Dillon Reservoir near full as last of spring runoff threatens flood in Summit County
The water just keeps rolling down the mountains. As the last of the spring runoff trickles into the tributaries that feed Dillon Reservoir, Summit County emergency officials continue to watch outflows below Dillon Dam as the reservoir reaches full capacity.
While there has been no major flooding in Summit thus far, heavy precipitation in late June and an extended winter led the National Weather Service to issue a river flooding advisory for the Blue River this past Thursday.
The dam — which was built in 1963 for water storage and not flood mitigation— does not have a whole lot of control on how much water flows out of it through its morning glory spillway. As a result, residents in Silverthorne on or near the Lower Blue River below the dam face flooding risks every summer as the reservoir fills.
Nathan Elder, water supply manager for reservoir owner Denver Water, reported Friday that the reservoir was just under a foot from being full, with 2,600 acre-feet of storage space remaining. Elder predicted the reservoir would fill in about two days.
The latest inflow data showed 2,219 cubic feet per second flowing into the reservoir, while 1,840 cfs is flowing out. Elder said that, while the dam wasn’t meant for flood control, the flows in the Lower Blue would be much stronger if the dam wasn’t there at all.
“We constantly try to balance inflows with outflows,” Elder said. “If the dam wasn’t there, flows below the reservoir would be close or at 3,000 cfs.”
Elder said the Roberts Tunnel, which channels water from the reservoir to the Front Range, was currently off and not bringing water to the Eastern Slope. Denver Water will continue adjusting flows for the reservoir to keep it at full capacity until Nov. 1, when the reservoir is lowered 3 feet to leave room for snow precipitation.
Elder said Denver Water has been conducting twice-daily briefings with county emergency officials, updating the forecast on flows into the Lower Blue. Summit County emergency director Brian Bovaird said that all tributaries in the county were at or just below “action stage,” or when county flooding preparations take effect.
Bovaird said there is a possibility Denver Water will increase flows below the dam to up to 1,900 CFS by this weekend, close to the highest flow recorded below the dam. However, he said there was good news from the National Weather Service, which predicted no heavy rain this weekend to push the rivers over the edge.
“A sustained rain event would definitely tip the scale toward flooding,” Bovaird said. “Right now we’re at peak flow. Everything is running as fast as it should for the rest of this year, barring anything super crazy. So that’s good.”
Bovaird said that emergency officials will start to get concerned if the outflows rise to 2,100 CFS. But for now, Bovaird said he didn’t expect any major flooding to occur when the peak flows finally peter out next week. Bovaird reported some “nuisance” flooding in Silverthorne’s South Forty neighborhood, but it did not cause any structural damage or threaten homes.
Bovaird added things were looking good at the Goose Pasture Tarn dam, which was built in Breckenridge in the ’60’s and has been a source of concern due to the potential for flooding or even collapse. Tenmile Creek, which approached flood stage a few weeks ago, peaked last week without any significant flooding or damage.
Regardless of the forecast, Summit County government still advises residents to be extra cautious the next few weeks with the elevated water flows. Residents wishing to prepare for flooding can go to SummitCountCo.gov/flood for the county’s swift water safety and flood preparedness guide.
Residents can also pick up sand bags to build flood barriers at the county’s Road and Bridge office located at the County Commons, 37 Peak One Drive in Frisco. Sand bags are also available at eight other Summit locations listed in the flood preparedness guide. The first 100 sandbags are free, while additional bags cost 50 cents each.
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