Five others and I have been detailed to Los Trancos Creek, a major tributary of the San Francisquito, to prepare a Thalweg profile. I ask Ariane Bertrand, assistant director of the Coyote Creek Riparian Station in San Jose, California, which trains the volunteers and oversees their efforts. A Thalweg is a basic step in understanding the physical characteristics of a stream, Ariane explains. It charts the deepest part of a river or channel throughout its course so that later observers can detect changes, track them to their source and, possibly, correct them. For instance, upstream construction can cause silting downstream.
Mud slides can alter the creek's course.
I stand in the stream holding a telescoping surveyor's rod, calibrated in tenths of a foot. Volunteer Roberta Bischel squints at me through the 20-power lens of a survey level mounted on a tripod. I move the offending fingers. The figure, when subtracted from the instrument height, Diana explains, represents the elevation of the streambed at that location in relation to a bench mark whose elevation is known. I look down at my feet.