1976 Big Thompson River flood

On July 31, 1976, heavy rainfall caused the Big Thompson River in Colorado to crest, causing at least 144 deaths, more than 250 injuries, and least 5 others to be missing. The crest was caused by a stalled thunderstorm complex that produced rainfall totals of 12-14 in near Estes Park, Colorado, including 7.5 in of rain falling in one hour. After cresting at 30 ft, widespread damage and flooding occurred along the river, with the damage totaling almost $150 million (2016 USD). The flood is considered one of the deadliest floods in the state's history.

Background
The Big Thompson River is a tributary of the South Platte River, approximately 78 mi long, in the U.S. state of Colorado. Originating in Forest Canyon in Rocky Mountain National Park, the river flows into Lake Estes in the town of Estes Park and then through Big Thompson Canyon.

Before the floods, 600 people lived at the Big Thompson Canyon, and between 2,500 and 3,500 people were also at the Big Thompson Canyon to celebrate 100 years after Colorado's statehood that would take place several hours later on August 1.

Meteorological synopsis
On July 31, a thunderstorm complex developed alongside the Front Range of the southern Rocky Mountains, near Estes Park, Colorado. The development was caused by a number of meteorological factors, including a shortwave trough, a polar front that moved across southeastern Colorado with the main polar airmass northeast of the Front Range, and moisture and unstable air behind a cold front extending across the Big Thompson Valley. Upper-atmospheric light winds caused the thunderstorm complex to stay stationary, which produced heavy rainfall across portions of the Big Thompson Canyon, including up to 7.5 in of rain falling in one hour. Over four hours near Estes Park, 12-14 in of rain fell, causing the Big Thompson River to overflow its banks and triggering a flood crest that moved through the river and the Big Thompson Canyon. The flood crest moved at an average speed of 15 mph, and crested at 30 ft. A flash flood warning was issued at 23:00 Central Time (5:00 UTC), but was criticized because the warning was issued only several hours following the flood.

Impact and aftermath
A portion of U.S. Route 34 was washed out by floodwaters, sustaining significant damage as visibility on the highway dropped to zero miles. The flood crest impacted most of Cedar Cove, Drake, and Midway, and causing extensive damage to buildings in Glen Haven. The floods caused more than $40 million in damage in 1976 USD, equivalent to nearly $150 million in 2016 USD, including 418 homes, more than 400 vehicles, and 152 businesses being destroyed, along with a further 138 buildings damaged. The flood swept people as far east as Interstate 25 and areas 25 mi away. Mudslides caused by heavy rainfall also closed Poudre Canyon. The flood is considered one of the deadliest floods in the history of Colorado, causing at least 144 deaths and 250 injuries, along with at least 5 missing.

On August 1, over 800 people were evacuated from flood-impacted areas via helicopter, and were taken to a high school in Loveland, Colorado that was established as a rescue center by the American Red Cross. U.S. Route 34 was reopened 86 days after it was closed after portions of the highway were washed out. United States President Gerald Ford designated Larimer County, Colorado a disaster area. In 2001, 25 years after the flood, a ceremony was held every year to remember the flood. A memorial was constructed off U.S. Route 34, several miles from Drake, Colorado, containing a list of people killed during the flood.

In popular culture
The Colorado country music singer Chuck Pyle wrote a song titled "Here Comes the Water" celebrating the actions of Colorado State Patrol Sgt. Willis Hugh Purdy who raced the flood down the canyon warning residents. On October 2, 2016, an episode of Colorado Experience premiered on PBS regarding the disaster.