The largest tributary of the Mississippi-Missouri, the Ohio River served as an essential link in the exploration and development of America west of the Appalachian Mountains. Formed by the confluence of the Allegheny and Monongahela rivers in downtown Pittsburgh, Pennsylvania, the Ohio flows 981 miles to join the Mississippi at Cairo, Illinois. It is believed that La Salle traveled down the Ohio to the vicinity of present-day Louisville around 1670. The French claimed the river and its valley in 1749 and built Fort Duquesne where Pittsburgh now stands. Near there, the French and their Indian allies defeated and killed General Braddock in 1755. The English took the site in 1758 and built their own Fort Pitt. Legal possession of the river passed to the English in 1763 and to America in 1783. For many years, the Ohio was the main artery bearing settlers into the midwest, although its dominance was impacted by the opening of the Erie Canal in 1825. Eventually, the railroads proved such strong competition that the era of steamboats ended, but locks and dams have created the opportunity for efficient commercial transportation of bulk commodities. Another result has been flood control.