Spatial and temporal variability of hydraulic conductivity in active reattachment bars of the Colorado River, Grand Canyon

Ground water/surface water interaction in rivers is dependent on the hydrau lic conductivity of sediments lining the streambed, This study was designed to determine the temporal and spatial variability of the hydraulic conduct ivity of active sedimentary deposits lining the streambed of the Colorado R iver in the Grand Canyon, These reattachment bars form aquifers and create return-current channels that are critical for supporting terrestrial and aq uatic ecosystems. Monitoring wells were placed in five separate reattachmen t bars over a 200 mile long reach of the Colorado River below Glen Canyon D am. Hydraulic conductivity was measured in all wells with the pneumatic slu g test method. There is no significant difference in hydraulic conductivity among the five reattachment bars in the Grand Canyon. Hydraulic conductivi ty is bimodally distributed within a reattachment bar because of differing sizes of sediments deposited under different eddy velocities. A major contr olled release of water from Glen Canyon Dam in March 1996 redistributed the sediments in the reattachment bars and compressed sediments deposited duri ng previous floods, Hydraulic conductivity was significantly lower in these sediments after the flood due to the increased effective stress from the n ewly deposited sediment. A year later, after the sediments had drained and some deflation had occurred, hydraulic conductivity of sand deposits return ed to values similar to pre-flood values, whereas fine-grained sediments th at compressed weren't able to elastically respond.