On June 9–10, 1972, extremely heavy rains over the eastern Black Hills of South Dakota produced record floods on Rapid Creek and other streams in the area. Nearly 15 inches (380 mm) of rain fell in about 6 hours near Nemo, and more than 10 inches (250 mm) of rain fell over an area of 60 square miles (160 km²). According to the Red Cross, the resulting peak floods (which occurred after dark) left 238 people dead and 3,057 people injured.^[8] In addition to the human tragedy, total damage was estimated in excess of $160 million (about $821 million in 2009 dollars), which included 1,335 homes and 5,000 automobiles that were destroyed. Runoff from this storm produced record floods (highest peak flows recorded) along Battle, Spring, Rapid, and Box Elder Creeks. Smaller floods also occurred along Elk Creek and Bear Butte Creek. Canyon Lake Dam, on the west side of Rapid City, broke the night of the flood, unleashing a wall of water down the creek. The 1972 flooding has an estimated recurrence interval of 500 years,^[9] which means that a flood of this magnitude will occur on average once every 500 years. Every year there is a 0.2 percent chance (1 in 500) of experiencing a similar event. To prevent a similar tragedy from occurring in the future, the city’s flood plain is no longer allowed to be built upon. Today the flood plain features golf courses, parks, sports arenas, and arboretums where neighborhoods and businesses once stood.

In 2007, the Rapid City Public Library created a 1972 Flood digital archive that collects survivors’ stories, photos and news accounts of the flood. The Journey Museum has an interactive display on the 1972 flood which is an ongoing project to give future generations the best idea of how the people were affected and the changes made to it because of the loss of 238 lives. It will in the future include the biographies of all of those who died so they will be remembered as more than names on a memorial.

More Info Here