20,000 grain-size observations from the bed of the Colorado River, and implications for sediment transport through Grand Canyon

In the late 1990s, we developed digital imaging hardware and software for in-situ mapping of sand-sized bed sediment of the Colorado River in Grand Canyon. This new technology enables collection and processing of hundreds of grain-size samples in a day. Bed grain size was mapped using this equipment on 8 surveys of the Colorado River in Grand Canyon, for a total of more than 20,000 observations spanning 8 years. These observations document the fining of the bed when fine sand is introduced from tributaries and document the winnowing of that new sediment in the mainstem during intervening periods. The observations show how grain size varies with depth and geomorphic setting (finer in shallow depths and in lateral separation eddies), and how it varies through time. The results document that mean grain size of sand covering much of the riverbed can change substantially through time (a factor of 3). Such changes in bed sediment can be expected to cause suspended sediment concentration and flux to change by an order of magnitude for a constant water discharge.