Grand Lake Clarity

Why Grand Lake Clarity Matters

Grand Lake, Colorado's largest and deepest natural lake, is located in the headwaters of the Colorado River in Grand County. A feature of the Colorado-Big Thompson Project (C-BT), Grand Lake is interconnected with Shadow Mountain Reservoir and part of the Three Lakes System that includes Lake Granby.

The 1937 Senate Document 80 broadly describes the C-BT Project and its intended water supply and power generation features. This document states that the Project must be operated in a manner to accomplish five primary purposes. One of those purposes is “to preserve the fishing and recreational facilities and the scenic attractions of Grand Lake, the Colorado River, and the Rocky Mountain National Park.” C-BT Project operations have impacted Grand Lake clarity and Grand County believes that this has affected the lake’s scenic attraction. As a result, Grand County engaged the U.S. Bureau of Reclamation, Northern Water and other stakeholders, to improve water quality conditions and preserve the scenic attractions of Grand Lake. 

To that end, Grand County has been working with Reclamation and other stakeholders to improve water quality conditions in Grand Lake. Stakeholders have agreed to utilize water clarity, as measured by Secchi depth, as the primary metric by which scenic attraction can be quantified. A Secchi disk is a circular plate divided into quarters painted alternatively black and white. The distance at which the disk disappears in the water is the Secchi depth. A higher Secchi depth equates to greater clarity. Visual water clarity, the distance at which objects can be seen through water, is typically described by Secchi depth measurements. In 1941, prior to completion of the C-BT project, a single Secchi depth of 9.2 meters (30 feet) was measured in early September in Grand Lake. As you can see from the graph below, clarity has degraded since the C-BT project began.Grand-Lake-Secchi_Historic

Taking Action

In 2008, to protect the lake Grand County, the town of Grand Lake, and Northwest Colorado Council of Governments sought a site-specific water clarity standard for Grand Lake of 4 meters. While the Colorado Water Quality Control Commission approved the standard, it imposed a deferred implementation date which never went into effect. Instead, a narrative water quality standard was adopted of, “The highest level of clarity attainable, consistent with the exercise of established water rights, the protection of aquatic life, and protection of water quality throughout the Three Lakes system.” (5 CCR 1002-33, Numeric Standards Table, at p. 13 (June 30, 2015)) This continues to be acceptable to the Commission and stakeholders including Grand County only because of the key MOU components, explained below.

In 2016, the Grand Lake Clarity Stakeholders Memorandum of Understanding was created “to establish an adaptive management process while Reclamation conducts a planning and NEPA process to evaluate alternatives to improve clarity in Grand Lake as described in the Clarity Supplement.”  The “MOU may be extended by mutual agreement of Reclamation, Northern Water, and the West Slope until Reclamation issues its decision document upon completion of the NEPA…”. This agreement was extended on December 22, 2021 and it is set to expire on December 31, 2026. 

The MOU established water quality “goal qualifiers” that were mutually agreed upon as targets to be achieved through adaptive management during the clarity period of concern, from July 1 to September 11 each year. The goal qualifiers are an average Secchi depth during the clarity period of 3.8 meters, and minimum Secchi depth of 2.5 meters. Since adaptive management began in 2016, C-BT pumping schedules in 2018 and 2019 have led to some quite unanticipated, promising, and positive water quality outcomes.

Grand County believes it was imperative that the Clarity MOU be extended:

  • To allow adaptive management to continue, so that stakeholders can determine whether recent results can be repeated, and pumping schedules can indeed be used as a long-term means to maintain optimal water quality and clarity in Grand Lake and Shadow Mountain Reservoir.
  • To allow Reclamation to complete its planning and NEPA process to evaluate alternatives to improve clarity in Grand Lake as described in the Clarity Supplement.

For more information:

Water Quality Summaries

Adaptive Management Reports

USBR Appraisal Study