ASSESSMENT OF STREAM WATER BYPASS THROUGH A MEANDER NECK IN A FLOOD PLAIN

An examination of water movement through a meander neck in a flood plain was performed to assess if a meander neck serves as a hyporheic pathway. Three wells were installed across a meander neck to a depth of 2.5 m. The screened interval, 1.8 to 2.5 m, coincides with a gravelly-sand zone that is at the same elevation as baseflow within the stream. The estimated hydraulic conductivity of the gravelly-sand, derived from core data, is 1 m/d. The wells were equipped with pressure transducers and temperature loggers.

A pressure gradient indicates that water is moving through the meander. With an average gradient during baseflow conditions of 0.0036, the specific discharge through the meander neck is 3.6 * 10^-3 m/d. The data also show that the subsurface water is responding to diurnal pressure variations similar to those observed in the stream, but with an average lag period of 4 hrs. Recharge events also indicate movement through the meander neck. During a recharge event, the pressure in the wells increased, with the upstream well responding within 45 min and the downstream well responding in 150 min. The increase in pressure is not uniform, and the gradient increases an order of magnitude. This elevated gradient is short lived, lasting on average five hours before returning to the baseflow gradient.

Temperature data also indicates that water is moving through the meander neck. Within the wells, the water temperature increased at different rates; the fastest rate occurred in the upstream well. No diurnal variations or responses to recharge events were noted with the temperature data, but this suggests that the large thermal capacity of the subsurface material is also being warmed by the water muting the variations due to recharge events and diurnal variation.

The data suggest that the meander neck serves as a hyporheic pathway. With the movement of surface water and no temperature disturbances, the subsurface material could serve as a habitat for invertebrates.