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Northwest hydropower output shrinks amid dry September

Pacific Northwest hydropower generation fell below historical averages in September amid drier-than-normal conditions.

Precipitation was below 50% of normal for the bulk of the Northwest in September, while temperatures were near normal to as much as six degrees Fahrenheit lower than average, according to the U.S. National Oceanic and Atmospheric Administration.

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Data from the U.S. Army Corps of Engineers shows that total net generation at 23 hydroelectric plants across the Pacific Northwest was 12.9% below the year-ago level and 4.2% lower than the 10-year average and reached 4.00 million MWh. That figure is down 16.9% from August.

Year-to-date hydropower production through September totaled 63.02 million MWh, down 2.6% versus the year-ago level but up 7.5% from the 10-year average for the period.

In the upper Columbia River Basin, the Grand Coulee Dam, the largest in the region, generated 1.09 million MWh, down 14.9% versus the year-ago month and down 3.1% versus the 10-year average for the period. The Chief Joseph Dam generated 602,227 MWh, down 13.1% versus September 2017 and down 2.4% versus the 10-year average for the month.

September flows in the lower Columbia River Basin were also lower versus the year-ago period. Located on the Washington-Oregon border, the Bonneville Dam produced 298,651 MWh, down 15.2% versus September 2017 and 7.3% lower versus the 10-year average for the month. The Dalles Dam, 50 miles downstream, produced 377,330 MWh, down 15.4% from the year-ago month and down 3.6% from the 10-year average for September.

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The Dalles Dam logs normal hydro generation despite above-normal inflow in 2018 water year

For the 2018 water year spanning October 2017 to September 2018, spilled flows kept power generation from The Dalles Dam "relatively normal," even as total water inflow at the facility reached 10% above normal, the U.S. Energy Information Administration said in its Sept. 28 "Today in Energy" report.

Below-normal winter temperatures and above-normal precipitation followed by rapid warming in May led to high springtime inflow at The Dalles Dam during the 2018 water year, but a combination of involuntary and increased voluntary spills, likely due to a new court order, also occurred.

The Dalles Dam is mandated by law to voluntarily spill inflow to facilitate fish passage during what is called the spill season, which runs from April 10 through Aug. 31. While the required spill was previously set at 40% of the inflow, the Corps' Fish Passage Plan that took effect for the 2018 water year changed the required spill levels for the first half of the spill season to "the maximum amount deemed safe for fish, based on measurements of total dissolved gas in the water," according to the EIA.

The new voluntary spill rules alongside involuntary spills, which are operational decisions undertaken by a facility to pass water in excess of its turbine capacity, drove daily spill levels ranging from 23% to 65% of daily inflow during the spring. Monthly inflow at the dam peaked in May as it reached 63% above normal, but daily spill for the month averaged 57% of the total inflow.