From the Summit County Citizens Voice (Bob Berwyn):
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…
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.
More Denver Water coverage here.