PLUM ISLAND — Mother Nature plays her part, of course.
But those who have seen Plum Island's beaches and dunes morph for decades say it isn't just the winds of nor'easters, the waves of the Atlantic Ocean and the natural characteristics of a barrier island that account for the erosion seen in recent years.
Man — or at least some of his inventions — is also to blame, they say.
Many locals say the jetties, groins and dredging — or in recent years, the lack of it — of the Merrimack River have contributed to the massive erosion of certain sections of Plum Island's beaches.
Paul Ivaska, the chairman of Newbury's Beach Committee, said nature's role is clear, but humans also contribute.
"A good part of (the erosion) is Mother Nature. It is a barrier beach, and the sand shifts," he said. But "my understanding is that there needs to be repairs to the jetty that have not been done to keep the erosion down."
The south jetty, which runs off Plum Island, is partly collapsing. Some argue the jetty plays an important role in shaping Plum Island, in part because of the way it interacts with natural phenomenons.
Scientists say mighty tidal rivers such as the Merrimack create two phenomenons outside their mouths.
The first is a massive sand hump that forms several hundred yards out in the ocean, caused by sand coming down the river, tides pushing water up and down the river, and sand migrating along the coast. In Plum Island's case, the movement of the sand along the shore is further complicated because it is funneled off the ends of the jetties.
Extending southward off that sand hump is the second natural phenomenon, a series of "bars," basically sand humps that run roughly parallel to the shore, caused by currents and sand migration. At low tide, those bars can be clearly seen a couple of hundred yards off Plum Island, running about 2 miles long and ending just south of the Center.