As the rainy season approaches, we can look forward to typical Oregon rain storms. Sometimes we get a series of moderate storms back-to-back (as happened last winter). In other cases, a single large event can affect us. In both cases, local or widespread flooding can occur.
But typically our weather would be characterized as “frequent light to moderate rains.” We get a lot of rainy days, but most daily totals don’t amount to much. Floods, when they occur, are seldom life-threatening.
Awhile ago I asked readers if they had experienced any extreme weather events and would be willing to describe them. A few people responded, describing thunderstorms and rain events. Among the responses was a phone call from Merrit Jensen of Corvallis, who lived through one of the biggest and deadliest floods in U.S. history: the Big Thompson Canyon Flood of 1976.
According to weather.com, “It was the height of tourist season in Big Thompson Canyon, a popular camping area an hour west of the city of Denver, Colorado.
“As several thousand people enjoyed hiking, fishing and relaxing at their campsites on the pleasant summer afternoon of July 31, 1976, they had no way of knowing that tragedy would strike in just a few short hours.
“A weak but moist easterly flow was forming on the east side of the Rockies. The air rose up the mountain slopes, enhanced by daytime heating which caused cumulus clouds to spring up. As the afternoon wore on, benign cumulus clouds developed into cumulonimbus clouds, then into a thunderstorm accompanied by heavy rain.”
The Big Thompson River basin is similar geologically to many river basins along the eastern side of the Continental Divide. Sheer rock forms the canyon walls, with little soil and vegetation to absorb runoff from storms. When heavy rain occurs, massive runoff follows.
Merrit was driving from Estes Park to Loveland. The rain began, soon getting so heavy he had to stop. He parked in a high spot near a cabin. He saw 4 people in the cabin. Soon the cabin was surrounded by water, and the lights went out. The water continued to rise. Merrit drove away, barely able to negotiate the road.
Down the road he found another high spot. Another car and a telephone repair truck were already there. The drivers watched, horrified, as the water continued to rise, into the parking lot, then halfway across the highway.
But at last the rains ended and the flood waters started to recede. The road to Estes Park was completely washed away. Many riverfront cabins were gone. Within two hours, the Big Thompson flood killed 145 people (including six who were never found), destroyed 418 houses and damaged another 138, destroyed 152 businesses and caused more than $40 million in damages.
During a four-hour period, 10″-14″ of rain fell on the area, an amount equal to a whole year of normal precipitation. And it couldn’t have come at a worse time. It happened in the middle of Colorado’s Centennial celebration, a three day weekend when the canyon was packed with tourists. Between 2500 and 3500 people were in the canyon, filling motels and camping spaces, relaxing in their summer retreats or retirement homes. Since the canyon is rather narrow, everyone was close to the river.
As a result of the flood, regulations were adopted to limit building along the Big Thompson River and other, similar rivers throughout the United States. The tragedy was also a major impetus nationwide for creating early warning systems for flash floods in mountainous cities and recreation areas.
Scientists have debated how rare the event had been. Ed Tomlinson, a scientist from Colorado who has studied floods extensively, told me “it was somewhere between a 500-year and a 10,000-year event – in other words, very, very rare.”
I’m grateful to Merrit for sharing his eyewitness account. Let’s just hope we don’t see the likes of that flood around here!
– originally published in 2006 in Weather Matters