A Historical Approach for Identifying Habitat Maintenance Flows, with Application to the Lower Duchesne River, Utah
Category: Watershed Science
Author(s): David A. Gaeuman
Author(s): David A. Gaeuman
Description: Channel migration, floodplain renewal, and fluvial disturbance are critical to the structure and functioning of many riverine ecosystems. Few existing channel- or habitat maintenance flow recommendations consider these dynamic processes. This dissertation presents a new strategy for determining flow recommendations that incorporates an analysis of historical channel change. The resulting recommendations consider large scale landscape elements and long-term or episodic processes. Geomorphic history was reconstructed by integrating an analysis of historical aerial photographs with stratigraphic, hydraulic, and hydrologic studies. Measurement of channel change generated by spatial overlay of independent air photo maps in a geographic information system is a fundamental component of the analysis. Because previous studies have neglected the planimetric errors generated by spatial overlay, it was necessary to develop a method for estimating the magnitudes of these errors and appropriate uncertainty margins. Channel responses to flow depletions in the study area over the past 100 years have been variable in both space and time. In general, gravel-bed reaches responded to perturbations with width adjustments, whereas sand-bed reaches responded with bed level adjustments. Stream flow reductions resulted in channel narrowing in gravel-bed reaches. The same reductions in stream flow caused bed aggradation and avulsions in sand-bed reaches. The effect of the avulsions varied depending on local valley slope. In low gradient reaches, avulsions created a nearly straight, stable channel with a low width to-depth ratio. An avulsion in a higher-gradient part of the valley caused a channel transformation, in which a sand-bed reach with a relatively low width-to-depth ratio was converted first to a wide braided gravel-bed reach, then to a meandering gravel-bed reach. Later, increases in flood magnitudes and durations caused bed incision in sand bed reaches, whereas gravel-bed reaches widened. Morphology-based gravel budgets coupled with a gravel routing procedure indicated that channel widening in the gravel bed reaches was accompanied by bed aggradation. Bank erosion transferred gravel stored in the banks and floodplains to the channel bed, and the resulting aggradation promoted continued bank erosion. This feedback between bank erosion and bed aggradation persisted until relatively large floods capable of re-incising the bed occurred. More recently, a reduction in the frequency of moderate floods has resulted in channel narrowing, even though larger floods have continued to occur with approximately their historical frequency.