In our one-on-one interactions with individuals in the community and presentations to civic organizations and environmental groups, we often hear the question about whether the water temperatures at the outflow will change and what impact that might have on aquatic life downstream. Here’s how we respond:

  • Environmental studies show that water temperature at the reservoir outflow is generally in the acceptable range for trout today, and will be similar in the future once the expansion is completed.
  • Currently, the biggest threat to aquatic life in South Boulder Creek (SBC) is lack of water, not water temperatures. As it stands today, there are times of the year when some portions of SBC run at water levels insufficient to maintain aquatic life. An expanded Gross Reservoir will include space for an Environmental Pool, which will be managed by the cities of Boulder and Lafayette. The intent of this is to enable release of water during low flow periods so SBC will have life sustaining flows year-round.
  • While a Multi-Level Outlet Works (MLOW) could increase water temperatures in SBC, the Colorado Department of Public Health and Environment (CDPHE) noted on Page 12 of its Rationale for Conditional 401 Certification that “The potential for environmental benefit from the MLOW applies to a relatively short stream reach (about 5 miles in length), and recent data suggest that the water warms noticeably over that distance.”

Ultimately, CDPHE did not include a MLOW as a permit obligation, but did require long-term monitoring of stream temperatures and aquatic life in SBC. CDPHE also recognized all the commitments Denver Water had made to enhance the environment on both sides of the divide and found the Gross Reservoir Expansion would result in a “net environmental benefit” to Colorado.