Daniel Scott, Ellen Wohl

Organic carbon (OC) in valley bottom downed wood and soil that cycles over short to moderate timescales represents a large, dynamic, and poorly quantified pool of carbon whose distribution and residence time affects global climate. We compare four disparate mountain river basins to show that mountain river valley bottoms store substantial estimated OC stocks in floodplain soil and downed wood. Although soil OC is generally young, geomorphic processes regulate soil burial and processes that limit microbial respiration, preserving aged OC in certain parts of the river network. Statistical modeling of OC stocks suggests that biogeomorphic processes and the legacy of past erosion regulate the modern distribution of OC in river networks. Our results suggest that although mountain rivers may accumulate large OC stocks relatively rapidly, those stocks are highly sensitive to alterations in soil and wood retention, implying both short- and long-term feedbacks between retentiveness and the distribution of OC between the land and atmosphere.