1. Debris flows triggered by the El Niño rainstorm of February 2-3, 1998, Walpert Ridge and vicinity, Alameda County, California [electronic resource] Yale Public content 2002. 2003-08-15T15:16:17Z. 2003-08-18T15:56:26Z. Geological Survey (U.S.). U.S. Geological Survey ; Information Services [distributor]. English. Public. California and Alameda County.