Geomorphology of undercut streambank habitat

Undercut streambanks are a physical element of habitat formed by bank erosion processes. They offer overhead cover for fish and other biota, yet little is known about their frequency, geometry (width, length and area) or spatial distribution. This study used field data from the gravel‐bed portion of a fluvial system with riffle‐pool‐bar bedforms in the northern California Coast Ranges to investigate (1) the frequency and geometry of undercuts, (2) the relation between reach‐scale fluvial variables and undercut frequency and geometry and (3) the spatial distribution of undercut streambanks. Results showed that undercut streambank frequency was 60% higher and median undercut widths were 15% higher in the relatively wetter coastal zone supporting a conifer forest. The longest undercut lengths were similarly present in this zone. These channel reaches had a higher frequency of large wood than in drier reaches farther inland. Nonetheless, undercut widths were relatively consistent in comparison to the variability of undercut lengths and areas. Probability density function (PDF) statistics illustrate the magnitude‐frequency distribution of undercut area. The power‐law exponent of the linear portion of this curve, representing the smaller number of undercuts with larger areas, equals −1.54, a dimensionless metric useful for comparison to other stream environments. Significantly, variability in the frequency and the geometry of undercuts results from their spatial distribution relative to geomorphic elements within the fluvial system. The majority of the undercuts were present in floodplain sediment, on the opposite side of the channel from bar bedforms and adjacent to pools and large wood. As components of physical habitat, undercuts were more closely spaced than pools, suggesting they are significant in contributing to the physical heterogeneity of gravel‐bed streams. Process‐based investigations are required to understand geomorphic and ecological interactions that promote the formation and longevity of undercuts in fluvial systems where habitat restoration is urgent.