Michael J. Breedlove | Geology Reports

USGS2010

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.

Arizona↗

USGS2008

Modeling Water-Surface Elevations and Virtual Shorelines for the Colorado River in Grand Canyon, Arizona

Using widely-available software intended for modeling rivers, a new one-dimensional hydraulic model was developed for the Colorado River through Grand Canyon from Lees Ferry to Diamond Creek. Solving one-dimensional equations of energy and continuity, the model predicts stage for a known steady-state discharge at specific locations, or cross sections, along the river corridor. This model uses 2,680 cross sections built with high-resolution digital topography of ground locations away from the river flowing at a discharge of 227 m3/s; synthetic bathymetry was created for topography submerged below the 227 m3/s water surface. The synthetic bathymetry was created by adjusting the water depth at each cross section up or down until the model?s predicted water-surface elevation closely matched a known water surface. This approach is unorthodox and offers a technique to construct one-dimensional hydraulic models of bedrock-controlled rivers where bathymetric data have not been collected. An analysis of this modeling approach shows that while effective in enabling a useful model, the synthetic bathymetry can differ from the actual bathymetry. The known water-surface profile was measured using elevation data collected in 2000 and 2002, and the model can simulate discharges up to 5,900 m3/s. In addition to the hydraulic model, GIS-based techniques were used to estimate virtual shorelines and construct inundation maps. The error of the hydraulic model in predicting stage is within 0.4 m for discharges less than 1,300 m3/s. Between 1,300-2,500 m3/s, the model accuracy is about 1.0 m, and for discharges between 2,500-5,900 m3/s, the model accuracy is on the order of 1.5 m. In the absence of large floods on the flow-regulated Colorado River in Grand Canyon, the new hydraulic model and the accompanying inundation maps are a useful resource for researchers interested in water depths, shorelines, and stage-discharge curves for flows within the river corridor with 2002 topographic conditions.

Arizona↗