Rivers route lahars and deliver sediment

When water combines with loose rocks and sediment in river valleys to form a flood or lahar, large areas downstream from a volcano can be buried with water and sediment several meters thick. Scientists monitoring an active volcano face the critical challenge of detecting a potentially dangerous lahar in real time so that a warning can be issued by public officials to people downstream.

An even more difficult and less obvious challenge for scientists, however, comes in the weeks and years after an eruption that significantly alters a volcano's watersheds--monitoring the long-term threat of sediment transport and increased flooding.  For example, annual sediment yields following the explosive 1980 eruption of Mount St. Helens were as much as 500 times greater than typical background level. After 20 years, the average annual suspended-sediment yield in the Toutle River downstream from the 1980 landslide deposit was still 100 times above typical background level. 

Why is this a potential problem? Such high sediment yields often cause river channels leading away from an active volcano to gradually fill with new, loose sediment. As such channels partially fill with sediment, their capacity to convey water within their banks is reduced, which commonly results in more frequent flooding during periods of intense rainfall. The experience at Mount St. Helens, and more recently with 1991 eruption of Mount Pinatubo in the Philippines, shows that effective mitigation measures must remain functional for decades following a major volcanic disturbance in order to reduce the likelihood of flooding.

The net effect of such changes to watersheds is that post-eruption stream velocities and peak discharges during rainstorms are temporarily much higher than during pre-eruption conditions. Streams typically respond more quickly to a given amount of rainfall and produce higher flows as rainfall is quickly flushed through a watershed.