The Influence of Disturbance and Reach-Scale Geomorphology on Depositional Patterns and Loads of Large Downed Wood in Semi-Arid Rivers, Colorado, USA

Abstract:

Large, downed wood (LW) can increase river corridor (channel and floodplain) physical complexity and provides important ecological functions. For example, LW accumulations, or jams, influence patterns of erosion and deposition during floods, create sites for seedling establishment, and act as erosion-resistant points on floodplains. Floodplain LW poses a promising opportunity for river restoration efforts because of the important ecological benefits and fewer related hazards relative to in-channel LW. However, few studies have quantified depositional patterns and loads of LW on the floodplain. We addressed this gap by measuring LW jam volumes and depositional characteristics on semi-arid mountain rivers in the Colorado Front Range. We surveyed sites in neighboring basins (West Creek, North Fork Big Thompson) to determine the importance of disturbance magnitude, geomorphic variables, and riparian forest characteristics on LW jam loads and characteristics. We conducted river corridor surveys of 16 reaches (up to 100m long) per river, measuring variables such as jam volume and composition, slope, valley bottom confinement, and forest density. We also measured floodplain jam fabric (i.e., LW piece orientation) across various geomorphic settings. Results indicate that total LW jam loads differ between the basins. Both streams were significantly impacted by a historic 2013 Front Range flood, but West Creek experienced a 400-year flood event, whereas the North Fork Big Thompson experienced a 50- to 100-year flood event. West Creek may have experienced a larger flood due to a recent forest fire (and therefore increased runoff), as well as having a more confined valley bottom relative to the North Fork Big Thompson. The differences in geomorphology and flood severity likely influenced the difference in LW jam loads between basins. Jam fabric varies by jam type and jam orientation relative to the channel. This work will ultimately inform river management decisions in the Colorado Front Range by providing measurements of floodplain LW jam volumes following disturbance events.