Prehistoric Rivers

Prehistoric Rivers. Three Rivers Heritage Trail. Three rivers draining the western slopes of the Allegheny Mountains meet at the Pittsburgh Point. The Monongahela flows from the mountains of West Virginia and the Allegheny comes down from northern Pennsylvania. Together the two rivers join to form the Ohio. From the Point at Pittsburgh the Ohio travels nearly 1,000 miles through the America heartland to the mighty Mississippi. , When the Appalachian Mountains formed 250 million years ago they probably looked like the Andes in South America a long chain of rugged snow-covered peaks stretching from Nova Scotia to Alabama. Rivers coming off the mountains shed sediment across the foothills and plains at least as far as what are, today, the Rocky Mountains. The rivers continued to cut down and erode to the level we see today. , This ancient river system once emptied northward into Lake Erie but reversed its flow approximately 900,000 years ago as glaciers were formed to the north. , Pittsburgh exists on these Ice Age terraces of sand and gravel left by prehistoric rivers. The famous "fourth river" beneath Pittsburgh is really a high underground water table called and aquifer created during this Ice Age. , Today the Army Corps of Engineers controls the river level by a series of flood control dams from distant places such as the Kinzua Dam near Warren and the Youghiogheny Dam near Confluence.