Assessing Potential Controls on River Bead Functionality in Mountain Streams

We evaluated flux attenuation potential, referred to as functionality, in laterally extensive, storage-dominated river reaches known as ‘beads’. 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 influence flux travel time and storage magnitude, assuming that functional beads contribute to higher travel times and storage magnitudes at the network scale. Geomorphic driver variables include drainage area, catchment slope, elevation, land cover and precipitation metrics, which represent water inputs into the stream corridor, as well as delta normalized burn index (dNBR) and catchment slope, which further represent sediment inputs into the stream corridor. Biotic driver variables within each bead include wood load, beaver modifications, and type of 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, USA. Statistical analyses examined relationships between drivers and responses and the effectiveness of grouping the beads in different ways (by dominant vegetation and by elevation). Analyses suggest that bead functionality is most strongly linked to bead ratio, or the ratio of bead size to catchment size. Functional beads are larger relative to catchment size. In addition, beads can be efficiently grouped by dominant vegetation; these different types of beads reflect significant differences in catchment geometry, geomorphic inputs, and biotic inputs, and display significant differences in bead geometry. Although functionality is 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 enhance river restoration by increasing the width of the active floodplain. Investigating drivers of functionality provides a crucial link between system inputs, restoration action, and desired response, allowing plans to be tailored to address targets. Because bead position and geometry cannot be feasibly modified, the functionality framework can be used to identify sites with the greatest potential for restoration.