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

Abstract

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.