ASSESSMENT OF RIVER-FLOODPLAIN AQUIFER INTERACTIONS

The interaction between the Brazos River and the adjacent floodplain a quifer was studied for 200 days in 1995 at a ground-water research sit e near College Station, Texas. Two In Situ Permeable Flow Sensors (ISP FS) 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 betwee n 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 p erpendicular to the river, decreased with increasing river stage and i ncreased with decreasing river stage, at both depths. However, the rat es 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 ch anged from upward to downward flow with long term river stage decline. Downward groundwater flow was not observed at the 18.3 m depth. Asses sment of river-aquifer interactions indicates that a direct and measur able relationship exists between river stage and ground-water flow com ponents at the site. The magnitude and direction of ground-water flow in the alluvial floodplain aquifer may be predicted if river stage is known.