Institute of Arctic and Alpine Research University of Colorado, Boulder
Recent increases in growing-season concentrations of nitrate (NO3-) in surface waters
(streams and lakes) of a headwater catchment near the continental divide raise concerns
of nitrogen saturation in this sensative alpine ecosystem.
While the sources of NO3- remain unknown, concentrations of NO3- are found to be higher
in ephemeral streams draining alpine talus landforms than in snowmelt, rainwater,
or proximal surface waters.
Talus accounts for more than half of the landcover of the Green Lakes Valley,
Colorado Front Range, and small patches of vegetated and unvegetated soils within talus
landforms have been found to contain sufficient inorganic and organic pools
of nitrogen (N) to produce the NO3- found in surface waters.
What remains unknown is the hydrologic and hydrochemical contribution of talus to
surface waters in the catchment.
Discharge from an ephemeral talus stream was measured continuously at a
V-notch weir following peak-snowmelt to the cessation of flow,
and periodically at other talus discharge streams within the valley.
Specific discharge, on average 0.3 cm hr-1 m-2 for late season flows,
was employed in a mass-balance appraoch to estimate that talus discharge contributed
70 to 80\% of total surface water discharge.
Hydrograph comparison between a talus stream and a nearby surface water gauge showed
strong correlation (r2 = 0.83) when a 9-hr lag time was applied.
A LiBr tracer study during summer baseflow conditions showed that only
10% of the lithium was recovered compared to bromide,
suggesting that lithium was removed by cation exchange reactions in the talus field
while bromide behaved conservatively.
The isotopic composition of talus waters suggests a complex storage and release chronology,
with rapid flow over saturated or impermeable layers at some times
and storage within the subsurface matrix at others.
Talus waters exhibit similar patterns of NO3- and silica concentrations,
suggesting that NO3- is leached from talus fine materials.
These results suggest that discharge from talus fields may contribute much of the water
quantity and solute loading in surface waters of high-elevation catchments
during the growing season.
Mark W Williams