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

Analyses of suspended sediment transport provide valuable insight into the role that sediment supply plays in causing geomorphic change. The sediment supply within a river system evolves depending on the discharge, flood frequency and duration, changes in sediment input, and ecohydraulic conditions that modify sediment transport processes. Changes in supply can be evaluated through analyses of coupled changes in suspended sediment concentration and grain size. The concentration of sand in transport in the Green and Colorado Rivers is most strongly controlled by discharge and the bed sand grain size distribution. Since the 1950s, sand loads have decreased in response to declines in peak discharge in the Green River and coarsening of the bed sand in the Colorado River. However, changes in the bed sand grain size distribution are associated with large changes in suspended sand concentration in both rivers; concentration varies by a factor of ~3 in the Green River and a factor of ~8 in the Colorado River, depending on the bed sand grain size distribution. Analyses of hysteresis in suspended sediment measurements show that sediment depletion during annual floods is most strongly controlled by flood duration, with peak discharge being nearly equally important in the Green River. Despite channel narrowing in both rivers, periods of bed sand coarsening and sediment depletion during annual floods indicate that these rivers are not necessarily in sediment surplus. Channel narrowing appears to be strongly controlled by short‐term declines in flood magnitude and the ecohydraulic effects of vegetation and may not be indicative of the long‐term sediment budget. Plain Language Summary: River channels change size and shape in response to changes in the amount of sediment transported downstream. Changes in streamflow and/or the upstream sediment supply are the cause(s) of such changes in sediment transport. The channels of the Green and Colorado Rivers near Canyonlands National Park, Utah, have both narrowed over the last century. We use measurements of suspended sediment transport to investigate how changes in the sediment supply influence sediment transport and channel change. In most cases, the transport of suspended sand is primarily controlled by the discharge of water and secondarily controlled by the bed sediment grain size distribution. Depletion of the upstream sand supply leads to bed sand coarsening and erosion, whereas enrichment of the upstream sand supply leads to bed sand fining and deposition. The sand supply is progressively depleted during annual snowmelt floods on the Green and Colorado Rivers, with greater depletion occurring during longer floods. Larger floods also cause greater depletion of the upstream sand supply in the Green River but are of less importance in the Colorado River. The size and shape of the present‐day river channels may therefore be maintained, and channel narrowing may be limited, if longer‐duration floods occur in the future. Key Points: Sand transport has declined because of reductions in discharge in the Green River and reductions in sand supply in the Colorado RiverSediment depletion occurs during annual floods and is determined by flood duration and magnitude, depending on the riverDespite channel narrowing, periods of bed sand coarsening and sediment depletion do not indicate conditions of sediment surplus [ABSTRACT FROM AUTHOR]