California’s coastal wetlands provide valuable ecosystem services to the environment, including sequestering carbon, improving water quality, and providing habitat for many endangered species. Yet due to sea level rise the future of coastal wetlands is in question. Sediment placement is a new and promising technique that aims to help coastal wetlands combat sea level rise. We measured suspended sediment concentrations (SSC) and volatile suspended solids (VSS) at the marsh surface of experimental and control sites one week, one month, three months, six months, and one year after sediment addition. SSC on the experimental and control sites was initially high, but then it decreased and remained low. VSS was lower at the experimental site than the control site for the first three sampling periods and similar for the remainder of the year. This is the first study to evaluate SSC and VSS changes associated with sediment placement.

Wetlands around the world are under threat from habitat destruction and degradation. In theUnited States, the Clean Water Act is the primary tool used to regulate impacts to wetlands. The U.S. Army Corps of Engineers (USACE) regulates Section 404 of the CWA, which allows for impacts to wetlands with appropriate compensatory mitigation. While permanent impacts and compensatory mitigation have been studied by researchers, temporary impacts to wetlands have not been examined. Temporary impacts to wetlands include any activity that temporarily discharges material into a wetland such as building a temporary access road, temporary water diversions, or excavating within a wetland to replace a pipe. This dissertation examines the nature, extent, and location of temporary impacts to wetlands in the southwestern United States and whether they are recovering to pre-construction conditions. First, we showed that hundreds to thousands of temporary impacts to wetlands are authorized each year within our study area of the Los Angeles District of USACE. Within our permit sampling, most (50%) of the temporary impacts were associated with construction activities, followed by sediment removal (17%). Nearly all sampled permits had a condition associated with the permit to restore the temporary impact to pre-project conditions, but only 20% were required to monitor the restoration of the temporary impact. Next, we used high resolution aerial imagery to assess hydrologic and vegetation recovery from temporary impacts from permits authorized in 2011. Vegetation cover of the impact site did not recover to 100% of pre-impact levels in 40% of our dataset. The overall riparian area recovery varied depending on the metric. From our analysis of 56 projects, four of the six metrics used had at least one site never recover and several sites that only slightly recovered. Sites with temporary impacts that never recover would be permanent rather than temporary impacts to wetlands. Finally, we analyzed permits from the past ten years for the land cover types in which the projects occurred. Between 63% and 74% of temporary impacts to wetlands were in natural lands and were individually small in impact size. We also specifically examined long, linear projects, which accounted for a disproportionate number of the impacts to wetlands in natural lands, but they were very small in total hectares. Given the hundreds to thousands of temporary impacts to wetlands permitted in this region each year, that 40% of our dataset did not fully recover vegetation cover, and only 20% of permits in our study had conditions to monitor the recovery of the temporary impacts, the USACE should update its regulations. Specifically, they should define how long an impact can occur while still being considered temporary, increase required monitoring of temporary impacts, and treat temporary impacts as permanent if they do not fully recover, requiring the necessary compensatory mitigation.