COMPARISON OF THE MAGNITUDE OF EROSION ALONG 2 LARGE REGULATED RIVERS

Historical inventories of sand bar number and area are sufficient to d etect large-scale differences in geomorphic adjustment among regulated rivers that flow through canyons with abundant debris fans. In these canyons, bedrock and large boulders create constrictions and expansion s, and alluvial bars occur in associated eddies at predictable sites. Although these bars may fluctuate considerably in size, the locations of these bars rarely change, and their characteristics can be compared through time and among rivers. The area of sand bars exposed at low d ischarge in Hells Canyon has decreased 50 percent since dam closure, a nd most of the erosion occurred in the first nine years after dam clos ure. The number and size of sand bars in Grand Canyon downstream from Glen Canyon Dam have decreased much less; the number of sand bars decr eased by 40 percent in some 8.3-km reaches, but by less than 20 percen t elsewhere. These differences are in part related to the fact that fl ood regulation is much greater in Grand Canyon than in Hells Canyon, a nd that downstream tributaries resupply sediment to Grand Canyon but n ot to most of Hells Canyon.