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A Century of Geomorphic Change of the San Rafael River and Implications for River Rehabilitation

<p> Beginning in the early 20th century and continuing into the 21st century, the lower 87 km of the San Rafael River in central Utah underwent rapid geomorphic changes. Extensive water development in the headwaters, invasion of the non-native tamarisk shrub, and man-made perturbations to the channel-floodplain system have been responsible for the changes that we documented in this study. We used a combination of spatially robust and temporally precise methods to reconstruct the modern history of channel change and identify the processes responsible for those changes. These methods include analysis of historic aerial photographs, analysis of USGS gage data, dendrogeomorphic analysis of floodplain stratigraphy, and comparison of historic and modern longitudinal profiles. </p><p> The San Rafael River changed from a wide, shallow, heterogeneous channel to a narrow, deep, homogeneous channel. Specifically, between 1938 and 2009, the San Rafael River along the length of the entire study area narrowed 83%. Additionally, the floodplain vertically accreted between 1.0 and 2.5 m. The majority of the channel narrowing occurred during two distinct time periods - 1952 to 1979 and 1987 to the present - when low, mean annual stream flow was low. Channel narrowing is primarily due to the reduction in transport capacity, but when coupled with tamarisk establishment, channel narrowing and floodplain aggradation has been rapid. </p><p> We documented the spatial extent of channel bed changes over the course of the 20th century. We found that the channel bed aggraded in five segments, lowered in one segment, and remained the same in the other portions of the study area. Analysis of historic, precise measurements of bed elevation at the USGS gage 09328500 revealed that the channel bed incised between Hatt&rsquo;s Ranch and MacMillan Lower Ranch Dam during two time periods: from 1952-1965 and 1983 to the present. Both episodes of incision were caused by unequal amounts of scour and fill. This imbalance in bed fluctuation was induced by human modification of the channel during the first episode and lowering of the local base control during the second episode of incision. </p><p> The changes to the physical template of the San Rafael River have implications for the management of three endemic fish &ndash; the roundtail chub (Gila robusta robusta), the bluehead sucker (<i>Catostomus discobolus</i>), and the flannelmouth sucker (<i>Catostomus Latipinnis</i>) &ndash; which currently utilize the study area. Future management of the river will benefit from the results of our study, which reveal the physical processes that are responsible for the historic and current condition of the river. </p>
Date07 July 2015
CreatorsFortney, Stephen T.
PublisherUtah State University
Detected LanguageEnglish

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