Abstract |
The Nishnabotna River Basin is situated in a glacial terrain underlain by a bedrock valley system that contains important volumes of pro-glacial sand and gravel. Study of low-flow river discharge, patterns of spatial variation of groundwater hydraulic head potential, surface and groundwater quality relations, and subsurface geology indicate that topography controls the pattern of groundwater flow near the Basin divide but that groundwater flow patterns within the Basin respond to both surface topography and the subsurface geology. Exportation of groundwater from the Basin at locations other than the surface outlet is limited to flow of about 500,000 gpd through the pro-glacial aquifer of a tributary bedrock valley. The major bedrock valley, the Fremont Channel, exerts strong potential drawdown effects within the Basin. Where this bedrock valley horizontally coincides with the West Nishnabotna valley, the drawdown effects of the two are superimposed, and a balance between stream flow and groundwater flow is maintained with groundwater discharge to the river. (Author) |