 The largest tributary is the Green River, which joins near the unincorporated town of Toutle.The North Fork's watershed was severely affected by the 1980 eruption of Mount St. Helens, which coated the valley with ash and filled it with successive mudflows.The result of the eruption was that many hillsides were stripped of forest, and as a result erosion has increased dramatically and the North Fork is now extremely laden with sediment.