Determining primary drivers of river bead functionality in mountain headwater streams

Abstract

I evaluated flux attenuation potential, or functionality, in laterally extensive storage-dominated reaches known as 'beads.' Beads are disproportionately important to river corridor health. Bead functionality was evaluated as a relationship between driver variables, which directly measure or measure proxies of geomorphic and biotic system inputs, and response variables, which are proxy variables believed to most accurately reflect flux travel time and storage magnitude, assuming that functional beads contribute to higher travel times and storage magnitudes. Geomorphic driver variables include catchment and reach-scale geometry and catchment hydrology, which approximate water inputs into the stream corridor, as well as delta normalized burn index (dNBR) and catchment slope, which approximate sediment inputs into the stream corridor. Biotic driver variables include wood load, beaver modifications, and riparian vegetation. Response variables include normalized difference vegetation index (NDVI), normalized difference water index (NDWI), patch density, and total sinuosity. Driver and response variables were measured through a mixture of fieldwork and remote data for 52 beads in 27 catchments in the Colorado Front Range. Statistical analysis examined relationships between drivers and responses and also examined the effectiveness of grouping the beads in different ways (by dominant vegetation, level of disturbance, and elevation). Analyses suggest that bead functionality is most strongly linked to bead ratio, or the ratio of bead size to catchment size. Generally, functional beads have a higher ratio of bead size to catchment size. In addition, beads can be efficiently grouped by dominant vegetation; these different types of beads display significant differences in catchment geometry, bead geometry, geomorphic inputs, and biotic inputs. Although functionality was found to be the complex result of numerous factors and may require case-by-case assessment efforts, restoration of channel-floodplain connectivity and facilitating greater retention of water will provide the biggest return on river restoration investment by increasing the width of the active floodplain. Researching drivers of functionality also provides a crucial link between system inputs, restoration action, and desired reaction, allowing plans to be tailored to address targets. In addition, certain aspects of functionality – namely position and geometry – cannot be feasibly modified, and thus the functionality framework can be used to pick a site with the greatest potential for restoration.