The role of headwater valleys within groundwater flow systems, and the manner in which gullies affect that role, need to be identified to assess the hydrologic effects of gully erosion in such valleys. This paper examines groundwater discharge to a gully in a small alluvial headwater valley in Northwestern Nevada, U.S.A., and qualitatively evaluates the effects of the gully on streamflow regulation.
Field observations of groundwater flow were made during and following snowmelt in Spring 1985. Measurements of hydraulic heads were made in wells and piezometers, and groundwater discharge from the gully was measured volumetrically. Horizontal and vertical hydraulic conductivities of meadow strata were determined with field and laboratory tests. Specific conductance of gully outflow was monitored and used to estimate components of groundwater discharge with a mass-balance technique. An approximate water balance was calculated based on measured snowmelt volume, precipitation, groundwater discharge, and water-table elevations.
Results indicate that despite the low primary permeability of the rhyolitic bedrock, a large proportion of groundwater discharged to the gully consists of water moving through fractures or faults in bedrock. Erosion of the gully has removed relatively low-permeability alluvium that formerly restricted groundwater discharge from the meadow. Groundwater discharge in response to snowmelt recharge is now probably more rapid and of shorter duration than prior to erosion.