Analysis of groundwater exchange for a large plains river in Colorado (USA)

Complete daily water budget information was assembled for a 105 km segment. of the South Platte River in the plains region below Denver, CO, for the p eriod 1983-1993. The data were used in testing the possibility that depende nce of alluvial exchange mechanisms on stage height, as shown by models of alluvial exchange, allows alluvial exchange to be predicted continuously ov er a given reach through use of statistical information on river discharge. The study segment was divided into an upper and a lower reach; daily alluv ial exchanges for each reach were estimated by the method of residuals. The two reaches show small (15%) but statistically significant annual differen ces in rates of exchange. For each reach, there is a seasonal pattern (2 .5 -fold oscillation) in alluvial discharge to the channel, reflecting seasona lity in recharge of the alluvium by irrigation. At discharges up to 40 m(3) /s (82nd percentile), alluvial discharge to the channel occurs at a rate in dependent of river discharge. Above 40 m(3)/s, net alluvial discharge into the channel is progressively reduced; at 60 m(3)/s (92nd percentile) there is no net alluvial exchange. At still higher river discharges, water is los t to the alluvium through bank storage at a rate that is linearly related t o the logarithm of discharge. Annually, alluvial discharge accounts for 15- 18% of water entering the study segment, and alluvial recharge through bank storage accounts for 2-48 of water leaving the segment. Alluvial recharge through bank storage at the highest discharges can, however, exceed low-flo w alluvial discharge rates by five-fold over short intervals. Even though d aily alluvial exchanges vary widely, they can be estimated at r(2) values a bove 80% on the basis of reach, season, and river discharge. Copyright (C) 2001 John Wiley & Sons, Ltd.