Patterns of Channel and Sandbar Morphologic Response to Sediment Evacuation on the Colorado River in Marble Canyon, Arizona

Abstract:

Management goals for the Colorado River in Marble and Grand Canyons include improving the condition of sandbars by the implementation of controlled floods. However, in this system that has been perturbed into fine-sediment deficit by an upstream dam, it is uncertain whether sand supply from tributaries is sufficient to support repeated sandbar building, since dam operations typically export sand. We report on a closed sand budget constructed from flux and morphologic measurements for a 50-km segment of Marble Canyon over a 3-year period. The results provide insight into the location and dynamics of sand-storage locations and places recent changes in sand storage in context with historic measurements of bed elevation.

The 2009-2012 study period included the largest annual water releases from Glen Canyon Dam since 1998. The measurements of sand flux and repeat morphologic measurements both indicate that these releases evacuated on the order of 300,000 m³ of sand, approximately equivalent to 20% of the total sand flux for the period. The pattern of sand storage change was different between eddies, where sandbars that are of management interest occur, and the main channel. Among eddies, deposition and erosion were approximately balanced and net change was negligible. Nearly all of the net sand evacuation was the result of erosion from the main channel. We estimate that a minimum of 250,000 m³ of sand remained in storage within eddies. Thus, if the high release volumes continued, sand evacuation would likely have been much larger. This is consistent with the measurements of sand flux, which did not indicate a decline in the rate of evacuation until the dam release rate was reduced.

Comparisons between the recent measurements of bed elevation with measurements made in the late 1990's indicate that the 2011 scouring event did not cause greater scour than occurred in the mid-1990s.