Using Gradients in Stream Power to Predict Log Jam Distribution and Characteristics

Log jams enhance the ecological and geomorphic resilience of a river by promoting floodplain development, increasing habitat heterogeneity and impeding the flow of water and sediment during floods. Wood additions and engineered log jams have become a common tool in restoration, but these efforts can be inefficient due to a lack of ability to predict where log jams commonly occur or how they interact with stream-longitudinal processes such as sediment transport. This study integrates understandings of sediment and wood transport to improve predictions of log jam distribution and geomorphic impact. We test whether downstream gradients in stream power, which are associated with the erosion and deposition or sediment, also influence log jam distribution and geomorphic impact. We hypothesize that in narrow streams, downstream increases in stream power are associated with wood recruited from eroding banks that forms log jams. Data from surveys of over 300 log jams on seven streams in Vermont show that downstream changes in stream power do influence the distribution and geomorphic effects of log jams. Downstream increases in stream power are disproportionately associated with log jams, particularly those formed by fallen streamside trees. Downstream decreases in stream power are associated primarily with log jams formed by transported wood. These findings demonstrate the importance of considering wood and sediment transport in tandem.