The Cowlitz has a 2,586-square-mile (6,698 km2) drainage basin,[5][6] located between the Cascade Range in eastern Lewis County, Washington and the cities of Kelso and Longview.

It rises 605 feet (184 m) from bedrock and created the 23-mile (37 km) long Riffe Lake (previously Davisson Reservoir).

The modulated inflow from the Mossyrock Dam allows Mayfield Lake to maintain a water level that rarely fluctuates more than a few feet.

Packwood Lake was dammed in 1964 by the Washington Public Power Supply System (now called Energy Northwest).

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.

At Castle Rock, the A St. bridge provides access from downtown to the school and residential areas across the river.

At the head of Riffe Lake, the 27 Road provides access to the forestland south of the Cowlitz from Morton and Glenoma to the north.

At Randle, SR 131 crosses the Cowlitz to provide access to the Cispus basin and the northern areas of the Mount St. Helens National Volcanic Monument.

The fish facility, along with the Cowlitz River Salmon Hatchery's diversion dam below Mayfield Lake, has permitted the reintroduction of salmon and steelhead in the upper Cowlitz River basin for the first time since the construction of the Mossyrock and Mayfield dams in the 1960s.

The Cowliz River's two hatcheries provide an exceptional sportfishing opportunity for recreational anglers in Washington and Oregon.