From the Goldthwait Reservation web pages:

“The Goldthwait Reservation contains a large area of salt marsh and a fringe of freshwater marsh around much of the perimeter. The Goldthwait salt marsh is connected to the Riverhead Beach area by a large diameter culvert which passes under Ocean Avenue. This culvert functions very much like a tidal creek, allowing water to flow into and out of the marsh with the tide.”

For more information about Goldthwait Reservation, see: