Date of Award:

6-2015

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Advisor/Chair:

John C. Schmidt

Abstract

Suspended-load rivers are subject to rapid geomorphic changes. In particular during the Holocene Epoch, arroyos of the Colorado Plateau experienced several periods of rapid erosion and aggradation. The most recent period of entrenchment occurred around the turn of the 20th century. The mechanisms responsible for the modern period of aggradation that has followed the most recent period of entrenchment have not been well documented. The research presented in this thesis reveals the mechanisms responsible for modern alluviation of the San Rafael River, which drains the Colorado Plateau

The lower 87 km of the San Rafael River, which enters the Green River south of the town of Green River, UT has experienced rapid geomorphic changes during the last 100 years. To quantify these changes, we used a complement of temporally precise and spatially robust methods. By understanding the rates, magnitudes and types of geomorphic changes, we could then identify the mechanisms of these channel changes.

The San Rafael River narrowed by 83% between 1938 and 2009 and the floodplain aggraded 1.0 to 2.5 m. Channel narrowing was caused by a reduction in the transport capacity of the river, and was accelerated by the establishment of vegetation, including the non-native tamarisk shrub, on active channel surfaces and the floodplain. Significant water withdrawals during the 20th century have primarily been responsible for the reduction in transport capacity by decreasing the magnitude and duration of the annual snowmelt flood. During this time period, monsoon floods continued to deliver large quantities of fine sediment to the channel.

During the 20th century, the channel bed incised in one segment and aggraded in five segments. The two periods of incision that we documented were related to human modifications of the channel and floodplain.

With the knowledge of the physical processes that have been responsible for the channel changes in the San Rafael River, prediction of future channel conditions can then be made. The changes to the physical template of the San Rafael River have implications for the management of three endemic fish – the roundtail chub (Gila robusta robusta), the bluehead sucker (Catostomus discobolus), and the flannelmouth sucker (Catostomus latipinnis) – which currently utilize the study area.

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