EFFECTS OF GLEN CANYON DAM ON COLORADO RIVER SAND DEPOSITS USED AS CAMPSITES IN GRAND-CANYON NATIONAL-PARK, USA

Glen Canyon Dam, located on the Colorado River 24 km upstream from Gra nd Canyon National Park, has affected downstream alluvial sand deposit s which are used as campsites by recreational boaters. Inventories of campsite numbers and sizes conducted in 1973, 1983 and 1991, and compa rison of aerial photograph series taken in 1965, 1973, 1984 and 1990 s how that there has been a system-wide decrease in the number and size of campsites. Campsites are unevenly distributed along the river, and availability is regarded as 'critical' along reaches comprising 45% of the river, based on interviews with river guides. During the first 10 years of Glen Canyon Dam operations, at least 30% of all campsites de creased in size. During the next 18 years, between 1973 and 1991, 32% of all campsites decreased in size, and campsite capacity decreased by 44%. High annual dam releases in excess of power plant capacity in 19 83 caused a net system-wide increase in the number of campsites, but d ecreased campsite capacity in two critical reaches. The 'benefit' of s and aggradation due to the 1983 high flow was short-lived, and by 1991 only a few campsites were larger than they had been in 1973. In contr ast, other sites, especially in critical reaches, were eroded by the 1 983 high flows and have not recovered in size. Options for future dam management must consider the variable response of campsites to high fl ows in critical and non-critical reaches and the duration over which ' beneficial' high flow effects persist.