Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Lahars are mixtures of ash and water that travel down river valleys. On Mount St. Helens, the eruption melted glaciers. The water mixed with ash from the eruption and created tremendous lahars. The lahars raced down valleys and knocked out many bridges. Answer: lahars
Output:
What do you call mixtures of ash and water that travel down river valleys?