The Monticello Dam at the southern end of Lake Berryessa is famous for its giant morning glory spillway, a vast concrete funnel that swallows excess water like a bathtub drain when the reservoir overtops. It is one of the largest such spillways in the world, and the rare overflow events draw photographers from across the state.
Most of the time the funnel sits dry and accessible from a viewing area on the dam. Lake Berryessa itself is a popular boating, fishing and swimming reservoir, and the drive from Napa takes about 45 minutes east through rolling oak country. Worth combining with a longer day-trip if the spillway is your main reason for the visit.