Anderson Valley stretches from Yorkville Highlands (located in a highland meadow straddling the upper Rancheria Creek and upper Dry Creek watersheds) through Boonville (located on Anderson Creek) and Philo (located on Indian Creek) to Navarro (located on Soda Creek). Rancheria, Anderson, Indian and Soda creeks are tributaries to the Navarro River, which flows north and west through the coastal range to the Pacific Ocean; Dry Creek flows south into the Russian River watershed in Sonoma County. The main stem of the Navarro River begins less than a mile south of Philo at the confluence of Anderson Creek and Rancheria Creek. The mouth of the Navarro is 10 miles (16 km) south of Mendocino, California. Encompassing 315 square miles (816 km²), the Navarro River watershed is the largest coastal basin in Mendocino County.
Such unique geography results in a wide diurnal range, with daily high and low temperatures occasionally diverging 40 or 50 degrees. This enables Pinot Noir growers to keep acid development in line with sugar and flavor formation through long, warm Indian summers. It also makes for superb Gewurztraminer and Riesling, giving rise to the valley’s annual Alsatian Varietals and Pinot Noir festivals.
The climate in the Anderson Valley appellation is tempered by cool marine air. Steep hills and mountains surround rolling to nearly level alluvial terraces. The dominant natural vegetation is a mixed forest of Coast Redwood, various native oak varieties, and Douglas-fir. Elevation ranges from sea level to 2,500 feet (760 m). The average annual precipitation ranges from 35 to 80 inches (900 to 2000 mm). The average annual temperature is about 53 °F (12 °C), and the average frost-free season ranges from 220 to 365 days. Towards the coast the summers are cool and moist with frequent fog, while the interior Anderson Valley proper features a warm to hot summer climate similar to nearby interior regions, with daytime highs occasionally in excess of 100 °F (38 °C).
Visitors to the Valley should come prepared for cool evenings and warm days. Locals dress in layers year round.