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Growing calls for the use of natural materials and processes to meet management goals have positioned artificial log jams as a compelling alternative to hard engineering instream and floodplain habitat. Deep uncertainties remain, however, about where and how wood should be placed to best mimic natural river processes. In this study, I test whether at-a-point or downstream gradients in unit stream power, an estimate of a river’s ability to do work, exert control over where and how log jams form. Using field observations of 360 log jams in New Hampshire and Vermont and an additional 320 previously published locations of log jams in Colorado, I find that jams are more prevalent where unit stream power increases in the downstream direction in small watersheds but more prevalent where unit stream power decreases in the downstream direction in larger watersheds. At-a-point unit stream power did not have a consistent relationship with jam presence. The disproportionate number of jams on reaches of increasing unit stream power in small watersheds indicates preferential recruitment on those reaches, likely from erosional processes. At larger drainage areas, jams on reaches of decreasing stream power are more likely to be structured around a key log that is disconnected from the bank and are commonly situated on gravel bars and floodplain margins or trapped on roughness elements. I interpret these results as demonstrating a transition in dominant jam-forming mechanism from (1) jams formed by the recruitment of trees by erosional processes on reaches of increasing unit stream power at smaller drainage areas to (2) jams formed by depositional processes on reaches of decreasing unit stream power at larger drainage areas. This study demonstrates that stream power gradients reflect a physical process that contributes to log jam formation and can help guide engineered approaches to river restoration.
Malakoff, Eliza H., "Downstream gradients in unit stream power influence log jam location and process domain" (2023). Dartmouth College Master’s Theses. 108.