UNDERCUT STREAMBANK HABITAT – AN INTERSECTION OF BIOTIC AND GEOMORPHIC PROCESSES

Undercut streambanks provide cover for aquatic invertebrates and fish. The fluvial landform provides a thermal refuge from direct sunlight and protection from avian predation. The physical characteristics and spatial extent of undercut banks are challenging to quantify--in part because they may not be visible in the field except at drought or anthropogenically reduced flow stages, and in part because they are not observable using remote sensing imagery. Nonetheless, quantification of the characteristics and spatial relationships of undercut banks is relevant to understanding the role of physical habitat in ecological restoration efforts. Geomorphic field maps and data from eight reaches with upstream drainage areas ranging from about 2.0 to 120 km² in a coastal California basin were analyzed to explore characteristics and extent of undercut banks. The basin’s drainage network extends across two zones with disparate climate, geology, and vegetation: a fog-influenced, wetter zone with a conifer forest, and a drier inland alluvial valley. Undercut widths perpendicular to the channel and lengths along the longitudinal profile were measured. The average width of the undercuts in reaches in both zones was similar, about 0.25 m. In the relatively wet, forested portion of the watershed, however, the length of undercut banks was about three times that in the drier, non-forested portion. Moreover, undercut banks in the forested areas proximal to large wood and roots from live trees were present about eight times more often than in the drier, non-forested reaches. Nonetheless, the presence, along with the similar widths of the undercuts in both zones suggests that morphodynamic processes leading to bank erosion contribute to formation of undercut banks, even in the absence of large wood or roots. Therefore, future process-based models must account for the complexity of interrelated and independent biotic and geomorphic factors that lead to undercutting. The characteristics and causes of undercut banks warrant interdisciplinary consideration in predicting and planning for ecological sustainability and resilience to a changing climate.