In 1976 the Big Thompson had another flood similar to this one. These canyons are extremely narrow, with nowhere for the water to spread out.
For those not around in the early evening of Saturday, July 31, 1976 :
a large stationary thunderstorm released as much as 7.5 inches of rainfall in about an hour (about 12 inches in a few hours) in the middle reaches of the Big Thompson River Basin and to a lesser extent in parts of the Cache la Poudre River Basin.
http://pubs.usgs.gov/fs/2006/3095/