Gov. Gavin Newsom declared a drought emergency last month in Sonoma and Mendocino counties because of severe drop-offs in the winter rains that once had been counted on to fill reservoirs in the Russian River watershed, north of the San Francisco Bay Area. Like most other California reservoirs, those human-made lakes were built in the 20th century, an unusually wet period when compared with more than a thousand years of climate records reconstructed from studies of ancient tree rings and geological evidence.