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 hydraulic conductivity of sediments lining the streambed. This study was designed to determine the temporal and spatial variability of the hydraulic conductivity of active sedimentary deposits lining the streambed of the Colorado River in the Grand Canyon. These reattachment bars form aquifers and create return-current channels that are critical for supporting terrestrial and aquatic ecosystems. Monitoring wells were placed in five separate reattachment bars over a 200 mile long reach of the Colorado River below Glen Canyon Dam. Hydraulic conductivity was measured in all wells with the pneumatic slug test method. There is no significant difference in hydraulic conductivity among the five reattachment bars in the Grand Canyon. Hydraulic conductivity is bimodally distributed within a reattachment bar because of differing sizes of sediments deposited under different eddy velocities. A major controlled release of water from Glen Canyon Dam in March 1996 redistributed the sediments in the reattachment bars and compressed sediments deposited during previous floods. Hydraulic conductivity was significantly lower in these sediments after the flood due to the increased effective stress from the newly deposited sediment. A year later, after the sediments had drained and some deflation had occurred, hydraulic conductivity of sand deposits returned to values similar to pre-flood values, whereas fine-grained sediments that compressed weren't able to elastically respond.