Use of Riparian-Geomorphic Relationships to Evaluate Flows Necessary for Riparian Restoration: A Case Study of the Diamond Fork River, Utah

Diamond Fork River, Utah has been used since 1915 to convey irrigation water from the Colorado River basin to areas along the Wasatch Front. These flows have increased summer base flows more than 10 times above natural levels and led to dramatic changes in channel and floodplain morphology. In 1939, Diamond Fork was a meandering, single-thread channel averaging 12 m in width. By 1952, the lower 6 km of the channel was braided, occupying the entire 150 m-wide valley floor. Concurrent with changes in floodplain morphology, there has been a 50 to 90 per cent reduction in the extent of riparian cottonwood forest. The remaining forest is dominated by late mature cottonwoods, with little survival into premature (2–7 yr) and early mature (8–30 yr) age classes. In contrast with previous patterns of establishment that were associated with frequent flood events, current cottonwood recruitment is limited to 50–100 year flood events. We are using analysis of a time series of aerial photographs, reconstruction of hydrologic data, and field studies of riparian vegetation structure and channel form to evaluate and recommend restoration flow regimes for Diamond Fork. The restoration objective is to provide an improved fishery, but the initially proposed flow regime did not address riparian needs, as it would have eliminated all potential for natural cottonwood establishment, or channel stability, as it did not consider flow duration or sediment transport. Recommended flow regimes need to consider the relation between hydrologic change and geomorphic process as well as the processes contributing to riparian establishment and maintenance.