Aquatic food base response to the 1996 test flood below Glen Canyon Dam, Colorado River, Arizona

We examined the impact of the 1996 test flood released from Glen Canyon Dam (GCD) on the aquatic food base in the Colorado River through Grand Canyon National Park, Arizona, USA, Benthic scour and entrainment of both primary and secondary producers occurred at all study sites along the 385-km river corridor. The majority of the organic drift occurred within the first 48 h of the test flood with the arrival of the hydrostatic wave. Recent macrophy te colonizers (Chara, Potamogeton, and Elodea) of fine sediment in the tail waters were scoured from the channel bottom, with recovery to pre-flood est imates within 1-7 months depending on taxa. Macroinvertebrates and filament ous algae recovered within three months depending on taxa. The test flood r emoved suspended particles from the water column and increased water clarit y, which enhanced benthic recovery. The test-flood hydrograph was designed primarily as an experiment in sand transport and occurred during a period o f sustained high releases from GCD starting in June 1995 due to above-avera ge inflow into Lake Powell. We discuss the implications of the hydrograph s hape, pre- and post-riverine conditions, and the slow response time of biol ogical resources for design of aquatic ecosystem experiments.