The Boulder County Commissioners on Tuesday unanimously approved a settlement allowing Denver Water to expand the dam and pool at Gross Reservoir, despite vocal opposition from some residents, after a $10 million mitigation deal was sweetened by $2.5 million to soften construction impacts for neighbors.

Denver Water is likely to vote Wednesday to approve a total of $12.5 million in mitigation and open space donations for Boulder County, after last-minute talks raised the sum.

The commissioners said they were heartsick at the destruction the dam expansion will cause for neighbors and for revered county open lands. But, they added, county attorneys advised them that federal laws preempt their planning process because the existing dam includes a hydroelectric generator and is therefore controlled by federal laws.

The attorneys said Boulder County would lose a federal suit filed by Denver Water and that the agency would withdraw its mitigation offer if they delayed a vote.

Denver Water already has the federal approval it needs to raise the dam on South Boulder Creek by 131 feet, and inundate the surrounding forest for 77,000 more acre-feet of storage, nearly tripling capacity.

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