Winding down the mesa below the Weather Trail is Bear Creek, normally a placid waterway. During a heavy rain, it becomes a raging torrent. The bare ground on the south side of the NCAR mesa, behind utility poles, indicates erosion caused by repeated heavy rains.

When a rainstorm is focused on a narrow canyon, the results can be devastating. A severe flash flood in the Big Thompson Canyon, about 30 miles (48 kilometers) to our north, killed 145 people on July 31, 1976. About every 100 years, the Boulder Creek floodplain near downtown can expect a flood several blocks wide with the potential for major damage and loss of life.

It is especially dangerous to drive into flood waters. Because water is so heavy, it only takes about 18 inches (46 centimeters) to make a typical vehicle float.