section_path: Navigation and flood control/20th century
section_name: 20th century
paragraph_text: 20th century
  
  In 1907, Congress authorized a 6-foot-deep (1.8 m) channel project on the Mississippi River, which was not complete when it was abandoned in the late 1920s in favor of the 9-foot-deep (2.7 m) channel project.
  
  In 1913, construction was complete on Lock and Dam No. 19 at Keokuk, Iowa, the first dam below St. Anthony Falls. Built by a private power company (Union Electric Company of St. Louis) to generate electricity (originally for streetcars in St. Louis), the Keokuk dam was one of the largest hydro-electric plants in the world at the time. The dam also eliminated the Des Moines Rapids. Lock and Dam No. 1 was completed in Minneapolis, Minnesota in 1917. Lock and Dam No. 2, near Hastings, Minnesota, was completed in 1930.
  
  Before the Great Mississippi Flood of 1927, the Corps's primary strategy was to close off as many side channels as possible to increase the flow in the main river. It was thought that the river's velocity would scour off bottom sediments, deepening the river and decreasing the possibility of flooding. The 1927 flood proved this to be so wrong that communities threatened by the flood began to create their own levee breaks to relieve the force of the rising river.
  
  The Rivers and Harbors Act of 1930 authorized the 9-foot (2.7 m) channel project, which called for a navigation channel 9 feet (2.7 m) feet deep and 400 feet (120 m) wide to accommodate multiple-barge tows. This was achieved by a series of locks and dams, and by dredging. Twenty-three new locks and dams were built on the upper Mississippi in the 1930s in addition to the three already in existence.
  
  Until the 1950s, there was no dam below Lock and Dam 26 at Alton, Illinois. Chain of Rocks Lock (Lock and Dam No. 27), which consists of a low-water dam and an 8.4-mile-long (13.5 km) canal, was added in 1953, just below the confluence with the Missouri River, primarily to bypass a series of rock ledges at St. Louis. It also serves to protect the St. Louis city water intakes during times of low water.
  
  U.S. government scientists determined in the 1950s that the Mississippi River was starting to switch to the Atchafalaya River channel because of its much steeper path to the Gulf of Mexico. Eventually, the Atchafalaya River would capture the Mississippi River and become its main channel to the Gulf of Mexico, leaving New Orleans on a side channel. As a result, the U.S. Congress authorized a project called the Old River Control Structure, which has prevented the Mississippi River from leaving its current channel that drains into the Gulf via New Orleans.
  
  Because the large scale of high-energy water flow threatened to damage the structure, an auxiliary flow control station was built adjacent to the standing control station. This $300 million project was completed in 1986 by the Corps of Engineers. Beginning in the 1970s, the Corps applied hydrological transport models to analyze flood flow and water quality of the Mississippi. Dam 26 at Alton, Illinois, which had structural problems, was replaced by the Mel Price Lock and Dam in 1990. The original Lock and Dam 26 was demolished.