The underlying soil layer, topped with pre-eruption vegetation and roots, prevented running water from eroding into even thicker tephra deposits erupted by the volcano hundreds of years ago. In many locations, however, these older deposits were also carried away by surface runoff during intense rainstorms and transported tens of kilometers downstream. Methods of hydrologic monitoring Detecting lahars in real time Installing system, Mount Pinatubo, Philippines Detection of lahars and other debris flows close to their sources provides an opportunity for timely warnings to people in downstream areas if adequate communication systems exist. USGS scientists have developed an inexpensive, durable, portable, and easily installed system to detect and continuously monitor the arrival and passage of debris flows and floods in river valleys draining active volcanoes. This system has the potential to save many lives from one of the most dangerous hazards posed to people who choose to live along rivers leading away from an active volcano.
Most sediment is transported from volcanically-disturbed watersheds during periods of heavy rain.
Scientists use stream-gaging instruments to measure the volumes of both water and sediment carried by rivers. Surveying eroded channel, Mount St. Helens New volcanic deposits consisting of loose, fragmented rocks are no match for the erosive power of running water, which can quickly carve wide and deep channels. In order to keep track of the erosion and corresponding sedimentation downstream, scientists make regular surveys across affected river valleys.