Lahars, which are relatively common in the Belham River Valley, are dense, concrete-like mudflows made of volcanic ash and rock. They form when water, especially during heavy rainfall, mixes with and mobilizes the ash and rock on the volcano's slopes. The volcano does not need to be erupting for lahars to form and so lahars can be expected to occur at any time regardless of the volcano's activity.
Because lahars carry a lot of ash and rock (a hyperconcentrated flow), they are much more dangerous than flows of water alone. Lahars can be powerful enough to move large boulders, trees and may even bury towns. For this reason, crossing a lahar is very dangerous and should never be attempted.