Allegheny Islands State Park
Allegheny Islands State Park is a Pennsylvania state park in Harmar Township, Allegheny County, Pennsylvania in the United States. The undeveloped park is composed of three alluvial islands located in the middle of the Allegheny River northeast of Pittsburgh. The islands are just north of the boroughs of Oakmont and Plum, and southwest of Cheswick. Bridges for the Pennsylvania Turnpike and the Bessemer and Lake Erie Railroad Branch of the Canadian National Railway cross the Allegheny River at the middle island.
Islands
The park land on the islands has a total area of, with two islands downstream of the Allegheny River Lock and Dam Three, and the other island upstream. Twelve Mile Island is the farthest downstream and has a number of houses and buildings on the western four-fifths of the island. The remaining two islands are both uninhabited and have no structures on them. They were originally one island named Fourteen Mile Island, but were split into two parts by the construction of the dam. Today the name Fourteen Mile Island is given to the middle island, just downstream of the dam, and it is the site of the highway and railroad bridges. According to the USGS GNIS, the last island is unnamed This unnamed island is the easternmost and the only one upstream of the dam. There are also five to seven shoals near the islands, which are occasionally exposed.Twelve Mile Island and Fourteen Mile Island are named for their distances, and, up the Allegheny River from the confluence of the Allegheny and Monongahela Rivers to form the Ohio River in downtown Pittsburgh. They are two of six alluvial islands in the Allegheny River near Pittsburgh. "The large number of islands in the Allegheny is a reflection of the large amount of glacially-derived sediment that was moved by the river."