Cumulative impacts of Section 404 Clean Water Act permitting on the riparian habitat of the Santa Margarita, California Watershed

Abstract

The Santa Margarita River is one of the few remaining free-flowing river systems on the Southern California coastal plain, and it provides one of the most expansive, unspoiled riparian habitats in Southern California. There is concern among resource managers that the cumulative impacts associated with rapid development of the upper watershed will degrade the ecological integrity of the entire watershed. In response to this concern, we assessed the impacts of development projects in this watershed, which were subject to regulation under Section 404 of the Clean Water Act. Cumulative impacts were assessed by comparing the conditions present at each project site prior to issuance of the Section 404 permit to conditions present after issuance of the permit. Changes between pre- and post-project site conditions were evaluated using a semantic categorization for the following six criteria: endangered species habitat, structural diversity, spatial diversity, undeveloped open space, linear contiguity, and adjacent habitat. The impact assessment showed that although the Section 404 permit program has attenuated project impacts, it has failed at minimizing cumulative impacts. Projects permitted under Section 404 contributed to the disruption of movement corridors linking the habitats of the upper and lower watershed and resulted in extensive floodplain encroachment, which may have hydrologic as well as ecological implications. More importantly, this assessment identified the types of activities and habitat functions that should be addressed during revisions in permitting procedures and should be emphasized in the watershed planning process. These conclusions will be incorporated into an ongoing Advanced Identification study sponsored by the U.S. Environmental Protection Agency and U.S. Army Corps of Engineers to develop a management plan for the Santa Margarita Watershed.