Geology Reference
In-Depth Information
mounted level, as another took our stadia rod—a giant collapsible ruler—out to the end of a
long tape measure. Using the level to read off the elevation every few feet as we moved the
stadia rod along the tape, we measured the elevation of the riverbed. Repeating the survey
over a number of years gave us a record of how the river ate down into the volcanic debris
as lahars—volcanic mudflows—surged downstream to bury villages and towns beneath a
blanket of sediment.
Just before lunch we noticed that an ominous black cloud had settled in over the volcano
several miles upstream. The river started rising as we kept surveying our way up through a
tight canyon. When the flow got deep enough to start moving the riverbed, grapefruit-sized
rocks rolled into our shins and we decided to break for lunch on a sand terrace five or six
feet above the water level. About halfway through lunch we noticed the water rising even
faster. As the river started lapping up onto our lunch-stop terrace, we retreated to the foot of
the canyon walls and watched six-foot-high waves cascade down the river we had walked
up all morning.
