From CBS Denver:
NOTABLE COLORADO FLOODS
June 3, 1921 – The city of Pueblo was devastated after a cloud burst west of town produced a major flash flood along the Arkansas River. Heavy rain north of town sent massive amounts of water rolling down Fountain Creek, which added insult to injury. The death toll was in the hundreds, with some estimates around 1,500. There was roughly 20 million dollars in damage. In some places, the water was 15 feet deep in the downtown area.
May 30-31, 1935 – Widespread slow-moving thunderstorms produced extensive flooding along Monument Creek in the Colorado Springs area. The water was estimated to be 32 feet deep near America the Beautiful Park. At least 18 people lost their lives. Record flooding also occurred along the Republican River of eastern Colorado with 6 people killed.
June 14-20, 1965 – Widespread flooding took place along the Arkansas River and South Platte River, claiming at least two dozen lives and causing millions of dollars in damage. The event changed the face of Denver as we know it and prompted huge upgrades to flood control, including the construction of Chatfield Dam.
July 31 – Aug. 1, 1976 – A catastrophic flash flood swept out of the mountains, through Big Thompson Canyon and onto the adjacent plains, killing at least 139 people and causing over 35 million dollars in damage.
July 28, 1997 – Six weeks of hot, dry weather gave way to a weather pattern that produced thunderstorms with torrential rains that unleashed a deadly flash flood in the city of Fort Collins.
April 29 – May 2, 1999 – Over a foot of rain created a major flood in southeast Colorado that impacted communities from Colorado Springs to La Junta.
Sept. 9-16, 2013 – A complex weather pattern produced torrential rain along the Front Range of Colorado, unleashing deadly flash floods in and near the foothills, which lead to a major river flood event for the South Platte River valley of northeast Colorado. Entire towns were cut off from the outside world for days, with many rescued by helicopter. Hundreds of miles of state highways were damaged or destroyed.