Erich R. Mueller

Sediment supply and channel morphology in mountain river systems: 2. Single thread to braided transitions

Differences in sediment supply between single-thread and braided channel types provide a long-recognized, though difficult to quantify, pattern discrimination. Building on the results from our preceding paper, we present a multiscale assessment of the sediment supply, geomorphology, and sediment transport characteristics of braided, gravel-bedded reaches in the northern Rocky Mountains, USA. First, we present a quantitative, theoretically based discriminant function that stratifies single-thread and braided reaches on the basis of variations in bankfull sediment concentration and dimensionless discharge following the work of Millar (2005) and Eaton et al. (2010). This function correctly classifies 50 of the 53 channel types where bed load concentrations are known. Second, while channel pattern transitions are often linked to changes in slope, field studies along Sunlight Creek, Wyoming, show that downstream transitions between single-thread and braided reaches are instead caused by valley constrictions and changes in the grain size of sediment from tributaries. Finally, we demonstrate that the two-dimensional variability in flow properties in braided reaches produces locally high values of shear stress and bed load transport. Yet bed load measurements and sediment transport calculations also show that sediment transport rates between adjoining braided and single-thread reaches may be approximately equal where channels are near the pattern threshold and downstream variations in bank fortitude and channel constriction force pattern transitions. Taken together, these results indicate that high bed load concentrations are fundamental to the braided channel planform and that braided channels likely reflect a quasi-equilibrium state within watersheds with persistent high sediment supply.

Climate, wildfire, and erosion ensemble foretells more sediment in western USA watersheds

The area burned annually by wildfires is expected to increase worldwide due to climate change. Burned areas increase soil erosion rates within watersheds, which can increase sedimentation in downstream rivers and reservoirs. However, which watersheds will be impacted by future wildfires is largely unknown. Using an ensemble of climate, fire, and erosion models, we show that postfire sedimentation is projected to increase for nearly nine tenths of watersheds by >10% and for more than one third of watersheds by >100% by the 2041 to 2050 decade in the western USA. The projected increases are statistically significant for more than eight tenths of the watersheds. In the western USA, many human communities rely on water from rivers and reservoirs that originates in watersheds where sedimentation is projected to increase. Increased sedimentation could negatively impact water supply and quality for some communities, in addition to affecting stream channel stability and aquatic ecosystems.

Durable terrestrial bedrock predicts submarine canyon formation

Though submarine canyons are first order topographic features of Earth, the processes responsible for their occurrence remains poorly understood. Potentially analogous studies of terrestrial rivers show that the flux and caliber of transported bedload are significant controls on bedrock incision. Here we hypothesize that coarse sediment load could exert a similar role in the formation of submarine canyons. We conducted a comprehensive empirical analysis of canyon occurrence along the west coast of the contiguous United States which indicates that submarine canyon occurrence is best predicted by the occurrence of durable crystalline bedrock in adjacent terrestrial catchments. Canyon occurrence is also predicted by the flux of bed sediment to shore from terrestrial streams. Surprisingly, no significant correlation was observed between canyon occurrence and the slope or width of the continental shelf. These findings suggest that canyon incision is promoted by a greater yields of durable terrestrial clasts to the shore.