Impact of In-Channel Geomorphic Structures on Surface-Subsurface Exchange of Water and Heat in Streams

In-channel geomorphic structures such as debris dams and steps are common in undisturbed streams, are often installed as part of stream restoration projects, and can significantly enhance exchange of water across the streambed. Surface-subsurface exchange is also an important cooling mechanism in summer for streams suffering elevated temperatures due to climate change or loss of riparian shade. However, the effects of many basic geomorphic structure characteristics (e.g., size and type) and hydro-geologic boundary conditions on water and heat exchange are poorly known but critical to understanding the impact of channel morphology on stream ecology. We combined surface and groundwater numerical hydraulic and heat flow models with field hydraulic and temperature measurements using a three-dimensional sensor network to quantify these relationships and their implications for stream functioning and restoration design. Results show that while water exchange with the subsurface generally increases linearly with geomorphic structure height, the residence time and size of the induced subsurface flow cell respond nonlinearly. Such hydrologic aspects of exchange also respond nonlinearly to various hydrologic and geologic controls, such as depth to bedrock, groundwater recharge, and channel slope. Finally, thermal effects, such as the degree of heat exchange via hydrodynamic mixing and heat conduction within the induced subsurface flow cell, and the corresponding temperature moderation of water as it returns to the surface, also appear to respond nonlinearly to basic descriptive parameters of in-channel geomorphic structures. These nonlinear trends have significant implications for understanding natural stream function as well as optimizing stream restoration design to maximize functions such as reactive time in the subsurface or reduction of maximum summer stream temperatures.