When first built in the late 1800s, the massive stone jetties were designed to create a funnel effect, which was intended to increase the velocity of water that moves through them and keep open a deep navigation channel through the river mouth. But they also have a tendency to erode the dunes at the northernmost end of Plum Island, where the funnel shape of the island’s jetty begins.
The jetties have not been repaired since 1970. In the years immediately following that repair, the northern end of the island underwent a similar phenomenon — dunes eroded through the funnel, pushing the shoreline southward toward hundreds of homes. Problems with erosion there peaked around 1975, when the City of Newburyport built an emergency wall made from concrete blocks to save homes, some of which were only 10 yards or so from the water’s edge. Since then, the dune has gradually grown, and now the water’s edge is about 200 yards or more from those homes.