Colorado: Dillon Reservoir dropping fast

Water level still 20 feet higher than during 2002-2003 drought

Even at a low level, Dillon Reservoir still shines at sunrise.
Dillon Reservoir’s water level has dropped to its lowest level since 2007.

By Bob Berwyn

SUMMIT COUNTY — Dillon Reservoir has dropped to its lowest level in about seven years — since May, 2007, when the elevation of the lake surface was at 9,002.21 feet and rising.

The current elevation of the reservoir (9,002.53) feet, is 15 feet below full and about 10 feet below the average elevation for this date, which is about 9,012 feet

Most recently, the reservoir dropped to near the current level in spring of 2009, when it hit 9006.72 just before the start of the runoff season in mid-March.

The current level may seem low, but the water dropped more than 40 feet lower in 2003 during Colorado’s last significant drought, reaching its lowest point on March 18, 2003, at 8,960.86 feet.

The reservoir will continue to drop the next few months, said Bob Steger, Denver Water’s manager of raw water supply, explaining that the combined current outflow through the Roberts Tunnel and the Blue River far exceeds the inflow of the streams that feed the reservoir.

On the shallow west end of Dillon Reservoir, miles of lake bottom are now exposed.

As of Aug. 28, Denver was was diverting about 380 cubic feet per second through the tunnel, with about 53 cfs flowing downstream into the Lower Blue.

The Snake, Blue Tenmile and other minor tributaries combined for an inflow of about 145 cfs for the month of August. The average August inflow is about 270 cfs, Steger said.

At this rate, the reservoir will drop about 12 feet by the end of October. If the weather turns very dry, it could drop another 10 feet; if late summer and fall are wet, the level could be a little higher going into the cold season.

The rate of diversion through the Roberts Tunnel could slow in the next few weeks as lawn watering demands decrease, but the immediate forecast for Denver is for continued hot and dry weather.

Steger said Denver Water plans to operate the tunnel throughout the fall and winter, which won’t affect Keystone’s ability to use some of the water for snowmaking.

Some years the tunnel is shut down for maintenance in the fall and part of the winter, but in many years — 16 of the last 20 — Denver Water continues to divert at least some water during the winter.

it’s going to user demand. By running the Roberts Tunnel, won’t have to take as much out of Cheeseman,” he said.

Steger said part of the idea is to balance water levels in Dillon Reservoir and the reservoirs on the South Platte River, where flows have been exceptionally low.

“We’re trying to manage system so that we can store as much water as possible during the spring runoff,” he said.

It’s too early to say whether Dillon Reservoir will fill next year.

“It depends what happens between now and next spring. If we have normal or wet weather between now and next spring, then an average 2013 would probably fill the reservoir,” he said. “If we have a dry fall and winter, then we’d probably need a better than average 2013 to completely fill the reservoir.”

Overall, Denver Water currently has 20 percent more water in storage than during the 2002 drought, in part thanks to conservation efforts by Denver Water customers, Steger said, adding that the utility’s storage system is still 79 percent full, compared to the historic median of 92 percent for this date.

Since the 2003 drought, Denver Water customers have cut their per capita water usage significantly. The average per capita use between 1993 and 2001 was about 211 gallons per day; post-drought (2005-2011) the average dropped to about 169 gallons per capita, per day.
Even in this record hot and dry year, with a record number of 90-degree days, Denver Water expects per capita water usage to stay close to that amount. By comparison, the amount was much higher, at about 192 gallons, in 2006, with similarly hot and dry summer.

Published by Bob Berwyn

Environmental journalist covering climate change, forests, water, mountains and biodiversity.

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