July 27 is the latest fill date in recent memory, but careful operations helped reduce downstream flooding risk while ensuring maximum usable supplies
By Bob Berwyn
SUMMIT COUNTY — After weeks of carefully juggling inflows, releases through the outlet works and diversions through the Roberts Tunnel, Denver Water officials said Dillon Reservoir filled completely July 27 and started spilling through the overflow drain, known as the glory hole.
It may be the latest the reservoir has ever filled, said Bob Steger, the Denver Water engineer who leads a team that computes the inflow forecasts and other information used to adjust the valves on the outlet works, sometimes on a daily basis.
Read more about the outlet works and the glory hole at this Summit Historical Society web page. And Denver Water also maintains an interesting website about the construction and history of Dillon Reservoirs with a gallery of historical photos.
The general idea is to fill the reservoir, which maximizes Denver’s water supply from the key reservoir, helping to maintain adequate supplies downstream in other storage buckets in the South Platte drainage. At the same time, Denver Water considers recreation needs in the reservoir (water levels at local marinas) and downstream in the Blue River (fishing and rafting) and the potential for flooding in Silverthorne neighborhoods. On top of all that, the water provider needs to anticipate downstream calls for water, from ranchers and fruit growers around Grand Junction and from hydropower providers.
This year was especially tricky. A near-record snowpack led to predictions of record runoff, but unseasonably cool weather lasted until late into the spring, delaying the runoff and setting up conditions for an almost unmanageable surge of runoff that, in the end, didn’t materialize.
To prepare, Denver Water drained Dillon Reservoir to levels not seen since the spring of 2003, following a historic drought, then slowly started to fill it again, all the while warily watching flows in the Lower Blue and in the key tributaries feeding the reservoir, where flows at times surged well above 2,000 cubic feet per second. Despite the drawdown, flows in the river below the dam ran perilously close to flood levels for several weeks, prompting warnings to boaters and even restrictions on river access through Silverthorne.
With the reservoir full, Denver Water cut the outflow to 500 cfs Wednesday morning.
“After we make that cut, the outflow should gradually increase as more and more water comes over the spillway,” Steger wrote in his regular email to update local officials on reservoir operations. “Unless the inflow drops below 500 cfs, we will attempt to keep at least 500 cfs outflow from now through the weekend for the sake of the rafting companies.”
By next week, Denver Water intends to reduce the outflow to 100 cfs through the power plan, with the remainder of the outflow coming over the spillway. At that point, the total outflow will depend on how much water is coming down the Blue and Snake rivers, Tenmile and Straight creeks, and some of the other smaller tributaries — and whether Denver Water starts diverting through the Roberts Tunnel.
As of July 27, the combined inflow was 936 cfs, still well above average for this date, but not a record. As recently as 1995, the July 27 inflow was 1,134 cfs. The lowest July 27 inflow in the last 10 years was during the epic drought of 2002, when all those streams and rivers were only delivering a combined flow of 75 cfs to the reservoir.
By comparison, the inflow last year at this same date was 232 cfs and the 10-year average is 295 cfs.
A few more for comparison:
2009 – 388 cfs
2008 – 424 cfs
2007 – 400 cfs
2006 – 376 cfs
2005 – 244 cfs
2004 – 237 cfs
2003 – 270 cfs
2002 - 75 cfs
2001 – 317 cfs
Some other fill-and-spill dates:
2001 – June 17
2002 – did not fill, highest elevation was 8,993.17 feet on June 12
2003 – did not fill, but got within about 1.5 inches of the lip of the glory hole
2004 – did not spill, highest elevation was 9,010.38 on August 10
2005 – June 19
2006 – June 4
2007 – June 5
2008 – June 27
2009 – May 25
2010 – May 24
The reservoir filled later during some wet years, when Denver Water draws the reservoir down to make room for more inflow, for example in 1996, when it filled June 21 and 1995 (July 12) and 1983 (June 25).
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