Recent Sedimentation along the Mudflats of an Urban Estuary

In southern California estuaries are often bounded by dense urban development, as much of the historical marsh areas have been lost to this urbanization over the past century. The impacts to the estuary from human activity are compounded by threats from sea level rise (SLR), as estuarine environments cannot migrate landward with increases in sea level. This on-going project seeks to investigate recent sedimentation along mudflats in Upper Newport Bay, an urban estuary located in Orange County, CA, in order to 1) better understand sediment deposition and accumulation throughout the estuary, and 2) assess the bay's vulnerability to SLR. Short sediment cores were collected in June 2016 from mudflats in three areas of the bay: a) proximal to the mouth, b) downstream of the primary fluvial channel, and c) near a secondary channel at the head of the bay. The cores were analyzed for grain size, total organic matter, and short-lived radioisotopes (7Be, 234Th, 210Pb, and 137Cs). Preliminary results showed the deepest penetration of 7Be was observed at the primary channel site (7 cm), with 3 cm penetration at the secondary channel site, and no penetration at the mouth site. While the 7Be data suggest centimeter-scale deposition over the previous winter and spring at the head of the bay, decadal-scale accumulation rates were only 2-4 mm/yr. These results demonstrate that wet-season deposition on the mudflats in the upper reaches of the estuary is nearly completely reworked and redistributed in subsequent seasons. Although this decadal accumulation rate is lower than the seasonal deposition rate, it is equal to or greater than recent estimates of relative SLR for Newport Bay (2.2 mm/yr based on nearby tide gage records from the 1950s through 1990s). To date these results suggest that sedimentation throughout the estuary is dynamic (multiple phases of deposition, erosion, and redistribution) on short time scales, but over time may be keeping up with/possibly even out-pacing relative SLR.