DENVER (CBS4) – There isn’t much time left for public comment on a U.S. Army Corps of Engineers project to increase the size of Chatfield Reservoir.

The comments period ends Sept. 6.

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The proposal would nearly double the amount of water stored in Chatfield Reservoir. It would flood 587 acres of the park, which would raise the water level up to 12 feet. That would put the existing swim beach and marina under water.

The Audubon Society is worried about the loss of habitat for migratory birds and the impact on recreation.

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“The recreational facilities will be rebuilt, so the swim beach is going to move up … the only problem is when the water isn’t here, which is going to be about seven years in 10, the facilities — the beach will be here and the water will be where it is now,” Polly Reetz with the Audubon Society said.

The reservoir was created for flood control along the South Platte River, but the expansion would allow the area to be used to hold excess water supply.

The Audubon Society says that would only happen in three years out of 10 and would not make the expansion worthwhile.

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Comments about the project are still being accepted. Visit the Chatfield Reservoir Reallocation Study website for more information.