Indicators of river system hydromorphological character and dynamics: understanding current conditions and guiding sustainable river management

The work leading to this paper received funding from the EU’s FP7 programme under Grant Agreement No. 282656 (REFORM). The Indicators were developed within the context of REFORM deliverable D2.1, therefore all partners involved in this deliverable contributed to some extent to their discussion and development

Evaluation of In-Channel Gravel Storage With Morphology-Based Gravel Budgets Developed from Planimetric Data

Gravel budgets developed from changes in river morphology have emerged as an important tool for exploring stream dynamics and sediment transport. In many cases, old aerial photographs are the only data available by which to evaluate past morphologic changes. Existing methods for building morphology-based gravel budgets from air photos are subject to several sources of uncertainty, including difficulty in identifying deposition within the active channel. In-channel deposition sites consist of submerged bars or general increases in bed elevation, both of which are virtually impossible to detect on air photos. However, bed deposition can be inferred when measured gravel storage losses are greater than the quantity of gravel exiting the study area at its downstream boundary. An integrated method combining measurements of gravel storage changes with a gravel routing procedure based on estimated gravel transport path lengths was developed to identify sites of bed aggradation. Inclusion of storage change results in the routing procedure reduces the uncertainty associated with the selection of appropriate transport path lengths. The method was applied to development of gravel budgets for a 50-year period in the lower Duchesne River, Utah. Areas in which the predicted bed aggradation was greatest displayed higher rates of channel activity and greater channel instability during subsequent time periods

Complex Channel Responses to Changes in Stream Flow and Sediment Supply on the Lower Duchesne River, Utah

Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events. A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply