The Herring River estuary was diked to tidal influence more than 100 years ago in an effort to control the mosquito population. Restricting tidal range in the Herring River estuary with a tide-control degraded one of the largest and most productive salt marshes in the Northeast, eradicated a significant herring run and ushered in new vegetation and development. This is the largest tidal restoration project in the region as it involves changing existing infrastructure as well as protecting buildings, homes, and roads from flooding as a result of the reestablished tidal flow.

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USGS New England Water Science Center has contributed research to this National Park Service (NPS) project since 2015 and recently published a new groundwater and surface-water monitoring data release of the area. This monitoring effort establishes the hydrologic characteristics of the Mill Creek watershed near the Herring River estuary prior to restoration. With this data, NPS has a characterization of the groundwater and surface water interactions and quality, and how those parameters are impacted by rainfall, tides, storm surge and other influences.

“It’s a baseline dataset to understand the interaction of groundwater, Mill Creek, the Herring River, and Wellfleet Harbor,” said project lead USGS hydrologist John Mullaney. “We have used statistics to help project managers understand that the water levels in Mill Creek are strongly affected by groundwater levels. Groundwater levels on the site are complicated because they, in turn, are affected by the ocean tides, storm surge, local rainfall, the growing season, and regional groundwater levels on Cape Cod.”

The NPS tasked the New England Water Science Center with collecting the groundwater levels and stream stage near or in Mill Creek, a tributary of the Herring River, to better understand the conditions of the water table and how the aquifer and stream interact prior to restoration. In cooperation with the Friends of the Herring River, 19 groundwater and surface-water sites were monitored. The most recent data release includes collection at these sites between 2018 and 2022, and this information was added to a USGS report originally published in 2019 that included data measured between 2017 and 2018.

The NPS used the data in the 2019 report to help design the tide gates on Mill Creek that will be used to restrict water flow onto private properties once the tidal range has increased on the Herring River.

Additionally, pre-restoration water-quality data were gathered to establish concentrations of nutrients, salinity, dissolved oxygen and other water-quality indicators. NPS will use the USGS data as the foundational water quality and water level data of the Mill Creek Basin upon which future data can be compared as tidal flow is slowly restored to the estuary.

This tidal restoration project is expected to improve soil and water quality, increase shellfish and river herring presence and expand healthy salt marsh wetlands, which sequester carbon, diminish flood and wave damage and provide habitat for an array of birds and other wildlife.

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