*Repost from Nantucket Conservation Foundation' Science and Stewardship Blog Nov 2012*
The Medouie Creek Wetland is a large wetland along the north side of Polpis Harbor on Nantucket Island. This wetland was historically one large salt water marsh, directly connected to the harbor, getting daily tides washing over it. Sometime in the 1930s, a dike was created to build a road, allowing people access to the harbor - you can see the Eastern Dike Road in the GIS map below.
This dike road completely cut off the flow of salt water into the back part of the marsh, and over time it converted to a freshwater marsh with plants and wildlife associated with freshwater wetlands.
A couple things came together all at once to prompt NCF to consider trying to restore this wetland to a salt marsh.
The first was a large population of Phragmites australis (the common reed) that was continuing to expand in the freshwater marsh (see the area in yellow on the above GIS map). Now if you haven't heard of Phragmites yet - I'll tell you its a very aggressive invasive wetland plant and some studies have shown that it doesn't do well in salt water. Instead of spending a lot of time and lots of herbicide trying to get rid of the Phragmites by spraying it - we thought it might be smart to restore the salt marsh and hopefully decrease the Phragmites by increasing salinity.
The second reason we started this restoration was purely to try and restore an important historic salt marsh back to its original function with all of the vital ecological benefits of a salt marsh. The Massachusetts Office of Coastal Zone Management Wetlands Restoration Program (MWRP) designated Medouie Creek as a high priority wetland restoration site increasing our desire to try and restore this important wetland.
Our restoration plan was actually fairly simple - we just set out to alter/restore tidal hydrology and let the vegetation and soils take care of themselves. Our actual construction method had three components:
1) Install a culvert under the current dike road
This culvert first allowed all of the freshwater that was impounded in the previous freshwater marsh to drain out and then it also allowed daily tides to push salt water into the previously restricted marsh.
To facilitate saltwater moving back into the wetland...
2) We dredge out an existing tidal creek and an old ditch in the restricted marsh to connect to the newly installed culvert.
Salt water can flow through this ditch into the back part of the marsh and move laterally out of the ditch into the rest of the marsh, bringing salt water to, hopefully, a large part of the marsh.
3) The last thing we did was lower the grade of the dike road to allow water of extreme high tide events to sheet flow over the road, further pushing salt water into the marsh.
All of this construction work took place in December of 2008 and the restoration has progressed rapidly since then, aided by some extreme high tide/storm events!