CONNECTING SURFICIAL GEOLOGY AND HYDROLOGIC FLUX IN LEAKY, SNOWMELT-DOMINATED CATCHMENTS; NIWOT RIDGE, CO

Alpine and subalpine snowmelt-dominated catchments are an important fresh water source in the western and northeastern United States, but understanding the hydrology of these catchments is challenging. Highly porous and permeable surficial deposits in some alpine areas provide subsurface pathways for snowmelt, resulting in “leaky” basins. We show that thickness and hydraulic conductivity of the shallow subsurface controls the expression of surface discharge.

This study investigated the surficial geology and hydrology of four alpine and subalpine catchments (total area 2 km²) on Niwot Ridge, Colorado Front Range to help characterize how surface and subsurface flow interact above the glacial limit. Martinelli, Saddle, Como, and Upper Fourmile basins support tundra communities and patchy forest that grows from thin soils developed on 2 to >5m of coarse, unconsolidated periglacial deposits (Leopold, 2008). Fractured igneous and high-grade metamorphic rocks underlie the layered surficial deposits. We have characterized local surficial geology with field mapping, inversion of electrical resistivity measurements, and well-log data. In areas without surface water expression, we measured a saturated/capillary fringe zone with low resistivity values of ~200-800 Ω m beneath a shallow vadose zone (1500-2500 Ω m) and above bedrock surfaces (>2500 Ω m) at depths of <1 to 5.5 m. We investigated basin hydrologic flux by measuring surface flow at selected sites by: (1) modeling snowmelt and (2) compiling groundwater and mass-budget measurements. Recent estimates of the hydraulic conductivity of unconsolidated deposits on the Niwot Ridge ranged as low as 9.82x10^-5 to 2.85x10^-4 m/s in the Martinelli basin (King, 2012), to as high as 1.1x10^-3 to 2.6 m/s in the nearby Loch Vale (Clow et al., 2003). Our initial calculations indicate that flow through the subsurface is delayed yet significant, and the expression of surface water is dependent upon subsurface geometry and hydraulic properties as well as the lateral and vertical extent of the saturated zone. This research has implications on catchment morphology, water quality, and future water balance studies.