From The Canyon Courier (Stephen Knapp):
Granted, the violent spectacle of Cactus Jack’s awash in a white-capped brown torrent, coupled with YouTube videos of pedestrian bridges being bullied down the middle of Upper Bear Creek Road, tend to make a body fretful. But reports of the dam’s impending demise have been greatly exaggerated at least once per generation for nearly 100 years, and they’re no more valid today than they were in 1934, when creek flows soared to more than 4,600 cubic feet per second (cfs), or in 1957, when they rose to 1,640 cfs, or last month when the canyon roared with up to 1,325 cfs. Fact is, when the sun finally burns out some 5 billion years from now, Evergreen Dam may still be here to mark the occasion.
“It’s not going anywhere,” says Evergreen Metropolitan District general manager Gerry Schulte. “It doesn’t really matter how much water is in Bear Creek. It has no real effect on the dam.”
To understand why, consider first the dam’s purpose. Unlike, say, Bear Creek Lake Dam, which is intended to catch and contain floodwaters, the Evergreen Lake Dam was designed by the engineers of Denver Mountain Parks simply to provide a static 900-acre-foot reservoir (about twice its current storage) for the dual purposes of aquatic recreation and visual appreciation. Because it’s always at capacity, adding more water at the top doesn’t meaningfully increase its load.
“It’s a flow-through dam, which means water coming into the lake just flows out again over the top of the dam,” Schulte explains. “Water flows into the tub, water flows out of the tub. It doesn’t make any difference to the tub.”
Now consider the Evergreen Dam’s construction. Begun in 1926 and completed the following year, it is of that breed sometimes referred to as a “gravity” dam, meaning that its own colossal weight makes it impervious to just about anything nature can throw at it. Engineers poured a whopping 12,000 cubic yards of concrete into its long central barricade and two monstrous 52-foot abutments, creating a tidy mass that tips the scales at something very near 39 million pounds of rock-hard resolution and was, at the time, considered proof against flows in excess of 50,000 cfs. And that seemed to be good enough for most folks until 1973, when moderate rains conspired with rapid spring snowmelt to dump water into Bear Creek at a punishing rate of 1,480 cfs, inspiring residents to suddenly notice three long cracks in the dam’s face. By the time the Big Thompson flooded three years later, folks were looking to the dam’s new steward — the fledgling Evergreen Metropolitan District — for reassurance…
Under normal circumstances, about 2 inches of water flows over the top of Evergreen Dam. Last month that flow topped out at 17 inches.