THE WEEKEND of July 31 to Aug. 1, 1976, marking the centennial of Colorado, brought thousands of visitors to Big Thompson Canyon, a rugged and scenic area where the ridge tops of Estes Park loom 9,000 feet high.The canyon walls are steep, with access provided by Highway 34, which parallels the Big Thompson River the entire length of the canyon.

The weekend began with a forecast of 30 to 40 percent chance of thunderstorms. The air was moist, but the day was beautiful, and the thousands of residents and tourists in Big Thompson Canyon saw no sign of two massive fronts moving into the area.

On Saturday evening, large thunderstorms -- some more than 50,000 feet high -- marched in, dumping as much as eight inches of rain in two hours. The normally placid Big Thompson River received a large infusion of water from numerous small tributaries between Lake Estes and Drake.

The first hint of trouble came when Colorado State Patrolman Bob Miller, on duty in Estes Park, was asked to follow up reports of rock and mud slides on the canyon road. At 8:35 p.m., Miller radioed this message for the campers thereabouts:

"Advise them we have a flood. The whole mountainside is gone. We have people trapped on the other side (of the river). I'm going to have to move out. I'm up to my doors in water. Advise them we can't get to them. I'm going to get out of here before I drown."

Meanwhile, Sgt. Hugh Purdy of the Colorado State Patrol headed for Drake, trying to find the headwaters of the flood.

His last transmission came at 9:15:

"I'm right in the middle of it. I can't get out . . . about one-half mile east of Drake on the highway. (Tell the cars) to get out of the low area down below."

Purdy's body was found about eight miles downstream from the site of his last radio report. His vehicle was ground up beyond recognition and was eventually identified only by a key ring inscribed Colorado State Patrol.

The floodwaters swept the canyon, carrying away homes and motels along the stream edge. The Loveland hydroelectric plant was removed in its entirety. Hundreds of automobiles were ground into fragments by tumbling, flood-borne rocks. At the mouth of the canyon, a battering ram of debris, including large propane tanks heard to emit an eerie whistle through punctures in their walls, struck and dislodged a gigantic water siphon installed as a Bureau of Reclamation water diversion project. The pipe weighed 227,000 pounds and was filled with an estimated 873,000 pounds of water.

The water siphon, deeply imbedded in the mountain wall, "came out like a big soda straw," according to Larimer County Sheriff Bob Watson.

The wall of water, as it passed the narrowest part of the canyon, was 19 feet above the original stream bed. Freed from the confines of the canyon wall, the flood then fanned out, devastating flatlands for miles downstream.

Cleanup and repair operations continued for more than a year after the Big Thompson Canyon floodwaters subsided. The death toll was 139, and property damage was estimated at $35 million.

Condensed from "Disaster by Flood," by Lee R. Hoxit, Herbert S. Lieb, Charles F. Chappell and H. Michael Mogil. Vol 1. "Thunderstorms: A Social, Scientific and Technologhical Documentary." Edwin Kessler, ed. Available through the Government Printing Office.