Abstract

The interaction between the Brazos River and the adjacent floodplain aquifer was studied for 200 days in 1995 at a ground-water research site near College Station, Texas. Two In Situ Permeable Flow Sensors (ISPFS) and a grid of well nests were used to correlate river stage to the magnitude and direction of ground-water flow at depths of 13.7 m and 18.3 m in the unconfined alluvial aquifer. Linear relationships between ground-water flow and river stage were determined at each depth. The floodplain aquifer responded differently to changes in river stage at the 13.7 m and 18.3 m depths. The horizontal velocity, parallel and perpendicular to the river, decreased with increasing river stage and increased with decreasing river stage, at both depths. However, the rates of change varied between the two depths. This caused the magnitude and direction of ground-water flow to be different at the two depths. The upward vertical velocity increased with increasing river stage at the 13.7 m depth and decreased with increasing river stage at the 18.3 m depth. At the 13.7 m depth, vertical ground-water flow gradually changed from upward to downward flow with long term river stage decline. Downward groundwater flow was not observed at the 18.3 m depth. Assessment of river-aquifer interactions indicates that a direct and measurable relationship exists between river stage and ground-water flow components at the site. The magnitude and direction of ground-water flow in the alluvial floodplain aquifer may be predicted if river stage is known.

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