Patterns of organic matter accumulation in dryland river corridors of the southwestern United States.

We use Google Earth imagery, drone imagery, and ground-based field measurements to assess the abundance, spatial distribution, and size of accumulations of organic matter in perennial, intermittent, and ephemeral channels in drylands of the southwestern United States. We refer to these accumulations as organic matter jams (OMJs). We examine correlations between OMJ characteristics and indicators of spatial heterogeneity within river corridors. We hypothesize that OMJs occur primarily in association with obstacles such as living woody vegetation and that spatially heterogeneous river corridors have greater numbers of OMJs per surface area of river corridor. Using data from 19 river reaches across four areas in Arizona and Utah, we find that OMJs are preferentially associated with bars in the active channel and with living woody vegetation in the channel and floodplain. We also find that whether greater spatial heterogeneity corresponds to greater spatial density of OMJs can be influenced by downstream distance from major sources of large wood and organic matter and whether the river corridor is supply- or transport-limited with respect to organic matter. Consequently, the strongest influence on OMJ location and abundance can vary between individual reaches of a river corridor and between watersheds. The abundance and size of OMJs in river corridors of sparsely vegetated drylands fall within the ranges of values published for perennial river corridors in wetter climates. We suggest that management of dryland river corridors explicitly include protecting and restoring organic matter accumulations in these environments.

Biogeomorphic influences on river corridor resilience to wildfire disturbances in a mountain stream of the Southern Rockies, USA.

We examine a 9.4-km-long portion of a montane river corridor in the Southern Rockies, the upper 8 km of which burned in 2020. We focus on sediment storage in logjam backwaters and how spatial heterogeneity in the river corridor attenuates downstream fluxes of material following the wildfire. Wider portions of river corridor exhibit greater spatial heterogeneity, as reflected in multithread channel planform and more closely spaced abandoned beaver dams and channel-spanning logjams. Logjams in multithread reaches have greater volumes of backwater storage and store finer sediment than logjams in single-thread reaches. Despite substantial turnover of sediment in backwater storage during the first runoff season after the wildfire, the cumulative volume of sediment stored at 11 monitored logjams following the 2021 runoff season was 71% of the cumulative sediment volume at the logjams immediately after the fire. Floodplain vegetation regrowth was also faster and more complete at multithread reaches. Vegetation recovery contributed to overbank deposition in these reaches, in contrast to the bank erosion observed in single-thread reaches. More spatially heterogeneous portions of the river corridor appear to be disproportionately important in attenuating enhanced inputs of sediment following wildfire, and the cumulative effect of this attenuation across a river network likely enhances watershed-scale resilience to wildfire disturbance.