Prior to the glaciers more than 780,000 years ago, the ancestral Monongahela River flowed from present-day north central West Virginia across Pennsylvania and northwest Ohio, but extensive ice sheets dammed the old north-flowing river and created vast Lake Monongahela, which stretched for about 200 miles from north of Beaver, Pennsylvania, to south of Weston, West Virginia, reached depths of more than 200 feet in some areas, flowed southwestward, and contained numerous narrow bays; drainage from this lake helped form the upper Ohio River Valley. The Monongahela River is 130 miles long with an average depth of 20 feet, was called "Menaonkihela" by Native Americans meaning "where banks cave in or erode," flows northward in West Virginia and Pennsylvania, and joins the Allegheny River at The Point in Pittsburgh to form the Ohio River. Its source is the confluence of the Tygart Valley River and the West Fork River at Fairmont, West Virginia, and two major tributaries are the Cheat River at Point Marion, Pennsylvania, and the Youghiogheny River at McKeesport, Pennsylvania. In October 1930, a severe drought reduced the river's flow to below 10 cubic feet per second, and in Morgantown and other locations it was low enough to cross on foot. Bituminous coal has been the most transported material on the river. In 1782, the Pennsylvania General Assembly declared the river a public highway, but because it was not navigable in all seasons, the General Assembly in 1817 authorized the Monongahela Navigation Company to build 16 dams with bypass locks between Pittsburgh and what would become West Virginia to overcome a 147-foot change in elevation and make the river navigable for its entire length. Locks and dams were in place from Pittsburgh to Brownsville by 1844; Lock and Dam No. 5 below Fredericktown and No. 6 in Rices Landing were completed in 1856; Lock and Dam No. 7 at the mouth of Georges Creek below Greensboro was completed in 1883; Lock and Dam No. 8 and No. 9 were built by the federal government in 1889 to open the river to Morgantown; the 1896 Rivers and Harbors Bill funded the federal purchase of all locks and other Monongahela Navigation Company properties for $3,601,615; and in July 1897 locks and dams became free of tolls, benefiting coal companies and making steamboat travel more accessible to passengers. Today, only two locks and dams remain in Greene County, the Grays Landing Lock and Dam and the Point Marion Lock and Dam, operated by the Pittsburgh District of the U.S. Army Corps of Engineers.