Hydrological and biogeochemical characteristics of alpine talus, Colorado Front Range

Among the most common landforms of high-elevation catchments in the Colorado Front Range are the unvegetated coarse regoliths that cover valley sides and collect below cliffs and steep slopes, commonly known as talus. Their widespread distribution and snow cover suggest that talus landforms may provide an important hydrological link between melting snow and high-elevation surface waters. Studies of alpine nitrogen dynamics point to talus as a potential source of nitrate (NO₃−) in these environmentally sensitive high-elevation ecosystems. This research investigates the physical, hydrological and biogeochemical characteristics of alpine talus of the Upper Green Lakes Valley, Colorado Front Range in order to quantify and better understand the hydrologic contribution of talus to the quantity and quality of high-elevation waters.; Talus covers 35% of the upper Green Lakes Valley, more than any other landform type. Talus discharge to valley accounts for about 68% of valley discharge during the post-snowmelt receding limb of the hydrograph. Infiltrating waters undergo rapid kinetic reactions with terrestrial materials in the talus matrix and acquire weathering-product solute chemistry before being discharged to the valley surface water system. Isotopic and chemical composition of talus water is significantly different from both precipitation inputs and catchment surface waters. Talus appears to have two flow regimes: rapid, convergent flow where the matrix is coarse, and slower, diffuse flow through subsurface fine materials.; Talus landforms are biologically active, and there is a sufficient amount of fine materials in surface and subsurface patches to influence the solute content of talus water. Total nitrogen storage in talus is at least half as much as other alpine environments on Niwot Ridge. Microbial N was found in all talus fine materials, and in some surface patches in quantities similar to nearby alpine tundra. Vegetation is sparse on talus, so assimilation and uptake of inorganic N are likely more limited than in nearby alpine tundra or valley bottoms. Rates of microbially mediated N mineralization and nitrification measured in surface plots during the growing season was similar to rates measured in alpine tundra, and are sufficient to suggest a plausible source for the elevated NO₃− concentrations found in talus water. Leaching of inorganic N and high nitrification rates during the growing season support the suggestion that nitrate production and export from talus may be important contributors of NO₃− to surface waters.