The hydrology of San Bernardino Valley, California

The San Bernardino basin lies near the eastern end of the valley of southern California.  Under the latter term is included that general lowland area which is definitely limited on the north by the San Gabriel and San Bernardino ranges and on the east by the latter range and the San Jacinto group, but whose southern boundary is irregular and difficult to define.  In this direction there is an interdigitation of ridges and valleys as the Sierra Madre Range of San Diego County dies out toward the north.  The more or less indefinite heights that represent its extension in this direction are separated by lowlands, which in turn are to be regarded as southwest lobes of the well-defined east-west valley that lies along the base of the San Gabriel Range.