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.