Abstract:
This study was undertaken to evaluate the occurrence and characteristics of undercut streambanks in forested headwater streams of the Oregon Coast Range. Undercut streambanks and associated reach characteristics were surveyed along 46 sample reaches (each 152 m in length) in 8 streams; all sample reaches occurred in unmanaged forested riparian areas. Drainage areas ranged from 0.3 to 16.6 km^2. At each undercut location length, surface area, volume, low-flow (summertime) volume, and maximum horizontal depth were measured. Individual undercuts had surface areas ranging from 0.3 to 27.7 m^2 with a mean value of 2.6 m^2, and lengths ranging from 1.2 to 15.2 m with a mean value of 5.0 m. The area undercut streambanks ranged from 0.0 to 27.4 m^2 / 100 m of stream, with an average value of 6.5 m^2 / 100 m. The proportion of bankfull channel area undercut ranged from 0.0 to 4.5%, with an average value of 1.1%. Reach length undercut ranged from 0.0 to 23.6%, with an average value of 6.2%. The values reported in this study are approximately mid-range in comparison to characteristics of undercut banks reported in studies from Alaska, Montana, and Wisconsin. The % of reach area covered by undercut streambanks in this study is approximately half of that provided by large woody debris. Outside channel bends had approximately 6 times more undercut streambanks than inside bends or straight sections. Streams having a sinuosity index greater than 1.15 averaged approximately twice as much % surface area undercut than streams having a sinuosity index less than 1.15. Number of undercuts and undercut characteristics were inversely correlated with channel gradient; significant differences occurring among 1%, 2-4%, and 5+% channel gradient classes. Streambanks ranging in height from 1-2 m had a higher occurrence of undercut streambanks than either lower or higher streambanks. Undercut streambanks were 4 times more common in "composite" than "non-composite" streambanks. Undercut characteristics appear to be correlated with valley segment type. Channel widths were, on average, significantly narrower at undercut sites when compared to reach average channel widths. However, it does not appear than width characteristics are a cause of undercutting. Based on field observations it appears that flow obstructions (gravel bars, boulders, large woody debris) have little impact on undercut characteristics. Both at-a-site and reach-level comparisons of undercut bank characteristics showed relatively strong correlations with streamside tree densities. Red alder (Alnus rubra) is the most prevalent species found in Coast Range riparian areas, and the most significant species in explaining reach-level differences in undercut characteristics. Sitka spruce (Picea sitchensis) is less common in riparian zones, but appears to be positively correlated with the proportion of reach area undercut.
