Evaluation of extremal hypotheses in an undeveloped alluvial river

Abstract

Recent work in the undeveloped Rio Murta Basin, located in Chilean Patagonia, identified an evolutionary trend in the fluvial system as it progresses toward and away from dynamic equilibrium. A location-for-time-substitution model employed over the longitudinal extent of a 16 km study site assessed the performance of extremal hypotheses in identifying dynamic equilibrium conditions. Numerous extremal hypotheses were successful in identifying the spatial trend, but no means were available to discern differences between them. Thus, this study uses field measurements within the evolutionary trend to propose a new metric for evaluating extremal hypotheses. A thorough review and synthesis of the extremal approach are additionally presented. The new method compares theoretical predictions against field-measured values to determine which extremal hypothesis is most effective in identifying the condition of dynamic equilibrium in a gravel-bed river. Channel width and depth are identified as the dependent stream variables that uniquely differentiate most extremal hypotheses from one another. The results indicate that extremal hypotheses based on energy metrics of the flow are most successful, with the strongest support for minimum kinetic energy and minimum specific stream power.

Channel Constriction Predicts Pool‐Riffle Velocity Reversals Across Landscapes

“…Even in bedrock‐influenced rivers, channel constriction is associated with pool location (Wohl & Legleiter, 2003). Beyond a specific channel maintenance mechanism, width variability has been linked to extremal hypotheses that have been proposed as the foundational mechanisms of dynamic equilibrium in alluvial rivers (Tranmer et al., 2019). Given that a velocity reversal is only one proposed mechanism for pool‐riffle maintenance, realization of extremal hypotheses of dynamic equilibrium is likely to be achieved by different processes (e.g., jet flow, turbulence, and flow convergence routing) at different locations along a river because of the numerous external forcings to which rivers are subjected.…”