Hailey Wilmer/Colorado Daily

Pictured is a view of Gross Reservoir taken on Tuesday afternoon. A proposal to enlarge the dam below Gross Reservoir would increase the water holding capacity in order to serve the needs of Denver water customers.

Gross Reservoir, west of Boulder, already holds lots of water – but a growing metro region needs stable water supplies, and growth at Gross could provide some system stability.

Plans are being considered to substantially expand Gross, which is located about 10 miles west of the CU campus along Flagstaff Road, and is also used for recreation. Gross currently has a capacity of about 41,000 acre-feet, and one option on the table would expand the capacity to about 113,000 acre-feet.

(An acre-foot, about 325,000 gallons, is the volume generated by flooding an acre of land with one foot of water.)

Despite its proximity to Boulder, Gross is a Denver Water Department holding that provides part of Denver Water’s supply. Also despite its proximity to Boulder, Gross is primarily filled with Western Slope water delivered through the Moffat Tunnel.

Travis Bray, Project Manager for Denver Water, said the current project has three primary objectives. One would be to increase Denver Water’s overall supply, and a second would be to ensure supply reliability on the district’s north end.

The third objective would be to decrease the vulnerability of its water supply, since Bray said about 80 percent of its water goes through the Strontia Springs Reservoir, located southeast of Conifer. He said an event that impacts the Strontia watershed could lead to serious problems for Denver Water and its customers.

For example, the loss of vegetation from the catastrophic 2002 Hayman Fire and the 1996 Buffalo Creek Fire exposed area terrain to water erosion, and the raw water supplies took on unwanted sediment. Bray said problems at Strontia could force Denver Water to rely on a limited number of sources, including Gross, to supply its thirsty customers.

But Bray’s Moffat Collection System Project is currently going through an Environmental Impact Statement (EIS) process. The U.S. Army Corps of Engineers is expected to release a Draft EIS (DEIS) this summer, and the process includes a period of time after the DEIS release for public comment.

While the possible Gross expansion is not exactly a City of Boulder project, it is on the agenda for Wednesday’s city Open Space Board of Trustees (OSBT) meeting, which starts at 6 p.m. in Council Chambers, 1777 Broadway.

The OSBT will receive a briefing on the project, and the agenda packet included a bullet-point list of “anticipated” environmental impacts on the South Boulder Creek (SBC) watershed.

For example, the list said reservoir enlargement might lead to inundation of additional portions of the SBC or gulches in the vicinity. It said a project might impact general riparian habitat, impact threatened Ute Lady’s Tresses habitat, impact stream hydrology or decrease stream flows at certain times.

“But it wouldn’t reduce minimum (SBC) flows that are already there,” said Bray. “We hold fairly junior water rights on the Creek, and the years that we remove water from the Creek would be mostly in wet years, or above-average years.”

Actual environmental impacts, of course, could depend on the details of any project that might be approved, and Bray said the DEIS will include a significant amount of environmental information.

The Denver Board of Water Commissioners recently selected one of five options as its preferred alternative. The option includes adding 118 feet in height to the existing dam, as well as adding 386 acres of surface area to the reservoir, at a 2006 estimated cost of about $140 million.

The Board-preferred option was less than half the cost of the second-least expensive option, although Bray said the cost of concrete and other raw construction materials are increasing. He also said construction could begin in 2012, and a project could be complete by 2016.

FYI

Wednesday’s Open Space Board of Trustees meeting begins at 6 p.m. in Council Chambers, 1777 Broadway. For Wednesday’s agenda, visit the City of Boulder’s Web site, www.bouldercolorado.gov, click “Calendar” and select the June 11 OSBT meeting. The Gross Reservoir update is not the only item on the agenda.

For more about the Moffat Collection System Project EIS, visit the Web site http://www.denverwater.org/MOFFAT—EIS.html. Note: the dash between MOFFAT and EIS should be an underscore.

Contact Richard Valenty about this story at (303) 443-6272 ext. 126, or at valenty@coloradodaily.com.