The Boulder County commissioners on Monday night declined to sign an intergovernmental agreement with Denver Water, withholding their approval for a planned expansion of Gross Reservoir.

About 40 residents and environmentalists, in a public hearing that lasted three-and-a-half hours, urged the county commissioners not to sign the agreement, citing concerns that ranged from noise to traffic safety to Denver Water's need to conserve instead of expand.

"We get some useful things out of this intergovernmental agreement, but fundamentally it's not enough in protecting the residents," said Commissioner Will Toor.

The project, known as the Moffat Collection System Project, would raise the height of the existing dam at the reservoir by 125 feet, allowing it to store an additional 72,000 acre-feet of water for use during droughts in the Denver area. Denver Water expects the project to require a two-year planning phase followed by two to four years of construction.

The intergovernmental agreement would have established a pool of at least $500,000 to compensate area residents and another $2 million the county could use to limit urban sprawl or pay out to residents. Another $4 million would have gone toward the preservation of land around Gross Reservoir.

Another $500,000 would have been contributed to use for forest treatments on private property, $1 million to assist the Coal Creek Canyon Parks and Recreation District with its master plan goals and $250,000 for recreation projects recommended by the Preserve Unique Magnolia Association.

Other conditions included requiring dust mitigation on gravel roads and improvements to Colo. 72 and affected county roads.

But, public speakers said, the limits were too vague and the amount of money that would be contributed much too low.

"Don't take the minuscule bribe that Denver Water is proposing," said Coal Creek Canyon resident Anita Wilks. "No amount of mitigation will be enough."

Residents' concerns included the potential for deadly accidents with construction trucks on narrow mountain roads and the environmental impacts, with people calling the project an "environmental catastrophe."

Others said Denver Water should better conserve water instead of expanding Gross Reservoir and noted that, based on climate change, there may not be enough water available from the Colorado River Basin to fill the expansion.

Residents also asked the commissioners to wait until the final environmental impact study is released -- a point that the commissioners agreed with. The final impact study is expected to be out soon.

Todd Adelman, who lives in Nederland, said Denver Water is working from short-term thinking. He said he's concerned that the additional water will be sold to hydraulic fracturing operators, given that Denver Water plans to have the expansion complete by 2020 but doesn't need the water until 2030.

"It is a flawed plan," he said.

As Denver Water moves forward with obtaining permits for the project, the Boulder County commissioners may renegotiate an intergovernmental agreement or invoke their state-granted "1041 powers" -- which give local governments say on projects that have statewide impacts.

County staff members, who recommended approval of the intergovernmental agreement, have said a 1041 permit denial likely would result in a legal fight.

County open space attorney Conrad Lattes previously said that Denver Water has contended that Boulder County's 1041 powers would be pre-empted by a Federal Energy Regulatory Commission permit that Denver Water is seeking. Although the county has not conceded that issue, he said the agreement would give the county more flexibility than the 1041 process and would avoid extensive litigation.