The Laramie-Poudre Tunnel is an early transmountain tunnel in the U.S. state of Colorado. The tunnel transfers water from the west side of the Laramie River basin, which drains to the North Platte River, to the east side Cache la Poudre River basin that drains to the South Platte River. The tunnel is about 11,500 feet (3,500 m) long with variable diameters with a minimum diameter of about 5.3 feet (1.6 m). The diameter varied due to the different material mined through and the erosion of almost 90 years of water flow. It is located at about 8,400 feet (2,600 m) elevation with about a 1.7 degree down slope. The Laramie River lies about 225 feet (69 m) higher than the Cache La Poudre River at this location separated only by a mountain ridge. The Laramie-Poudre Tunnel is located about 45 mile