Chimney Hollow, Northern #Colorado’s biggest new reservoir, will likely be one of its last — The #FortCollins Coloradoan #ColoradoRiver #COriver #aridification

From The Fort Collins Coloradoan (Jacy Marmaduke):

Northern Colorado is getting its biggest new reservoir in about 70 years, at the cost of diminished Colorado River flows.

Construction of Chimney Hollow Reservoir will begin in August southwest of Loveland, just west of Carter Lake. An April legal settlement between project proponent Northern Water and environmental advocacy groups cleared the way for the project, which began the permitting process in 2003.

The 90,000-acre-foot reservoir is the main component of the Windy Gap Firming Project, a plan to increase the reliability of Colorado River water rights in the Windy Gap Project. The project’s 12 participants include Platte River Power Authority, Loveland, Broomfield, Longmont and Greeley. Construction is expected to take until August 2025, after which it will take about three years to fill the reservoir.

The reservoir’s water will come from the Colorado River, decreasing flows below Lake Granby by an annual average of 15%. Most diversions will take place in May and June.

The 18-year journey toward construction demonstrates the extensive maneuvering required to build new reservoirs in Colorado as rivers become increasingly stressed from climate change and heavy diversions as growing Front Range communities seek to shore up their water supplies. Northern Water won approval from key government agencies and some advocacy groups with a suite of mitigation measures and spending commitments for areas impacted by the project.

Northern Water spokesman Jeff Stahla described Chimney Hollow as “in the right place at the right time.” The reservoir site has a few qualities that have helped Northern Water avoid some common setbacks for new water project construction: It’s near existing Colorado Big Thompson Project infrastructure, so Northern Water won’t have to build much new infrastructure for water deliveries, and there are no homes or businesses at the site, which Northern has owned since the 1990s.

“The one assumption you have to make is that water storage is part of the future way that we’re going to provide water,” Stahla said, and he thinks it is. “If you get past the ‘Do we need storage’ question, this ends up being an incredible site that will meet lots of needs, including the ancillary needs of recreation, into the future.”

[…]

Northern Water Engineering Director Jeff Drager acknowledged the new reservoir’s impact on Colorado River flows, but he said the project’s targeted mitigation efforts still offer a major value and are a key reason why it crossed the regulatory finish line.

One of the most significant mitigation measures, known as the Colorado River connectivity channel, will involve shrinking the existing Windy Gap Reservoir in Grand County to about half its current size and building a new channel around it. The Windy Gap dam currently blocks the Colorado River, preventing movement of fish, silt and sediment.

The connectivity channel will allow the river below the reservoir to act more like “a stream without a reservoir on it” when Northern Water’s water rights aren’t in priority, Drager said. The mitigation measures will also open up a mile of stream to public fishing in an area where private landowners possess most of the land adjacent to riverbanks…

During wetter years, Lake Granby can overflow and the water that would’ve been delivered to Windy Gap users flows downstream. During drier years, Northern Water is often unable to divert the full extent of its water right because it is a junior right, meaning more senior water users get access to water first. During the 23-year period between 1985 and 2008, for example, no Windy Gap water was delivered for seven of those years.