Cumulative impacts to riparian wetlands in the Aliso Creek and San Juan Creek watersheds

Although riparian wetlands have recognized ecological functions and values, there have been cumulative impacts to riparian wetlands in Southern Californian. Most cumulative impact studies in Southern California have focused on assessing the areal and lineal loss of riparian wetlands or evaluated the ineffectiveness of compensatory mitigation. This dissertation developed an alternative approach towards assessing changes to riparian wetlands beyond metrics measuring areal and lineal loss to include other attributes with subsequent regulatory applications.;The alternative assessment methodology, termed the Aerial Photography Based Assessment Methodology (APBAM), relied only on aerial photographs for assessing riparian condition. APBAM exhibited high correlations and low numbers of gross disagreements with two other field-based assessment methodologies, validating its value as an assessment tool that provides flexibility and time savings over field-based methods. APBAM was effective with moderately sized riparian reaches, unaffected by watershed disturbance, and insensitive to aerial photograph resolution.;Implementation of APBAM for the Aliso Creek and San Juan Creek Watersheds documented the removal of large lengths of riparian wetlands and changes to the remaining riparian wetlands from 1974 to 2000. In addition to losses of 24% and 8% of the total length of riparian wetlands in the Aliso Creek and San Juan Creek Watersheds, respectively, there have been declines for indicators measuring connectivity, channel alteration, native vegetation buffer, and upstream watershed condition. Overall, the extent of direct and indirect impacts to riparian wetlands has been more substantial in the Aliso Creek Watershed than the San Juan Creek Watershed.;Section 404 permits had a small contribution towards cumulative impacts to riparian resources in the study watersheds. In the Aliso Creek and San Juan Creek Watersheds, Section 404 permits accounted for 8% and 55% of the cumulative lineal impacts to riparian reaches, respectively, and comparable fraction of the cumulative decreases in riparian reaches with high APBAM indicator scores.;As a framework for identifying restoration sites using landscape considerations, APBAM identified potential restoration opportunities at 30 riparian reaches in the Aliso Creek Watershed and 52 riparian reaches for the San Juan Creek Watershed. Proposed restoration activity sites addressed restoration of connectivity and restoration within open spaces.