Natural and built features have shaped the St. Louis region as ice during the last Ice age blocked the Mississippi River channel here and forced the river to carve a new route over bedrock, leaving a chain of strong stone ledges in the river for several miles north of St. Louis that remained dangerous for boat navigation during most river stages and impossible during low water. To accommodate riverboat traffic, the Army Corps of Engineers built an 8.4 mile canal in 1953 to bypass the chain of rocks, and the low water dam just south of the bridge was built by the Army Corps of Engineers in 1964 to raise water high enough to cover the upstream rocks and maintain levels at the top of the canal for boat travel. In 1894 and 1915, two structures in the river that look like castles were built to draw drinking water from the Mississippi River. In 1929, the Chain of Rocks toll bridge, funded by local entrepreneurs, opened to the public as a one-mile bridge with a 22-degree bend designed so boat captains would not have to navigate bridge piers and water intake towers at the same time; it originally featured an ornate toll booth beneath an amusement park on the Missouri side and a row of 400 elm trees on the Illinois side. Route 66 was rerouted over the bridge in 1936, increasing toll profits, and in the early 1940s its bright red sections were painted dull green so it would be less visible from the air during World War II. The bridge closed in 1970 shortly after Interstate 270 opened, planned demolition in 1975 was judged not cost effective because scrap steel values were low, and in 1999 the bridge reopened for walking and bicycling. Since prehistoric times, the Mississippi River has also served as a major migration route for birds, with 40% of all migrating birds in North America making the annual round trip along the Mississippi Flyway between breeding grounds in Canada and the northern United States and wintering grounds along the Gulf of Mexico and in Central and South America, relying on the river’s waterways, wetlands, and woodlands as important resting points.
