Cowlitz River
Source Confluence of muddy and clear forks
Mouth Columbia River
Length 170 km
Basin Area 6,700 km²

The Cowlitz River is a river that flows from the slopes of Tahoma to the Columbia River at Longview. Its tributaries are sourced from glaciers on all three southern Washington strato-volcanoes, Tahoma (Rainier), Pahto (Adams), and Loowit (St Helens). Shipping traffic on the lower Cowlitz River was ended after the 1980 eruption of Loowit sent large amounts of ash and debris into the river channel. The Cowlitz watershed demarks most of the northern extent of the Columbia watershed west of the Cascade crest. Its flow is impeded by three dams, including the second tallest in the Cascadian bioregion, the Mossyrock Dam.