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Big Thompson's Sudden Tragedy
Combine the celebration of Colorado's statehood centennial with the grandeur of summer in the Colorado Rockies and you can well imagine the scene along Highway 34 in the Big Thompson Canyon as July 1976 ended. Many tourists and Coloradans, estimated at several thousand people, had congregated in the river valley for some hiking, fishing, camping and relaxing in the cooler mountain air. Many were settling into the valley's campgrounds at the popular site as night fell.
The Big Thompson River basin is typical of many river basins along the eastern slopes of the Continental Divide. The river begins high in the Rockies near Estes Park in Colorado flowing eastward through a rugged, steep-walled canyon of sheer rock, where in places, such as "The Narrows" at the canyon's east end, the canyon walls rise almost vertical. Elsewhere, the canyon's walls slope more gently, supporting a forest of Douglas fir and ponderosa pine. Overall, little soil and vegetation cover the canyon floor to absorb runoff from storm rainfall. From its head, the river drops vertically over 800 metres (half a mile) and exits the canyon onto the rolling, forested plains west of Loveland. US Highway 34 stretches the length of the canyon with homes, restaurants and other businesses along the highway.
Thunderstorms often form on the slopes of Colorado's Rockies as most air ascends the Continental Divide. Winds found at these mountain ridges are usually strong enough to push thunderstorms eastward over the Plains. On 31 July, 1976, moist easterly winds rose up the Big Thompson canyon and surrounding mountain slope. Combined with daytime heat, the moist air spawned thunderstorms along the Front Range. But something was different this day; the upper-level winds were extremely weak, not strong enough to push the storm away from the Big Thompson Valley.
That evening, a tremendous thunderstorm had formed over the Continental Divide near the headwaters of the Big Thompson River. Unlike most storms, this one was caught in a wind pattern that held it nearly stationary over the valley. For over three hours this evening, the storm sat and dumped 300 mm (12 inches) of rain into the canyon. Two hundred millimetres (eight inches) of rain fell in a one hour-long stretch.
The deluge turned the normally placid two-foot-deep river into a raging torrent of water 5.8 metres (19 feet) high. At Drake, about halfway down the canyon, the river flow was estimated at 3.84 cubic metres (137 cubic feet) of water a second before the rain started at around 6 pm. By 9 pm, the gages showed it carried 874 cubic metres (31,200 cubic feet) of water a second.
A deadly flash flood was sweeping down through Big Thompson Canyon. Mountainous regions are ideal locations for disastrous flash flooding because of their steep slopes and narrow stream and canyons. And the situation was looking grim for the unsuspecting campers below. Sweeping 3-metre (10-ft) boulders in front of it, the wall of water raced down the canyon toward the highway and campgrounds. By 7:30 pm, mud and rock slides had blocked Highway 34, and the Big Thompson's waters were rapidly rising.
The flood's victims had little or no warnings of the approaching torrent. The first official alert came around 8:30 pm when Colorado State Patrolman Bob Miller radioed to headquarters: "Advise them we have a flood. I'm up to my doors in water." Those in cars, campers, and buildings in the torrent's immediate path had no chance of survival, for in the canyon, the only avenue of escape was up the nearly vertical canyon walls. Those who stayed in their cars died. Rushing water smashed vehicles against rocks and ripped them to pieces. A State Patrol vehicle manned by Sgt. Hugh Purdy was one of those demolished, the car only identifiable by a key ring labelled "Colorado State Patrol." Others, unable to escape the rising water which rose over 4 m (14ft), scrambled up the steep canyon walls and spent the night in the chilling rain before being rescued the next day.
Some of the survivors commented that the roar of the river as sounding like jet engines. At the canyon's east end, the water piled up into a 5.8-metre (19-foot) wall in The Narrows composed of car, building and other debris, including hissing propane tanks. The torrent wretched a 114-ton water pipe from it supports at the mouth of The Narrows, and carried the pipe and its estimated 237,000 tons of water, from the canyon's walls a quarter of a mile downstream.
In two hours, the Big Thompson Canyon flood killed 145, including six persons whose bodies were never found, and injured 88 people. It demolished 418 houses and damaged another 138. It destroyed 152 businesses with the damage estimate exceeded $40 million. Three kilometres (1.9 miles) of US 34 were washed out.
Yet the situation could have been worse. The torrent rushing down the Thompson River's North Fork peaked 40 minutes after the Main Fork peak; had they coincided, the wall of water would have been much higher.
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