Dam its Cold: Spatial and Temporal Changes in Fish Occurrence in the Grand Canyon

Authors: Crosby Hedden, David Rogowski, Jan Boyer, Lauren Mason-Sarantopulos

Year: 2021

Abstract

In the arid American Southwest, the rivers are regulated to manage water required to sustain agricultural, municipal, and industrial needs. A consequence of this intensive management is the largescale alteration to the riverine habitats in the region. Glen Canyon Dam on the Colorado River has lowered mean temperature of the river, reduced annual variability in water temperature, decreased turbidity, hindered sediment transport, and caused a deviation from the natural flow regime. Initially the effects of the dam resulted in largescale declines in the native fish assemblage within the Grand Canyon and caused spatial structuring of fish distributions due to the artificial temperature regime. In recent years, widespread drought throughout the Southwest has led to reservoir level declines, exposing riverine habitats in areas that were previously inundated by reservoirs. Water levels on Lake Mead have declined by over 40 meters since 2000, exposing over 80 km of newly emerged riverine habitat in the western Grand Canyon that possesses higher temperatures and increased turbidity relative to upstream portions of the Grand Canyon. This newly emerged riverine habitat has resulted in an increase in native fish abundance. Our objective was to observe how the probability of fish occurrence has changed within the Grand Canyon as abiotic conditions have changed in relation to the declining reservoir levels and emergence of previously unavailable habitat. We used 21 years of longterm monitoring data on the mainstem of the Colorado River in Grand Canyon collected by the Arizona Game & Fish Department to map the probability of occurrence based upon modelled water temperature for native and common nonnative fish species through space (river kilometer) and in relation to declining water levels in Lake Mead. Our results indicate that as reservoir levels declined and driver habitat re-emerged in the Western Grand Canyon, the probability of native fish occurrence increased while nonnative occurrence decreased longitudinally in the river. Information on past distribution of fish prior to reservoir declines, and how the community has responded to these changes can be a useful tool for future management of native and nonnative fish in highly regulated river systems.