RIVER-AQUIFER EXCHANGES IN THE TWISP RIVER, WASHINGTON

摘要

The U.S. Geological Survey has been investigating river-aquifer exchanges along the lower 15 km of the Twisp River of north-central Washington to elucidate the seasonal patterns in ground-water discharge to the river and the effects of irrigation-canal seepage on aquifer recharge and base flow. The Twisp River consistently gained flow from Nov. 1, 2000 to Apr. 23, 2001 with a mean net ground-water discharge to the river of 0.13 m~3/s or 11 percent of mean discharge of the Twisp River near Twisp for the period. River-aquifer exchanges fluctuated during the high-flow period from Apr. 23 to July 25, 2001, with alternating periods of gains and losses in streamflow. The river returned to a gaining condition on July 26, with a mean net ground-water discharge to the river of 0.41 m~3/s from Aug. 15 to Oct. 15 or 60 percent of mean discharge of the Twisp River near Twisp for the period. The combined seepage rate from two irrigation canals in the lower Twisp River valley was estimated to be 0.2 m~3/s in June and July 2001 but likely decreased during the late summer. Seepage from the irrigation canals recharged shallow aquifers and, in turn, may have marginally increased the rate of ground-water discharge to the river during the late summer and early autumn. Snowmelt and bank storage during high flows also recharged aquifers helping to support ground-water discharge to the river during low-flow periods.