INTEGRATING HIGH-RESOLUTION VEGETATION DATA AND 14 YEARS OF TOPOGRAPHIC SURVEYS TO QUANTIFY IMPACTS TO SANDBAR CAMPSITES IN GRAND CANYON NATIONAL PARK

Sandbars along the Colorado River in Grand Canyon National Park, USA are an important recreational resource used as campsites by over 25,000 river runners and hikers annually. The number and size of campsites decreased following the completion of Glen Canyon Dam in 1963 caused by reductions of sediment supply, vegetation encroachment, and streamflow regulation. Controlled floods conducted since 1996 are currently the primary strategy to rebuild sandbars and improve campsites. Using high-resolution (0.20 m) 4-band aerial imagery along 360 km of river corridor, 1-m DEMs of sandbars derived from total station surveys, and 14 years of campsite surveys, we quantify the relative magnitude of factors that contribute to changes in campsite size. These factors include fluvial deposition and erosion, gullying from hillslope runoff, and vegetation expansion. Campsite area declined by 37 percent between 2002 and 2016 (an average of 161 m² per site at long-term monitoring sites). Two drivers contributed to campsite area change: (1) short-term gains and losses associated with controlled floods and flood-deposit erosion and (2) long-term one-directional loss caused by vegetation encroachment. Although controlled floods have contributed to short-term increases in sandbar size, long-term increases in campsite area have not occurred because of sandbar erosion between controlled floods and vegetation expansion. Manual vegetation removal may need to be considered in future management strategies.