Event Title

Maintaining Gravel Habitat in a Sand-Dominated River: A Field Experiment

Presenter Information

Samuel Lyster
Peter Wilcock
John Schmidt

Location

USU Eccles Conference Center

Event Website

http://water.usu.edu

Start Date

4-5-2016 5:09 PM

End Date

4-5-2016 5:12 PM

Description

Gravel is an essential component of fish habitat, providing spawning and rearing habitat and favorable conditions for benthic production. When alterations in the supply of water and sediment to a river cause accumulation of fine sediment in the river bed, habitat for fish is degraded. If reductions in the supply of fine sediment are possible, along with adequate high flows to mobilize the bed, reduction in fine sediment content and recovery of gravel habitat can be achieved. Such changes in watershed conditions are often not immediately possible and the potential for habitat restoration depends on the extent and mobility of exposed gravel and the conditions needed to preserve or increase it. We report on a restoration experiment that includes the addition and tracking of gravel to a river that has become predominantly sand-bedded. Our goal is to learn the conditions under which gravel is transported and the fate of the transported gravel. With active gravel transport, the prospect exists that exposed gravel may persist. With marginal transport and general burial of gravel, exposure of the gravel resource depends only on conditions of sand supply and deposition. We report on a restoration experiment on the San Rafael River (SRR), a large (6200 km2) right-bank tributary to the Green River in southcentral Utah. Stream-flow diversion in the upper watershed has reduced floods and base flows, while monsoon precipitation in the lower watershed continues to deliver sand from ephemeral tributaries. Flood reduction and persistent sand supply has led to channel narrowing and simplification. A gravel bed is present along much of the river channel in the lower watershed, but the gravel is largely covered by a mantle of sand. Exposed gravel is found in occasional riffles and near the mouths of tributaries that have a small gravel component in their supply. Two gravel bars were constructed in May 2015 with the goal of exploring whether gravel augmentation might trigger bed mobilization, bar migration, and increased channel complexity. Immediately after the gravel bars were placed, a series of three short-duration floods (each with a magnitude close to the two-year flood) produced (i) scour of 30% and 80% of the bars with negligible bar migration, (ii) short-term bed scour adjacent to the bars, and (iii) negligible bank erosion. The bars were constructed with 120 Mg of exotic gravel imported from outside the watershed. Use of such a large quantity of tracer gravel allows the fate of the transported gravel to be evaluated using systematic sampling of the downstream river channel. Transport of larger gravel sizes eroded from the bars was limited to about one or two channel widths; finer gravels eroded from the bar travelled as far as 700 m (~70 stream widths). We will report on the gravel travel distance and fate in the context of an independent estimate of the natural supply of gravel to the system in order to evaluate whether the gravel resource and habitat is likely to persist under a flow regime dominated by water diversion.

Comments

A poster by Samuel Lyster, who is with Utah State University, Watershed Sciences

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Apr 5th, 5:09 PM Apr 5th, 5:12 PM

Maintaining Gravel Habitat in a Sand-Dominated River: A Field Experiment

USU Eccles Conference Center

Gravel is an essential component of fish habitat, providing spawning and rearing habitat and favorable conditions for benthic production. When alterations in the supply of water and sediment to a river cause accumulation of fine sediment in the river bed, habitat for fish is degraded. If reductions in the supply of fine sediment are possible, along with adequate high flows to mobilize the bed, reduction in fine sediment content and recovery of gravel habitat can be achieved. Such changes in watershed conditions are often not immediately possible and the potential for habitat restoration depends on the extent and mobility of exposed gravel and the conditions needed to preserve or increase it. We report on a restoration experiment that includes the addition and tracking of gravel to a river that has become predominantly sand-bedded. Our goal is to learn the conditions under which gravel is transported and the fate of the transported gravel. With active gravel transport, the prospect exists that exposed gravel may persist. With marginal transport and general burial of gravel, exposure of the gravel resource depends only on conditions of sand supply and deposition. We report on a restoration experiment on the San Rafael River (SRR), a large (6200 km2) right-bank tributary to the Green River in southcentral Utah. Stream-flow diversion in the upper watershed has reduced floods and base flows, while monsoon precipitation in the lower watershed continues to deliver sand from ephemeral tributaries. Flood reduction and persistent sand supply has led to channel narrowing and simplification. A gravel bed is present along much of the river channel in the lower watershed, but the gravel is largely covered by a mantle of sand. Exposed gravel is found in occasional riffles and near the mouths of tributaries that have a small gravel component in their supply. Two gravel bars were constructed in May 2015 with the goal of exploring whether gravel augmentation might trigger bed mobilization, bar migration, and increased channel complexity. Immediately after the gravel bars were placed, a series of three short-duration floods (each with a magnitude close to the two-year flood) produced (i) scour of 30% and 80% of the bars with negligible bar migration, (ii) short-term bed scour adjacent to the bars, and (iii) negligible bank erosion. The bars were constructed with 120 Mg of exotic gravel imported from outside the watershed. Use of such a large quantity of tracer gravel allows the fate of the transported gravel to be evaluated using systematic sampling of the downstream river channel. Transport of larger gravel sizes eroded from the bars was limited to about one or two channel widths; finer gravels eroded from the bar travelled as far as 700 m (~70 stream widths). We will report on the gravel travel distance and fate in the context of an independent estimate of the natural supply of gravel to the system in order to evaluate whether the gravel resource and habitat is likely to persist under a flow regime dominated by water diversion.

https://digitalcommons.usu.edu/runoff/2016/2016Posters/14