Since its creation, the Ohio River has continually changed through seasonal floods and dry spells that made it both feared and respected. During droughts, the river could shrink to a shallow course only one to two feet deep, making it possible to wade across, and in the 1800s several shallow points became important crossings for the Underground Railroad. Because of the low depth, boats were built with very shallow hulls, yet they were often grounded for months or damaged by rocks and fallen trees; in 1888 the U.S. snag boat E.A. Woodruff removed 1225 snags, 127 large rocks, 46 new wrecks, and 13 older wrecks. Efforts to control the river began as early as the 1820's to create a safe, reliable transportation route, and in 1885 the first Ohio River dam was built below Pittsburgh, Pennsylvania, by the U.S. Army Corps of Engineers; over the next several decades, 51 dams were constructed from Pittsburgh to Cairo, Illinois, later replaced by fewer, larger dams such as the Meldahl Locks and Dam. Flooding remained common, and the worst flood on record came in January 1937, when heavy rains and melting snow pushed the river to a record 73 feet at Chilo, leaving only foundations where houses and businesses had stood and, combined with the burden of the Great Depression, driving many residents away permanently. The river continues to change as dams have transformed the natural river into a series of lakes, altering the dispersal of aquatic species, changing habitat, impeding the flow of silt and other material downstream, raising water levels, ending the possibility of walking across the river, and backing water into tributaries to create new wetlands and backwater areas.