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Come hell or high water, Evergreen Dam isn’t going anywhere

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By Stephen Knapp

It is an apparently immutable law of hydrology that each uncommon rise in the waters of Bear Creek is attended by a simultaneous and equal rise in public anxiety regarding Evergreen Dam.

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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.

“There was a lot of fear in the ’70s,” says Schulte. “There was even talk of tearing down the dam all together.”

Instead of depriving Evergreen of its lake, the district called in the Army Corps of Engineers, which gave the dam a thorough physical and pronounced it fit as a fiddle. The cracks, it said, were purely superficial and in no way compromised the dam’s basic structure. But the corps is by nature a cautious organization, and it identified a potential weakness that had to be addressed before it would sign off on the patient’s chart. Specifically, corps number-crunchers postulated that, in the event of Bear Creek flows over 60,000 cfs, it was remotely possible that “hydrostatic uplift” could scour the “granular backfill” out from under the abutments, leaving the dam’s flanks effectively untethered to terra firma.

“They were concerned that maximum flows could cause erosion behind the abutments,” Schulte clarifies, “and that could cause the dam to pivot on its axis.”

The solution was simple enough, but it wasn’t easy. To ease the corps’ concerns, in 1980 the folks at EMD drove 18 vertical cores down through the center of the structure — five through each abutment and eight through the dam itself — and kept on drilling another 20 feet into the bedrock below. Into each core they threaded about two dozen sturdy steel cables, cementing them into the bedrock below and fastening them to a foot-thick steel plate above. Then, using a powerful hydraulic ram, they tightened those hefty bundles until not a micron of slack remained, essentially anchoring the entire dam structure to the Earth with bands of steel. The project took about three months and $500,000, and when it was completed the Army Corps of Engineers declared Evergreen Dam capable of withstanding what it calculated to be Bear Creek’s “maximum probable flow” of — get this — 122,000 cfs.

That’s an interesting term, “maximum probable flow.” The largest recorded flood event in Bear Creek Canyon occurred in 1896, with streamflows in the 9,000 cfs range. The Colorado River drains about 250,000 square miles and averages something like 17,500 cfs. The mighty Missouri River collects itself from more than 1.3 million square miles and typically carries about 86,000 cfs. Even the Nile, which drains an area of almost 3.5 million square miles — about 10 percent of the African continent — only manages (after paying necessary dues to Egyptian agriculture) to increase the wine-dark Mediterranean’s stock by just 100,000 cfs. How, one can’t help but wonder, could little Bear Creek, which draws its wherewithal from a hinterland (above Evergreen Dam) of just 164 square miles, possibly be induced to out-flow the River of the Pharaohs?

“It’s hard to say,” Schulte admits. “It would have to be an event where it rained a lot harder than what we just saw, combined with extremely rapid snowmelt.”

Imagine an April downpour of biblical stubbornness happening at the same time that an extra-snowy (and by all accounts geologically stable) Mount Evans Wilderness breaks out in a severe rash of volcanoes, and you might be creeping up on the 120,000-cfs mark. In any case, should such a terrible conjunction of calamities occur, the good people of Bear Creek Canyon and points east would have more important things to worry about than how Evergreen’s dam is getting along.

Under normal circumstances, about 2 inches of water flows over the top of Evergreen Dam. Last month that flow topped out at 17 inches. Should the Army Corps of Engineers’ “maximum probable flow” ever actually materialize, the increase of water flowing out of the tub would be, er, pronounced.

“There’d be a 30-foot wall of water coming over the top of the dam,” says Schulte. “Downtown Evergreen would be long gone.”

Along with Kittredge, and Idledale, and Morrison, and a big chunk of Lakewood, and all the works of Man within flooding distance of the South Platte all the way to Fort Lupton and beyond.

But — and this should be comforting to anyone still harboring doubts — Evergreen Dam would remain exactly where it’s always been, standing proudly erect at the bottom of that enormous river, unmoved and unmoving, indifferently holding its post as an ocean of destruction swept past. Indeed, that decidedly improbable “probable” flow could strip away the very shoulders of native rock that grasp the dam on either side — a very real possibility — without troubling it in the least.

“It wouldn’t even pivot on its axis,” Schulte smiles. “Like I said – it’s not going anywhere.”