Geomorphic controls on hyporheic exchange

Abstract

Water moves continuously between rivers and the gravel aquifers beneath their floodplains, sustaining cold-water refuges, aquatic life, and water quality. Stream restoration projects reshape channels and floodplains to recover this subsurface exchange, but practitioners cannot predict how specific design choices change it across the range of flows that matter ecologically. This dissertation closes that gap through three studies at Meacham Creek, a salmon-bearing tributary in northeastern Oregon. The first study evaluates a temperature-based analytical method for characterizing the gravel aquifers beneath floodplains. Heat-transport simulations show that the method yields reliable estimates in a "proximal zone" near the channel. Farther away, heat exchanges with the atmosphere and bedrock distort the temperature signal and degrade the estimates. The second study uses a three-dimensional hydrogeologic model to quantify how restoration changes subsurface exchange across baseflow, bankfull, and flood conditions. The framework separates total exchange into two parts: the wetted area available for exchange, and the rate at which water moves through each unit of that area. The third study attributes the restoration effect to three geomorphic outcomes: channel and floodplain area, channel complexity, and alluvial depth. Expanding wetted area produces the largest gains at most flows. Channel complexity dominates at intermediate flows and fades during floods. Alluvial depth contributes modestly and is fixed by site geology. Together, these studies form a connected toolkit for measuring, quantifying, and attributing restoration's effects on subsurface exchange. The framework lets practitioners ground restoration design in a physical accounting of which choices yield the largest subsurface returns at the flows most relevant to a project's ecological goals.