DEVELOPMENT OF A GEOMORPHIC BASEMAP OF SAND DEPOSITS OVER A 460-KM LONG REACH IN GLEN, MARBLE, AND GRAND CANYONS, ARIZONA

Most large sand deposits along the Colorado River in Grand Canyon occur in zones of recirculating flow (eddies) that form downstream from debris fans. These sand deposits have decreased in size and abundance since the closure of Glen Canyon Dam (GCD), which effectively cut off upstream sediment supply to the downstream Colorado River Ecosystem (CRE), decreased seasonal flow variation, and increased daily flow fluctuation. Despite regular monitoring at a collection of long-term study sites, and periodic inventories of sand extent in aerial photographs, a comprehensive map of sandbars and the geomorphic settings in which they occur has never been compiled. Previous studies identified the locations of geomorphically significant tributaries which create debris fans throughout Glen, Marble, and Grand Canyons. But maps of entire debris fan-eddy sequences, which link debris fans, eddies, and associated sandbars, have been limited to discrete study reaches comprising less than 20% of the system.

In this study, we developed a map of alluvial sand deposits and their geomorphological context for the entire 460-km segment of the CRE from GCD to Pearce Ferry. Photogrammetrically corrected 25-cm resolution aerial imagery collected in May 2009 was used as the base for delineating 29 different geomorphological units, including 16 categories of bare and vegetated sandbars, gravel bars, debris fans, and 13 categories of channel units, including eddies. The map includes over 2,700 individual sand deposits, 1,320 of which occur in eddies downstream from debris fans, 289 in other types of eddies, and 1,112 in other geomorphological settings. This map has multiple uses as a base reference, including tracking trends in the area of exposed sand within eddies using high-resolution remote sensing data, characterizing the geomorphic setting of riparian vegetation communities, segregating measured changes in sand storage volume by depositional setting, and providing habitat units for studies of aquatic ecology.