Disturbance and valley confinement: Controls on floodplain large wood and organic matter jam deposition in the Colorado Front Range, USA

Abstract

AbstractOrganic matter dynamics (entrainment, transport, deposition) can influence ecogeomorphic complexity in rivers. However, the depositional controls on downed large wood (LW) and coarse particulate organic matter (CPOM) on floodplains have rarely been assessed. We investigate the influence of different disturbance histories, as well as geomorphology and forest stand density, on the deposition of LW and CPOM accumulations, or jams, between two adjacent drainages (West Creek, North Fork Big Thompson) in the semi‐arid Colorado Front Range, USA. Both basins recently experienced an extreme flood, but West Creek experienced a larger flood peak as well as a recent upstream fire. We also analyse jam fabric (LW piece orientation) between floodplain and in‐channel jam types. We measured LW and CPOM jams on the North Fork to compute jam frequencies (jams ha−1) and loads (volume per area; m3 ha−1), and we compare these data to a previously published dataset from West Creek. Average floodplain LW jam frequencies (70.4 jams ha−1) and loads (678.6 m3 ha−1) on West Creek were significantly higher than on the North Fork (frequency: 14.8 jams ha−1; load: 94.3 m3 ha−1). Average floodplain CPOM jam frequencies (West Creek: 19.0 jams ha−1; North Fork: 15.1 jams ha−1) and loads (West Creek: 10.70 m3 ha−1; North Fork: 9.98 m3 ha−1) were not significantly different between basins. A larger flood peak, likely enhanced by a recent upstream fire and a more confined valley bottom, resulted in greater deposition of LW jams on West Creek compared to the North Fork. Geomorphic characteristics, such as valley confinement and slope, also influence jam frequencies and loads. Floodplain jam types oriented parallel to the stream were significantly different than other jam types. This work will inform river restoration efforts using wood and organic matter and provide insight on the transport and depositional processes influencing floodplain LW jam formation.