Selective Withdrawal at the Flaming Gorge Dam

Alexa La Plante

On the first day of our trip, we received a specialized group tour of the Flaming Gorge Dam by Ms. Johnita Morton, an amiable and inquisitive 19-year old woman. Johnita’s father, Mr. Morton kindly met with me in the control room after the tour and talked with me for quite some time about the Selective Withdrawal System that was installed in 1978 for temperature control of dam releases.

Originally, I assumed the Selective Withdrawal Devices (SWD’s) to make up a rather simple system in which three gates lined the dam wall; one gate in the hypolimnion (lowest water layer), one in the thermocline (middle water layer), and the third in the epilimnion (surface water layer). However, this is not the case at all. In fact, three additional shutter gates were installed between 5913-6002 ft. elevation. Four adjustable 20’X 30’ gates facing upstream are moved much like shutters that are lowered onto one another one by one to reach lower release depths. The lowest gate, which is the original gate used before the SWD’s, is rusted shut and no longer used. The gate above it is raised up 30 ft. and then hooks to the second gate and shelves it another 30 ft. when it locks onto the upper gate. When fully raised, the gates cover the opening up to 6,000 ft. of the 6,048 ft. elevation dam. During high flow conditions, the gates cannot be raised above 40-50 ft. from the top of the dam so as not to create a vortex condition on the generating units due to excessive air being sucked through the gate openings. The Bureau of Reclamation has expressed the desire to unclog and fix the lowest gate to help control oxygen levels in the reservoir, but there is not enough money to send a diver down to do the work. The water from the hypolimnion is typically lacking in dissolved oxygen in the summer, but it is also extremely cold and may have negative implications on tailwater fish.

The SWD’s are used from May until October to maintain a release temperature of 50- 55°F. There are temperature sensors located at each gate. The gates are adjusted by estimating where to lower or raise the gates so as to achieve the desired release temperature. Mr. Morton mentioned that it would be more efficient to install a temperature sensor that spanned the length of the entire selective withdrawal system so that they would not have to guess and check for temperature releases. Again, there is not enough reason to justify the expense. During our trip the gates were open to 72 feet below the top of the dam and the temperature is 50°F. By July, Mr. Morton will raise the gates another 10 ft. to release water of 55°F to abide by the USBR 2000 Flow and Temperature Recommendations (see my paper on Water Quality of the Green River for more information).

After my informational interview with Mr. Morton, he took me inside the selective withdrawal system, which consists of rather immense steel structures connected to the penstock gates. I could see that the selective withdrawal system is a much more complex system than I originally imagined, but after considering my discovery, the system is rather straightforward and engineered quite well. I appreciated the time Mr. Morton and his daughter spent to satisfy my curiosity.