Electric power generation from Missouri River dams fell below average in 2015, as water was kept in upstream reservoirs to balance levels in the river system, according to the U.S. Army Corps of Engineers.
The Corps, which manages dams and reservoirs along the 2,341-mile river, said energy production from the six projects in the Dakotas, Montana and Nebraska was 8.5 GW of electricity last year, down from 9.6 GW in 2014.
The plants have generated an average of 9.3 billion kilowatt hours of electricity since 1967, including a high of 14.6 GW in 1997, said Mike Swenson, a Corps engineer in Omaha, Nebraska.
The Western Area Power Administration, which buys and sells power from 56 hydropower plants around the nation, says the six Missouri River facilities are WAPA's second-largest producer of energy.
Kara Lamb, a WAPA spokeswoman in Lakewood, Colorado, said the shortfall in electricity production from hydropower meant WAPA had to get energy from other, more expensive sources. That will ultimately mean higher costs for ratepayers as those costs are passed on. WAPA spent about $67.5 million to purchase power in the open market to help make up the shortfall, she said.
Purchasing power to fulfill contracts is not unusual. WAPA has spent more than $1.5 billion since 2000 to fulfill contracts due to shallow river levels caused by drought. During the driest years in the past decade, power plant output dwindled below 5 GWh in 2007 and 2008, the Corps said.
The Corps is charged with finding a balance between upstream states, which want water held in reservoirs to support fish reproduction and recreation, and downstream states, which want more water released from the dams, mainly to support barge traffic.
Swenson said 2015 was about average for rain and snow runoff in the Missouri River system, though releases were reduced at some upstream reservoirs to bring levels up.
The 118.2-MW Oahe plant near Pierre, South Dakota, which holds Lake Oahe in the Dakotas, and 583.3-MW Garrison, which creates Lake Sakakawea in western North Dakota, are typically the biggest power producers in the Missouri River system.
Oahe generated 2.3 billion kilowatt hours last year, below the long-term average of 2.6 GWh. The plant recorded a low of 1.1 GWh in 2007.
Garrison Dam also generated 2.1 GWh of electricity last year, data show. The long-term average at the dam is 2.2 GWh.
The water storage level of the six upstream reservoirs in the Missouri River system is about 56.4 million acre-feet at present, or slightly above the ideal level, Swenson said. An acre-foot is the amount of water covering one acre, a foot deep.
"We essentially have a full reservoir system to start the year," Swenson said.
Based on runoff estimates for 2016, the Corps has forecast 9.6 GWh of electricity this year, or slightly above the long-term average.
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