How does 'The Glory Hole' at Lake Berryessa work?

The Morning Glory Spillway at Lake Berryessa is an impressive, and unusual, sight. But how does the spillway, more commonly known as 'The Glory Hole,' actually work? And why did engineers build such an unusual spillway for Monticello Dam?
Published: 9:42 AM PDT March 15, 2019
Updated: 3:37 PM PDT March 15, 2019