Lakes and Drainages

Magma began intruding into the Mount St. Helens edifice in the late winter and early spring of 1980. By May 18, the cryptodome (bulge) on the north flank had likely reached the point of instability, and was creeping more rapidly toward failure.

Streams that head on the volcano enter three main river systems -- the Toutle River on the north and northwest, the Kalama River on the west, and the Lewis River on the south and east. Lahars can form on any of these.