Rates and probable causes of freshwater tidal marsh failure, Potomac River Estuary, Northern Virginia, USA

Dyke Marsh, a distal tidal marsh along the Potomac River estuary, is diminishing rapidly in areal extent. This study documents Dyke Marsh erosion rates from the early-1860s to the present during pre-mining, mining, and post-mining phases. From the late-1930s to the mid-1970s, Dyke Marsh and the adjacent shallow riverbottom were mined for gravel, resulting in a ~55 % initial loss of area. Marsh loss continued during the post-mining phase (1976–2012). Causes of post-mining loss were unknown, but were thought to include Potomac River flooding. Post-mining areal-erosion rates increased from 0.138 ha yr⁻¹ (~0.37 ac yr⁻¹) to 0.516 ha yr⁻¹(~1.67 ac yr⁻¹), and shoreline-erosion rates increased from 0.76 m yr⁻¹ (~2.5 ft yr⁻¹) to 2.60 m yr⁻¹ (~8.5 ft yr⁻¹). Results suggest the accelerating post-mining erosion reflects a process-driven feedback loop, enabled by the marsh's severely-altered geomorphic and hydrologic baseline system; the primary post-mining degradation process is wave-induced erosion from northbound cyclonic storms. Dyke Marsh erosion rates are now comparable to, or exceed, rates for proximal coastal marshes in the same region. Persistent and accelerated erosion of marshland long after cessation of mining illustrates the long-term, and potentially devastating, effects that temporally-restricted, anthropogenic destabilization can have on estuarine marsh systems.