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Restoration of northern Coggeshall salt marsh on Prudence Island

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PI: Kenneth Raposa, NBNERR
Funding: RI Coastal Resources Management Council

Salt marshes throughout southern New England are deteriorating rapidly in response to high rates of sea-level rise.  Researchers are now documenting die-off of high marsh plants, pond formation on the marsh surface, and overall losses of marsh area.  At the same time, marsh drainage is being reduced as historic ditches continue to fill with sediment.  The result is that more water is flooding the marshes due to sea-level rise, and less is escaping back out due to limited drainage.  This leads to the waterlogged conditions seen in marshes throughout Narragansett Bay and southern New England.[/vc_column_text][vc_empty_space][vc_column_text]

One way to increase marsh resiliency to sea-level rise is to excavate new channels into waterlogged marshes to facilitate drainage of tidal water.  Projects like this are now underway in Rhode Island, but the effects of this approach are poorly known due to a limited number of published studies.  To improve our understanding of how these kinds of projects affect waterlogged salt marshes, the Reserve secured a $23,000 grant from the RI Coastal Resources Management Council to dig new sinuous tidal creeks into a waterlogged section of Coggeshall salt marsh on Prudence Island.  We are monitoring ecological conditions before and after the project (and at a nearby reference marsh) to understand how the marsh responded to our work.

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Our monitoring documented an immediate drop in marsh water levels.  Before the excavations, low tide water levels averaged 1.5 cm below the marsh surface, indicating that the marsh was not draining well.  After the excavations, low tide water levels dropped an additional 21 centimeters, illustrating that we were successful at relieving the waterlogging.  More alarming, we also detected substantial drops in marsh elevation in unvegetated areas with soft, organic soils.  If our project is ultimately successful, it will provide coastal resource managers with a vetted tool for improving salt marsh resiliency to sea-level rise and other stressors.

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