 Previously, Spirit Lake was the source of the river, but eruptions blocked the outlet and Spirit Lake no longer drains directly into the North Fork; instead, a tunnel was excavated in 1985 to connect the lake with South Coldwater Creek, a tributary of the North Fork.Once a mature river, the North Fork has transformed into a braided stream of relatively young origin, and its valley floor is less than three decades old.The Toutle River Sediment Retention Structure was built by the U.S. Army Corps of Engineers on the river in 1989 to prevent this sediment from traveling downstream and clogging the shipping channel of the Columbia River.