Spatio-temporal Dynamics of Edge Habitat in Natural and Altered Spring Snowmelt Recession Flow Regimes, Sierra Nevada, California.

Edge habitat, defined as the relatively shallow, slow zones in channel margins, provides important flow refugia and nursery habitat for juvenile fish, amphibians and riparian species. The Mediterranean-Montane rivers of the Sierra Nevada foothills, California, have a spring snowmelt flow regime. The timing and rate of change of the recession limb has been shown to shape local ecological processes, such as reproductive cues for the threatened foothill yellow-legged frog (Rana boylii). It was hypothesised that flow regulation would alter the availability, location and spatial dynamics of edge habitat during the recession limb. A hydraulic patch dynamics framework, was applied to a regulated (Middle Fork) and unregulated reach (North Fork) of the American River, California to evaluate potential differences in edge habitat dynamics. Repeat surveys of the location and hydraulic characteristics (depth, mean column streamwise and lateral velocity and velocity variability), of the edge habitat zone, delineated in the field by relative differences in depth, velocity and/or surface flow type conditions from the main channel, were conducted throughout the spring snowmelt recession limb (April–August 2011) using a 2D Acoustic Doppler Velocimeter. This paper presents the results of the patch dynamics analysis, highlighting differences in the number of hydraulic patches and their associated hydraulic characteristics as well as the change in location and extent of edge habitat throughout the recession limb. The ecological implications of flow regime alterations on edge habitat dynamics and suggestions for how these could be minimised will also be discussed.