The clarity of Grand Lake would forever be impacted June 24, 1937, when Congress signed Senate Document 80 to pursue, at the time, the largest ever transmountain water diversion project in world called the Colorado-Big Thompson Project.

The Colorado-Big Thompson Project, which is known as the C-BT Project, was created to move water from the headwaters of the Colorado River to the Front Range to support farmers, ranchers and future development. Eighty-seven years after Senate Document 80 was signed and more than 1 million people now depend on the water delivered from the CB-T Project. The water users include cities and towns such as Boulder, Fort Collins and Loveland, and approximately 615,000 acres of farmland.

According to the United States Bureau of Reclamation, the C-BT is one of the largest and most complex projects it has undertaken. With more than 100 integrated structures including pumps, canals and reservoirs, water could now be captured from the upper Colorado River watershed and diverted. The water is stored in three reservoirs: Willow Creek, Lake Granby and Shadow Mountain.

---

---

The three reservoirs constructed for the project were connected to Grand Lake so water could be transferred through the Alva B. Adams Tunnel, which is located on the east end of Grand Lake and runs under the Continental Divide to the Front Range. 

Water first flowed through the nearly 13.1-mile tunnel on June 23, 1947, and cost $12.8 million, according to Northern Water, which jointly operates the project with the federal government. This process of diverting water from reservoirs and into Grand Lake changed the natural flow of the water.

---

---

Senate Document 80 laid out the development plans and cost estimates for building the C-BT Project and it included a list of five “primary purposes.” Two primary purposes are important for Grand Lake, which is the largest and deepest natural lake in Colorado. One purpose states that fishing, recreation and scenic attractions of Grand Lake and surrounding areas should be preserved. Another states that the water elevation and normal fluctuations should be preserved.

Grand Lake was historically known for its pristine water. However, the process of moving water through Grand Lake into the tunnel for 77 years has impacted Grand Lake’s water quality.

“C-BT Project operations have impacted Grand Lake clarity and Grand County believes that this has affected the lake’s scenic attraction,” Northern Water’s website states.

Grand Lake is fed by two pristine tributaries that originate in Rocky Mountain National Park, which are the East and North inlets. The tributaries still reveal very clear water today. One of the clarity issues is caused by the third tributary, which is the unnatural flow that comes from the project pumping reservoir water into Grand Lake. The algae and sediments coming from Shadow Mountain, especially during the hot summer months of July and August, factor into the lake’s loss of clarity.

Water clarity is measured by a method called Secchi, which is when a black and white circular disk is lowered into water. The clarity measurement is taken in meters at the depth when the disk can no longer be seen.

The earliest recordings of water clarity in Grand Lake were taken before the project was operational. On Sept. 6, 1941, Grand Lake’s clarity was documented at 9.2 meters which is about 30 feet. Today, clarity goals for Grand Lake range from 2.5 to 3.8 meters depending on the time of year.

Some of the first water clarity complaints about Grand Lake were received by U.S. Sen. Eugene Millikin shortly after the system became operational. The letters made it clear that reversing the natural flow of Grand Lake had impacted water quality.

This remains true today. 

In 2024, a joint resolution was passed by Colorado lawmakers to increase clarity restoration efforts. The resolution urges that effective and achievable solutions become established to meet state-issued clarity standards.

Three Lakes Watershed Association is a key organization that has been pushing for cleaner water and better clarity in Grand Lake for the past 50 years. In addition, the association donates thousands of dollars annually for water and algae monitoring. 

Grand Lake is an important asset and natural wonder to the state and nation. It should be restored, preserved and protected. 

Freshwater ecosystems, like Grand Lake, provide more than just water to the state’s growing population. They also provide nourishment for plants, animals and are the lifeblood of Colorado. Grand Lake can recover and will recover if efforts are focused on fixing the root cause — the C-BT Project reversing natural water flows. Bringing back the lake’s natural flow can help restore Grand Lake’s ecosystem for generations to enjoy.