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.

Associations between riparian plant morphological guilds and fluvial sediment dynamics along the regulated Colorado River in Grand Canyon

“…Vegetation expansion has been greatest on the portions of the riparian zone that are never inundated or inundated infrequently since dam closure (Sankey et al, 2015). These sandbars are an important resource in the managed river system because they are used heavily by river recreationists (Hadley, Grams, & Kaplinski, 2018) and add to shoreline complexity that forms aquatic habitats used by native fish (Dodrill et al, 2015). 226.5 and 707.9 m 3 /s.…”

Geomorphic response to historic and ongoing human impacts in a large lowland river

“…Point bars became much larger over the decades of the Navigation Project, despite the presence of dams reducing sediment supply, which typically causes bars to decrease in area (e.g. Hadley et al, 2018; Pavlek & Faivre, 2020). River bar area reflects hydrologic changes (Hazel et al, 2010; Wiele et al, 1996), a legacy of landscape disturbance, or historical/modern human activities (Jacobson & Gran, 1999; Martin & Pavlowsky, 2011; Mossa & Marks, 2011).…”

Hydrologic and geomorphic effects on riparian plant species occurrence and encroachment: Remote sensing of 360 km of the Colorado River in Grand Canyon

“…Vegetation encroachment within hydrologic zone 1 sandbars is a resource management concern, because these features create backwaters that support native fishes (Dodrill et al, 2014) and are particularly susceptible to encroachment and channel narrowing (Butterfield et al, 2020; Sankey et al, 2015a). Additionally, sandbars are the most important recreational camping sites along the river (Hadley et al, 2018), and they also serve as a source for aeolian sand that protects archaeological features throughout the river corridor (Cook et al, 2019; Sankey, Caster, et al, 2018; Sankey, Kasprak, et al, 2018).…”

“…Sandbar habitats are of particular interest because they are an important cultural and biological resource in Grand Canyon. Sandbars provide valuable recreation and camping locations for the approximately 25,000 river runners and backpackers who visit the Colorado River in Grand Canyon each year (Hadley et al, 2018; National Park Service, 2017). Backwaters and eddies created by sandbars provide habitat for native endangered fish such as humpback chub ( Gila cypha ) (Dodrill et al, 2014).…”