Today, the island's composition has entirely switched — the southern two-thirds of the island has only one house; the once-wild northern third has over 1,200 houses.
The year 1883 was a landmark year in the history of Plum Island's erosion. That's the year that the Army Corps of Engineers began building the north and south jetties.
The jetties were intended to allow ships to enter the river by holding a channel in place. Jetties help control and funnel the flow of water and sand. They weren't the first jetties to be built on the U.S. coast, said Ed O'Donnell of the Army Corps of Engineers. Nor were they the first jetty-like structure built to protect the Merrimack River mouth. A long wall was built around 1827 near Woodbridge Island, just west of Plum Island's tip, but it failed to work.
The jetties held the river mouth in place, but they also created new land. On the Plum Island side, tons of sand piled up against the jetty, extending the island into what was once the Merrimack River.
Matthews noted a large part of the jetty is now completely buried hundreds of yards from the river.
"Old timers told me that where the (Plum Island Point) parking lot is now, they used to catch mackerel there," said Matthews. "It's changed that much."
Around the time when the jetties were being built, Plum Island's population began to change rapidly.
Historian Nancy Weare documents it in her book "Plum Island: The Way It Was," describing the increased flow of traffic along Plum Island Turnpike due to new horse and electric rail lines, the expansion of the elegant Plum Island Hotel, and, in the 1920s, the creation of the Plum Island Beach Company. The company subdivided the island into 70-by-70-foot lots, and marketed them heavily to a beach-hungry public. Hundreds of simple cottages sprang up. That lot and street layout is clearly seen on the island today.