1827 — The Army Corps of Engineers draws its first map of the Merrimack River entrance. It is located more than a half mile south of its 2008 position.
around 1827 — A jetty system is built near the mouth of the Merrimack River to control the route of the river mouth. It is a failure. Its ruins can still be seen today near Woodbridge Island.
1839 — Three hurricanes cause catastrophic damage to Plum Island and Salisbury Beach. A half-mile stretch of both beaches is swept away. The river mouth splits in two.
1840s — An island that has formed in the middle of the river attaches to Plum Island.
1851 — A map drawn by the Army Corps of Engineers shows the river course has changed dramatically. A thin, 1.5-mile-long finger of land has formed on the north end of Plum Island. It is separated from the island's northwestern point by a new bay, Plum Island Basin.
1880 — The construction of two jetties is authorized to hold the river mouth in place.
1881 — Construction begins on the north jetty
1883 — Construction begins on the south jetty
1883-5 — Dyke built across Plum Island Basin to prevent the river from breaking through and creating a new entrance. Ruins of the dyke can still be seen today.
1905 — South jetty is completed. It is 2,415 feet long.
1914 — North jetty is completed. It is 4,118 feet long.
1920s — Erosion becomes noticeable along a mile stretch on either side of the island center.
1937 — Jetties are repaired.
1940 — Three storms (Feb. 14, 19 and April 22-22) sweep away a cottage and damage several others.
1945 — Three storms (Nov. 20, 22, and 28-30) peel away up to 50 feet of shoreline, undermining 18 cottages.
1950 — Four storms (Feb. 15, March 21, Sept. 11-12, and Nov. 25-26) cause up to 65 feet of dune to disappear, several cottages are swept out to sea, and others are damaged. The narrowest strip between the Basin and the ocean has receded to the point where a breakthrough is feared.