Date of Award:

12-2022

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Phaedra Budy

Committee

Phaedra Budy

Committee

David Speas

Committee

William Macfarlane

Abstract

Rivers comprise some of the most unique and biodiverse ecosystems on the planet with their waters supporting both human societies as well as the organisms that make these rivers their home. Large rivers like the Colorado are often highly regulated and diverted in order to support human residence in arid regions like the desert Southwest, and these water diversions often have dramatic, negative impacts on the natural flow regime of the river. These impacts leave large reaches of the river dry, reduce the river’s capacity to transport sediment, cause channel and habitat homogenization, and significantly reduce the amount of suitable habitat available to aquatic organisms. A reduction in critical habitat has, in particular, led to the population reduction, localized extinction and federal listing of many native fish species. The San Rafael River in southcentral Utah is a highly-degraded tributary of the Upper Colorado River Basin, and has historically provided critical spawning, rearing and resting habitat for native and endemic fish species. A 2010 flooding event caused an otherwise narrow, homogenous and degraded reach of the San Rafael to change into a wide, diverse, multi-threaded river system and wetland complex. We used drone-collected and historic imagery to collect data on how this event has impacted habitat complexity and resilience. We also tracked the movements and habitat usage of tagged, critically endangered bonytail (Gila elegans). Our findings have shown that the wash reach of river has become substantially more diverse than any other reach along the lower 64 km of the San Rafael River. Additionally, the wash reach of river has provided resilience in the face of seasonal drying, retaining water for periods longer than expected during summer months. Our findings also suggest native, endangered bonytail prefer and favorably utilize the complex habitat that is a direct result of the affected reach of river. We conclude that process-based desert river restoration is crucial to the persistence of many unique, endemic and native Southwestern fishes, and that large-scale disturbances like the one that has occurred in the San Rafael may be a key method for restoring these highly-degraded desert tributary systems.

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