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Physical
Cowlitz River
The Cowlitz River is a Washington river, roughly 105 miles long, that drains volcanic slopes and flows into the Columbia.
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The idea
The Cowlitz River is a river in the state of Washington and a tributary of the Columbia River, which means it flows into that larger river. It is roughly 105 miles long, not counting its tributaries. Its drainage basin, the area of land whose rain and snowmelt end up in the river, covers 2,586 square miles between the Cascade Range in eastern Lewis County and the cities of Kelso and Longview.
The river gathers water from a large region that includes the slopes of three volcanoes: Mount Rainier, Mount Adams and Mount St. Helens. Its major tributaries include the Cispus River and the Toutle River. When a volcano erupts, lahars, which are fast-moving mudflows of volcanic ash, rock and water, can rush down valleys like these.
The Cowlitz is also known for its smelt, small silvery fish that come into the river to spawn. When the smelt arrive, gulls gather and feed on them in a frenzy that lasts for weeks. Because of these runs, the nearby city of Kelso is known as the Smelt Capital of the World.
Check yourself
Question 1 of 2
What overtook the Toutle River during the May 1980 eruption of Mount St. Helens?
Composition
what it's made of
- Main channel — Roughly 105 miles of river, not counting tributaries, ending in the Columbia River.
- Drainage basin — A 2,586-square-mile area between the Cascade Range in eastern Lewis County and the cities of Kelso and Longview.
- Volcanic headwaters — Tributaries that drain the slopes of Mount Rainier, Mount Adams and Mount St. Helens.
- Major tributaries — The Cispus River and the Toutle River.
- Smelt run — Smelt spawn in the river, and gulls feed on them for weeks.
Connection
how it relates
Change
how and why it evolves
- Past
- During the May 1980 eruption of Mount St. Helens, volcanic mudflows called lahars overtook the Toutle River, one of the Cowlitz's major tributaries. Mossyrock Dam began making power in 1968 and created the 23-mile-long Riffe Lake, which flooded the towns of Riffe and Kosmos.
- Now
- As of 2025, the dam built on the Toutle River to trap ash and sediment from the 1980 eruption was catching only 20% of it, down from 80%.
- Next
- Runoff and sediment from the volcanic slopes in its basin will likely keep shaping the river. A future eruption of Mount St. Helens could again send mudflows down its tributaries.