Rapid spontaneous restoration of Glen Canyon ecosystems as Lake Powell dries

Seth Arens1*

1Seth Arens, Western Water Assessment, University of Colorado – CIRES, University of Utah - GCSC

Decades-long drought intensified by climate change significantly reduced Colorado River streamflow and caused severe declines in the amount of water stored in Lake Powell, the nation’s second-largest reservoir. As Lake Powell reached a record low elevation (3,519 feet and 181 feet below full capacity) and storage capacity (24%) by April 2023, media attention and resource management planning related to the Colorado River crisis focused, understandably, on water supply. A poorly understood impact of low water in Lake Powell is the fate of the over 100,000 acres of land that was uncovered by the drying reservoir. Terrestrial desert ecosystems are now establishing on landscapes that have been out of water for up to 24 years. Glen Canyon, the canyon along the mainstem of the Colorado River that comprises the majority of Lake Powell, is home to over 100 tributaries, many of which have perennially-flowing water. In 2022, a four-year study began to assess how ecosystems and plants are re-establishing on previously inundated Glen Canyon landscapes and how these ecosystems change over short time periods. In spring 2022 and 2023, 74 plant survey sites were established in 20 locations within Glen Canyon and along the Escalante River. The majority of sites were established in tributary canyons with perennially or intermittently flowing creeks, but 12 sites were established in drying bays of Lake Powell. Preliminary results from the surveys showed that native shrubs (Salix exigua, Baccharis salicifolia and B. salicina), grasses (Vulpia octoflora, Juncus arcticus) and forbs (Veronica americana, Artemisia ludoviciana) are establishing rapidly and are generally out-competing non-native plants (such as Salsola tragus, Tamarix ramosissma), especially on landscapes that emerged from the reservoir more than 3 years ago. Despite the proliferation of native plants on newly exposed landscapes, non-native plants are present at sites above and below 3,700 feet (the elevation of Lake Powell when full) and plant survey sites below 3,700 feet (areas once inundated by Lake Powell) have significantly greater cover of non-native plants compared to sites above 3,700 feet. The three most common non-native plants were Salsola tragus, Tamarix ramosissma and Bromus tectorum. Salsola tragus is extremely abundant on sites that recently emerged from Lake Powell, but its abundance appears to rapidly decline on landscapes that have been exposed for more than three years. Tamarix ramosissma, an extremely common non-native shrub in the Colorado River Basin, is present in very low abundance along riparian corridors of tributaries to Glen Canyon; most of the Tamarix stands observed established during times when Lake Powell was much higher and are now stranded from water sources and severely drought stressed or dead. Riparian forests with Populous fremontii, Salix gooddingii and occasionally Acer negundo are establishing on landscapes within 3-5 years of emergence from Lake Powell and mature Populus fremontii trees up to 50 feet tall and 16 inches in diameter are present at many locations that have been out of water for 20 years or more. Unique hanging garden ecosystems and cryptobiotic crust, typically considered slow to re-establish on disturbed landscapes, are present at some locations within 5 years of a landscape’s emergence from Lake Powell.