Allegheny Islands State Park - Islands

Islands

The park land on the islands has a total area of 43 acres (17 ha), with two islands downstream of the Allegheny River Lock and Dam Three (officially named the C. W. Bill Young Lock and Dam), and the other island upstream. Twelve Mile Island is the furthest downstream (west) and has a number of houses and buildings on the western four-fifths of the island (the park land is the eastern end). 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 (west) of the dam, and it is the site of the highway and railroad bridges. According to the USGS GNIS, the last island (which was once part of Fourteen Mile Island) is unnamed (although it is still labeled "Fourteen Mile Island" on the U.S. Army Corps of Engineers navigation maps of the Allegheny River.) 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 and Fourteen Mile Islands are named for their distances (12 miles (19 km) and 14 miles (23 km)) along the Allegheny River to 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 (for comparison, near Pittsburgh the Ohio River has only three islands and the Monongahela River has none). "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."