Cowlitz River - Dams

Dams

The Cowlitz River currently has 3 major hydroelectric dams, with several small-scale hydropower and sediment retention structures within the Cowlitz Basin.

The Cowlitz Falls Project is a 70 megawatt hydroelectric dam that was constructed in the early 1990s and completed in 1994. The dam is 140 feet (43 m) high and 700 feet (210 m) wide. The Cowlitz Falls Project produces on average 260 gigawatthours annually for Lewis County PUD. Its reservoir, Lake Scanewa, is located at the confluence of the Cowlitz and Cispus Rivers downstream of Randle.

Mossyrock Dam began generating power for Tacoma City Light in 1968. It rises 605 feet (184 m) from bedrock and created 23-mile (37 km) long Riffe Lake (previously Davisson Reservoir). It is the highest dam in the Pacific Northwest. The dam is named for the nearby city of Mossyrock, and the lake for the town of Riffe, which, along with Kosmos, was destroyed by the flooding of the Cowlitz River valley above the dam.

The Mayfield Dam is 850 feet (260 m) long and 185 feet (56 m) high. An 860-foot (260 m) tunnel connects the reservoir to the powerhouse. The dam began producing electricity in 1963. Mayfield Lake offers many recreational opportunities due to the presence of several county and state parks and its location below the Mossyrock Dam. The modulated inflow from the Mossyrock Dam allows Mayfield Lake to maintain a water level that rarely fluctuates more than a few feet. It is located several miles downstream of Mossyrock.

Packwood Lake was dammed in 1964 by the Washington Public Power Supply System (now called Energy Northwest). As an ancient landslide used to, the dam supplanted the job of holding back the lake, redirecting streamflow to a 27 megawatt hydroelectric generator in the Cowlitz River valley floor 2,000 feet (300 m) below just outside the town of Packwood. When designing and building the dam, care was taken so as not to affect the abundant wildlife that calls the lake and surrounding area home; the dam raising the water level by only a few feet.

A serious side effect of the Mount St. Helens 1980 eruption has been the downstream movement of enormous amounts of sediment through the North Fork Toutle River. After the eruption, river-borne sediment increased over five thousand-fold, making the Toutle River one of the most sediment-laden rivers in the world. The Toutle River Sediment Retention Structure was constructed to trap this sediment before it was carried farther downstream, where it could clog the river channel, exacerbate floods along the lower Toutle and Cowlitz Rivers, and fill the Columbia River shipping channel, which still requires periodic dredging. An overflow channel has been added to divert lahars around the dam.