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.