The 1939 Flood
On March 17, 1936, Johnstown experienced a devastating flood caused by heavy runoff from melting snow and three days of rain. Before the waters receded the following day, the flood had risen to 14 feet in some areas. About two dozen people died in the flood, while 77 buildings were destroyed-- nearly 3,000 more were severely damaged. Property damages were estimated at $41 million.
The disaster became the catalyst for major federal support to rehabilitate Johnstown. The Works Progress Administration (WPA) mustered every available man in a four-county area to provide assistance-some 7,000 men and 350 trucks were set to the task of digging out the town. After the flood wreckage had been cleared, long-term public works programs began, such as replacing sidewalks, roads and bridges.
But Johnstowners wanted more, and the White House was swamped with 15,000 letters from local people pleading for help. President Franklin D. Roosevelt responded by touring Johnstown, and authorizing the U.S. Army Corps of Engineers to channelize the rivers through town, at a cost of $8.7 million. The goal of the Local Flood Protection Program was to increase the capacity of the rivers to prevent future flooding.

Taken from: The Johnstown Area Heritage Association Website