Long‐Term Evolution of Sand Transport Through a River Network: Relative Influences of a Dam Versus Natural Changes in Grain Size From Sand Waves

Abstract

Temporal and spatial nonuniformity in supplies of water and sand in a river network leads to sand transport that is in local disequilibrium with the upstream sand supply. In such river networks, sand is transported downstream as elongating waves in which coupled changes in grain size and transport occur. Depending on the magnitude of each sand‐supplying event and the interval between such events, changes in bed‐sand grain size associated with sand‐wave passage may more strongly regulate sand transport than do changes in water discharge. When sand transport is controlled more by episodic resupply of sand than by discharge, upstream dam construction may exacerbate or mitigate sand‐transport disequilibria, thus leading to complicated and difficult‐to‐predict patterns of deposition and erosion. We analyzed all historical sediment‐transport data and embarked on a 4‐year program of continuous sediment‐transport measurements to describe disequilibrium sand transport in a river network. Results indicate that sand transport in long river segments can evolve over ≥50‐year timescales following rare large sand‐supplying events. These natural changes in sand transport in distal downstream river segments can be larger than those caused by an upstream dam. Because there is no way to know a priori whether sand transport in a river has changed in response to changes in the upstream sand supply, contemporary continuous measurements of sand transport are required for accurate sand loads and budgeting. Analysis of only historical sediment‐transport measurements, as is common in the literature, may lead to incorrect conclusions with respect to current or future sediment‐transport conditions.

Does Channel Narrowing by Floodplain Growth Necessarily Indicate Sediment Surplus? Lessons From Sediment Transport Analyses in the Green and Colorado Rivers, Canyonlands, Utah

“…documented throughout the upper Green River and Yampa River watersheds by Manners et al (2014) and Topping et al (2018) and are similar to those detected in the Colorado and Snake river basins by White et al (2005).…”

“…annual sand and annual silt-and-clay loads (Topping et al, 2018). Annual loads are provided in Text S3 and Table S1.…”

Causes of Variability in Suspended‐Sand Concentration Evaluated Using Measurements in the Colorado River in Grand Canyon

“…This process occurred in 1983, when a factor of 2 decrease in D b was accompanied by a factor of ~30 increase in predicted sand concentration (Topping et al, 2007). Topping et al (2018) describe examples from other rivers where grain size of sand in suspension tracks better with the fine tail of the bed‐sand grain‐size distribution than with D b .…”

Measuring channel planform change from image time series: A generalizable, spatially distributed, probabilistic method for quantifying uncertainty

“…The flow regimes of the Yampa and Little Snake rivers are largely unregulated and dominated by spring snowmelt floods. The mean annual flood at the Deerlodge gauge is 408 m 3 s −1 , and late summer is a time of low discharge (Manners et al ., 2014; Topping et al ., 2018). Both rivers in the study area have wide active channels with many active bars, as well as bars adjacent to the channel that were formed by floods of different magnitudes.…”

“…Both rivers in the study area have wide active channels with many active bars, as well as bars adjacent to the channel that were formed by floods of different magnitudes. The Little Snake River is the primary source of fine sediment to the Yampa River in Yampa Canyon, Dinosaur National Monument (Topping et al ., 2018), and provides a disproportionately large supply of fine sediment relative to the river's contribution of streamflow (Andrews, 1980). We selected this location for our channel change case study because the National Park Service is concerned about the maintenance of valued park resources that might be affected by upstream water development and recognizes the need to distinguish natural patterns of channel change from changes associated with anthropogenic perturbations.…”