Lake Berryessa is near San Francisco and exists as a result of the Monticello Dam, which was completed in 1958.
The dam has a spillway which releases excess water and prevents overflows. The spillway is 72 feet in diameter, and feeds into a tunnel 28 feet in diameter and 700 feet long. It’s nicknamed ‘Morning Glory’ or simply, the ‘Glory Hole’ (other forms are available, I’m told).
This captivating 13-second youtube clip shows it in action:
Sadly it seems that the spillway doesn’t come into use all that often: several pictures date from 2006.
Links to selected pictures:
Blog links for further reading:
I’m studying (the mathematics of) fluid flow with the Open University, and am grateful to a fellow student for bringing Lake Berryessa to my attention.