Quantifying geomorphic and vegetation change at sandbar campsites in response to flow regulation and controlled floods, Grand Canyon National Park, Arizona

Abstract

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 due to reductions of sediment that replenish sandbars and increases in vegetation cover caused by flow regulation. Campsite area continues to decrease despite the use of controlled floods to rebuild sandbars. We quantify the relative magnitude of factors that contribute to changes in campsite size, such as fluvial deposition and erosion, gullying, and vegetation expansion with analysis of four‐band aerial imagery and digital elevation models. Campsite area declined by 37% between 2002 and 2016 (an average of 161 m 2 per site at long‐term monitoring sites). Two drivers contributed to campsite area change: (a) short‐term gains and losses associated with controlled floods and flood‐deposit erosion and (b) long‐term one‐directional loss of campsite area caused by vegetation encroachment. There was more erosion and slope change at sites in critical reaches—sections of river where campsites are infrequent or in high demand—than in noncritical reaches. Vegetation continues to expand at campsites under flow regulation, particularly in noncritical reaches. 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.