The broad level lands that gave Illinois the nickname Prairie State were fashioned by late Cenozoic glaciation, which leveled rugged ridges and filled valleys over the northern and central parts of the state. The fertile prairies are drained by more than 275 rivers, most of which flow to the Mississippi-Ohio system; the Illinois is the largest river in the state.
These rivers provided early explorers a way SW from Lake Michigan into the interior of the continent and later, in the days of canal building, played a big part in hastening settlement of the prairies. The completion of the Erie Canal linked Illinois, through the Great Lakes, to the eastern seaboard of the United States. The Illinois Waterway links Chicago to the Mississippi basin as the old Chicago and Illinois and Michigan canals once did, and the St. Lawrence Seaway provides access for oceangoing vessels. The waterways are but a part of a transportation complex that includes railroads, airlines (Chicago's O'Hare airport is one of the busiest in the world), and an extensive modern highway system.
The state's climate is continental, with extreme seasonal variations of temperature in parts of the state. Among Illinois's many tourist attractions are Shawnee National Forest, with recreational facilities; the Cahokia Mounds; and many state parks and historical sites, including New Salem and Lincoln's home and burial place in Springfield. An additional summer attraction is the Illinois State Fair. Springfield is the capital; Chicago, Rockford, and Peoria are the largest cities.