Research about Herring River estuary

USGS2026

Ecogeomorphologic response of a tidally restricted salt marsh to tidal restoration and sea-level rise

Diked marshes with decades of restricted tidal flow exhibit fundamentally altered hydrologic and ecologic dynamics that result in limited vertical accretion, land subsidence, and underdeveloped tidal channels. These changes complicate marsh restoration efforts and constrain the marsh's ability to build elevation in response to future sea levels. In this study, we developed an integrated hydrodynamic and marsh accretion model to conduct the first assessment of the restoration potential of a diked marsh in response to reintroduction of tidal flow and sea-level rise (SLR). We applied our model to evaluate changes in biomass production in the Herring River estuary, Cape Cod, MA, USA—which has experienced significant ecological shifts due to more than a century of tidal restriction—in response to a range of SLR scenarios, tidal flow conditions, management interventions, and biomass production rates through 2100. We found that with restoration of tidal flow, the simulated marsh extent expanded in areas with efficient drainage, increasing biomass production by up to 4.4 times. However, when SLR was imposed, parts of the marsh gradually transitioned to open water, leading to a decline in marsh coverage from 19% to as low as 12%. Management interventions aimed at enhancing marsh elevation and improving the drainage capacity of tidal channels slowed marsh loss, supporting marsh area through 2100. Our findings provide insight into the key drivers of marsh resilience and loss, and our approach can be applied across tidally restricted and restored systems to develop targeted, adaptive restoration strategies that achieve desired ecological outcomes.

Massachusetts↗

USGS2026

Groundwater–surface water exchange at a New England salt marsh: The interplay between tidal pumping at multiple frequencies and terrestrial groundwater discharge

Groundwater–surface water exchange plays a fundamental role in the transport of energy and solutes in coastal salt marshes, which are positioned at an intersection between terrestrial and marine hydrologic environments. Hydraulic gradients in shallow marsh sediments continuously change due to tidal water level variations that alternately flood and drain the marsh platform, often superimposed on terrestrial groundwater drainage. We use timeseries data from Vertical Temperature Profiler and Conductivity-Temperature-Depth instruments to investigate the space–time patterns of groundwater–surface water exchange at a hillslope-adjacent fringing salt marsh on the Herring River in Wellfleet, MA. Analyses of data acquired from September to December 2023 demonstrate that time-averaged exchange rates systematically vary from recharge near the tidal channel to saline groundwater discharge towards the upland fringe and systematically decrease during the transition from summer to winter conditions. Exchange rates were continuously modulated by tides, including high-frequency compound and harmonic components generated by non-linear interactions as the astronomical tide propagated into the estuary. The exchange rate sensitivity to tidal forcing decreased with both frequency and distance from the tidal channel, consistent with the lateral diffusion of pore pressure perturbations through the high permeability layer underlying the surficial, low permeability peat. We find that tidal loading of the marsh sediments is a two-stage process consisting of direct, instantaneous loading during inundation and indirect, time delayed loading via lateral diffusion of pore pressure variations in-between inundation intervals. The combination of terrestrial groundwater entering the marsh through the high permeability layer beneath the peat, seawater flooding the marsh platform during inundation, and tidally induced pore pressure variations diffusing into the marsh generates a complex space–time exchange pattern, including bi-directional exchange at tidal periods, with poorly understood but important implications for biogeochemical processes in the marsh sediments and constituent fluxes to the coastal ocean.

Massachusetts↗