Rapid City Flood, 1972, and Cloud Seeding: 'Oops'

"Rapid City flood"
Jay Trobec

"Rapid Creek normally flows peacefully through Rapid City, fueled mostly by the runoff of rain and snow from the surrounding hill. But the creek became a killer on the night of June 9, 1972. The forecast called for 'variable cloudiness with a chance of scattered showers and thunderstorms,' and the Institute of Atmospheric Sciences was conducting routine seeding experiments on some small clouds west of Rapid City. Previously, they had mostly used silver iodide as the freezing nuclei in the clouds, but this day they sprinkled three to four hundred pounds of finely ground table salt from the plane. The final seeding flight concluded about five o’clock...."

About foot - 10 to 15 inches - fell on the Black Hills of South Dakota in about 6 hours. The Canyon Lake dam broke, 375,000 gallons of water flushed down natural waterways, and somewhere around 250 people were killed. The exact number given seems to depend on what resource you read.

One reason for the high death toll was that people had built houses in Rapid Creek. The neighborhoods were quite scenic - and Rapid Creek ran down a channel that storm waters had cut in earlier times.

The government agency that was running the experiment said it wasn't their fault. They could be right. Some of the survivors sued, but that didn't go anywhere.

It's possible that the storm would have dumped that rain, no matter what the researchers did. But at the time, people didn't want to think that.

Interest in - and support for - weather modification dropped.

A little more:
"Cloud-Seeding Experiment / Lawsuit"
Vanderbilt University

An abstract of a June 9, 1975, CBS News story.