SEDIMENT ACCUMULATION IN A MANIPULATED BAY OF PUGET SOUND, BELLINGHAM, WA

Mud Bay, located in Bellingham, Washington, at the north end of Chuckanut Bay, is infilling at a rate greater than expected sea-level rise. This study will quantitatively determine the rate of bay infilling. In 2013, based on qualitative studies, the City of Bellingham identified the pocket estuary of Mud Bay as a 10-year restoration priority due to degradation of habitat from sediment accumulation. Mud bay is an important habitat for eelgrass, shellfish and birds. Chuckanut creek, the primary fluvial input into the bay, has historically been a steelhead spawning ground. Anthropogenic factors that could have increased sediment input include logging, mining and quarrying in proximal areas during the late 1800s to early 1900s. A rip-rap train trestle, constructed across the mouth of the bay in the 1920s and still in use today, limits energy from tides, waves, and storm surge. During the construction of Interstate 5 in the early 1960s, additional sediment could have been transported to the bay by Chuckanut Creek. To determine the degree of bay infilling, cores were collected from four sites in the intertidal zone of the study area, near the mouth of Chuckanut Creek. Using ²¹⁰Pb geochronology, sediment-accumulation rates were measured as 0.2 to 1.07 cm yr^-1, with spatial variations attributed to differences in tidal currents and fluvial supply at each coring location. Accumulation rates were corroborated using ¹³⁷Cs at one coring site. Both geochronological analyses established sediment-accumulation rates of ~0.8 cm yr^-1 at this location. Sedimentary structure was examined using X-radiographs, and grain-size distributions were incrementally conducted for each coring site using a Beckman Coulter LS 13-320 laser diffraction particle-size analyzer. Relative sea-level rise projections for the next century for this location are ~30 cm, with a probability of exceedance of 95%. Accommodation space is decreasing because rates of infilling exceed the rate of local sea-level rise. Additional work is needed to determine what fraction of the observed infilling is attributed to anthropogenic stressors, as opposed to natural causes.