Why do we see such extreme changes in Oxbow and Hells Canyon reservoir levels during the course of a single day?
In water terms, neither Oxbow nor Hells Canyon reservoirs are very big. The average outflow from Brownlee Dam can fill Oxbow Reservoir in a little more than six hours. Because Oxbow Dam is smaller than Brownlee, only about 25,000 cubic feet of water per second (cfs) can pass through its turbines. During the night we usually draft Oxbow Reservoir down to create space for water from the Brownlee Dam. In the morning when electricity demand rises and more water is passed through Brownlee Dam, we have room in Oxbow Reservoir for this increased volume of water. In this way, water level changes at Oxbow and Hells Canyon dams reduce the impact of flow changes on the river below Hells Canyon that can reach as much as 10,000 cfs a day.
