Pool Spacing in San Gregorio Creek, California

Abstract

Field surveys in pool-riffle channel types within mainstem San Gregorio Creek shows that pool formation are driven in large part due to flow obstructions that were exposed as a result of channel incision and widening, such as exposed bedrock outcrops and undercut banks hardened with exposed root systems.  The depth, cover and complexity of these pools are generally much lower than the few pools formed by Large Woody Debris in the study reaches.  The pool spacing for the three sites is relatively high, averaging 3.5 pools per channel width which can be contributed in part to the low LWD frequency which averages 0.17 LWD pieces/m) across the three sites.  When observed pool spacing and LWD frequency values in San Gregorio Creek are compared with those predicted using a regression equation based on a variety of Washington and southeast Alaskan stream channel types, the pool spacing is overestimated and LWD frequency is underestimated by a factor of roughly 3. This is due in large part to the high proportion of often low quality pools formed by obstructions rather than LWD.