Gradients in stream power influence log jam locations

Log jams are integral to river systems, yet deep uncertainties remain about the processes that control their formation. We tested whether downstream gradients in unit stream power exert control over where and how natural log jams form. Using data from 680 natural log jams in New Hampshire, Vermont and Colorado, we found that jams are more prevalent where unit stream power increases in the downstream direction in watersheds smaller than a critical drainage area of 29.5 km² (95 % CI: 11.3–62.0 km²), but more prevalent where unit stream power decreases in the downstream direction in larger watersheds. The critical drainage area was not statistically different between the Northeast and Colorado and at-a-point unit stream power did not have a consistent relationship with jam presence. We interpret these results as demonstrating a transition in dominant jam-forming mechanism from (1) jams preferentially formed by the recruitment of trees by erosional processes on reaches of increasing unit stream power at smaller drainage areas to (2) jams preferentially formed by depositional processes on reaches of decreasing unit stream power at larger drainage areas. Tree dimensions relative to channel dimensions likely influence the critical drainage area at which this transition occurs. This study demonstrates that stream power gradients reflect a physical process that contributes to natural log jam formation and can guide river management.