Ground-water discharge and aquifer recharge zones near Four Corners, Gallatin County, Montana
Schaffer, Mark Andrew
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The West Gallatin River is closed to new surface water appropriations. Montana law recognizes the potential for new ground-water developments to compromise existing surface-water rights. Determining the effects of new ground-water development upon existing surface-water users requires a detailed understanding of the hydrologic system and the surface and ground water exchanges, which is not available in the Four Corners area of Gallatin County Montana. Hydrologic data was collected during the 2006 water year. Data was collected on stream discharge, streambed hydraulic gradients, temperature, ground-water levels, and specific electrical conductance to characterize zones of upwelling ground water and downwelling surface water. The river and aquifer share a complex hydrologic connection. Some reaches of river gained streamflow from the aquifer and contained zones of downwelling surface water. Other reaches of river which lost water to the aquifer, contained zones of upwelling ground water. Water moved back and forth between the river and aquifer through a variety of hydrologic pathways including; seepage below irrigation canals and irrigated land, drainage ditches from irrigated lands, spring channels, and the porous alluvial riverbed. Ground-water discharge to the stream contributed 8% of the streamflow when no irrigation was occurring in November 2005. In August 2006 during the irrigation season, the volume and relative contribution of ground-water discharge increased to 28% of the streamflow. The data collected from this study was compared to historic water-level data collected during the 1950's and later in the 1990's. This study's analysis benefited from the comparison of surface-water levels to ground-water levels, streambed hydraulic gradients, continuous records of ground-water levels and temperatures from near stream wells. Data from this study indicates that a consistent zone of downwelling surface water was present north of Four Corners which had not been identified in past surveys. However, the baseline data was found to be insufficient for a comparison to this analysis due to a lack of surface water data collected in the previous studies. As a result, it remains unclear whether the zone of downwelling has always been present or represents a change to the hydrologic system at Four Corners.