The Black Belt is blessed with two great river systems, the Alabama and the Tombigbee. The Alabama River flows west along the northern rim of the region until it is joined in Dallas County by the Cahaba River which originates in the mountains near Birmingham. The Alabama River then turns south and cuts through the Black Belt. Impressive white bluffs are often visible where the rivers cut through the Selma chalk that underlies the Black Belt prairies, but when the Alabama enters the Coastal Plain, its current slows and its channel widens. West of the Alabama, the Tombigbee River follows a similar course. At Demopolis, it joins the waters of the Black Warrior River which originates in the hills of the Cumberland Plateau. The Tombigbee and Alabama River eventually merge to form the Mobile River in Clarke County on their way to the Gulf of Mexico. The Cahaba River has been designated an official "Alabama Natural Wonder" and is currently the state's only large free-flowing river. Since the rest have been dammed for hydroelectric power, the Cahaba in many ways is a window on past river conditions throughout the region. It is the most biologically rich river of its size in the nation with over 131 different species of fish. It is a safe harbor for a variety of freshwater mussels and snails, and in May it puts on a spectacular show when the beautiful Cahaba lily blooms among the rocks at the fall line near West Blocton and Piper in Bibb County. Change in the landscape has been a constant in Alabama's Black Belt Region, and perhaps the longest ongoing change has been the meandering of these rivers. This process is slow and subtle but associated flooding can be rapid and dramatic. For eons, muddy waters have been shaping and reshaping wide flood prone areas around each river, creating abandoned channels, oxbow lakes, shallow swamps, seasonal ponds, low flood deposited sand ridges, and extensive canebrakes. Alabama's native people were naturally attracted to the abundant plant and animal life located in these meander belts. The flood deposited sand ridges were well drained and easy to work, so they became ideal soils for growing corn, beans, squash and other cultigens of the great mound building cultures of prehistoric Alabama. Floods regularly refreshed their fields with nutrients and also deposited fish into the oxbow lakes and swamps where they could spawn and be easily trapped. Add migrating waterfowl and a wide variety of mussels in the clean free-flowing rivers, and it is easy to see how the Black Belt river bottoms provided a nearly unlimited supply of protein. For centuries before European contact, towns and homesteads of the Moundville and Pensacola cultures blossomed in the river bottoms that cut through Alabama's Black Belt prairies.