A Hierarchical Modeling Approach to Simulating the Geomorphic Response of River Systems to Climate Change

Abstract:

Anthropogenic climate change is expected to change the discharge and sediment-transport regime of river systems. Because rivers adjust their channels to accommodate their typical inputs of water and sediment, changes in these variables can potentially alter river morphology. Here, I developed and applied a hierarchical modeling approach to examine potential changes in reach-averaged bedload transport and spatial patterns of erosion and deposition for three snowmelt-dominated gravel-bed rivers in the interior Pacific Northwest (the Tucannon River in southeastern Washington and the South Fork Coeur d’Alene and Red rivers in Idaho). The modeling hierarchy was based on discharge and suspended-sediment load from a basin-scale hydrologic model driven by a range of downscaled climate-change scenarios. In the field, I collected channel morpholohy and sediment grain-size data for all three rivers. To estimate changes in reach-averaged bedload transport, I used the Bedload Assessment of Gravel-bedded Streams (BAGS) software. I then used the Cellular Automaton Evolutionary Slope and River (CAESAR) model to simulate the spatial pattern of erosion and deposition within each reach to infer potential changes in channel geometry and planform. Results from the BAGS sediment-transport formulas indicate that changes in the duration of the critical discharge needed to mobilize bed sediments are the primary drivers of changes in reach-averaged sediment transport. CAESAR modeling results include changes in river morphology for the two higher-energy river reaches, but no significant morphological changes for a lower-energy reach with steep, cohesive banks, suggesting that the geomorphic response of river systems to climate change may depend on how reach characteristics affect a river’s relative stability or mobility. Changes in sediment transport and river morphology resulting from climate change could affect the management of river systems for human and ecological uses.