Abstract

Estuaries worldwide have been altered by anthropogenic modifications including land clearing, dredging, and land reclamation, which impact sediment routing and accumulation on tidal flats. Numerous studies have explored tidal flat and marsh vulnerability to submergence or “drowning” under accelerating sea-level rise, but recent work along the Oregon coast suggests estuaries are maintaining positive accretionary balances (at least in marshes) despite ongoing sea-level rise. In this study, accretion rates (sediment accumulation rates) were evaluated from 210Pb profiles at eight sites on broad intertidal flats in Coos Bay, one of the largest estuaries on the U.S. West Coast and a site of substantial development and logging since the mid-1800s. Based on the century-scale record of sediment accretion represented by 210Pb profiles, tidal flats have been accreting at rates of ~ 1–3 mm/yr with little spatial relationship to relative sea-level rise or patterns of tectonic uplift. Thus, accretion is generally not accommodation- or supply-limited, and therefore likely not regulated by sea-level rise. Peaks in sediment accretion are well-preserved from the last 30 years, and accretion rates averaged over this more modern time span tend to be four times greater than rates averaged over the whole-core (century-scale) 210Pb records. It is unclear whether the higher, more modern rates represent a real change in estuarine accretion patterns over the past decades or a Sadler effect (i.e., an apparent but not real increase in accretion in younger sediments). The results highlight the spatial variability in accretion rates within a single estuary, the potential resiliency of this tectonically active estuary to sea-level rise (in the form of a positive accretionary balance), and raise the issue of whether management decisions are best made based on century-scale accretion rates or multi-decadal accretion rates.