On July 31, 1976, the natural and man-made basins and reservoirs above Colorado’s Thompson Canyon reached a critical tipping point. Several unusual rain systems seemed to have snagged on the rugged Rocky Mountain peaks. It was as if the clouds were punctured by the craggy spears of rock and the rains gushed out relentlessly. Finally, somewhere up there, a tipping point was reached. A “tipping point” occurs when clusters of small events — like repeating cloudbursts — become weighty enough to cause sweeping destruction. So, somewhere in the Estes Park region a critical mass of rainfall had accumulated in a...