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River Managers Extend Spill Experiment At Little Goose Dam To Encourage Spring Chinook Passage
Posted on Friday, June 16, 2017 (PST)

A new plan to reduce spill eight hours a day at Little Goose Dam to encourage passage of adult spring chinook salmon seems to be working.


At one point two weeks ago, there was a difference in adult passage between Lower Monumental Dam and Little Goose Dam on the lower Snake River of nearly 9,000 spring chinook. In essence, the fish were trapped between the two dams and were not continuing their migration upstream through lower Snake River dams because they had difficulty with passing Little Goose.


At that time, the interagency Technical Management Team worked with the U.S. Army Corps of Engineers to reduce spill to 40 percent of the river at Little Goose Dam for six hours a day, but that spill level had not produced results.


So fishery managers and the Corps last week dropped spill to 30 percent of the river for eight hours a day. That seems to have hit the sweet spot. The deficit, or difference in the number of fish passing Lower Monumental Dam downstream to the number of fish passing Little Goose Dam, dropped to about 3,200 fish, according to Paul Wagner of NOAA Fisheries. Fish are moving, he said.


Spill at the dam this year has typically topped more than 50 percent of the river’s flow and at Little Goose that high spill and flow has been causing a downstream eddy that dam operating agencies believe is discouraging spring chinook from passing.


Although spill will drop naturally and may eventually be just 30 percent over the next 10 days, TMT at its meeting this week opted to continue the 30 percent spill, which was to end Wednesday, June 14, for one more week to Wednesday, June 21.


The management effort by TMT is significant. High river flows and resulting spill at Snake and Columbia river dams have been considered “unmanageable” by TMT fisheries managers. However, through tweaking the amount of spill and reservoir levels, TMT may have found one way to pass fish when high spill is inevitable.


The number of fish passing the dam each day this week averaged about 1,300 fish, with a total passage of spring chinook as of Wednesday, June 14, of 26,152 adults and 8,171 jacks. Passage at the downstream dam, Lower Monumental, on the same day was 28,545 adults and 8,270 jacks.


As the first operation at 40 percent spill began, some 15,101 adults and 5,439 jacks had passed Little Goose Dam as of June 5, but 23,665 adults and 6,618 had passed Lower Monumental Dam, leaving almost 9,000 fish lost in the Little Goose pool.


As has been typical of the run this year on the entire Columbia and Snake river systems, this year’s run has been lower than last year’s and lower than the 10-year average. Last year, some 62,016 adults and 6,301 jacks had passed Little Goose by June 14, 2016, and the 10-year average on that date is 62,698 adults and 11,705 jacks, according to the Fish Passage Center (


For Lower Monumental 66,115 adults and 6,266 jacks had passed last year by June 14, and the 10-year average is 68,087 adults and 10,905 jacks.


The operations – both the 40 percent and the 30 percent operation – violate the minimum operating pool requirements, largely set to aid the downstream passage of juvenile salmonids. MOP calls for a one-foot range in each of the lower Snake River reservoirs from April 3 to about September 1 while juvenile migrants are present (see the 2017 draft Water Management Plan at


The range for MOP is 633 to 634 foot elevation in Little Goose Dam’s forebay.


The operation approved at TMT this week calls for 30 percent spill for eight hours, 4 am to noon. Noon to 4 pm, spill is increased to pass inflow. 4 pm to 4 am, spill is increased to draft the reservoir back to the MOP range of 633 – 634 feet, if possible. The operation also limits spill to 40 percent during the 4 pm to 4 am increased discharge period in order to avoid exceeding 130 percent total dissolved gas levels in the Little Goose tailrace. The operation also recognized that “if inflows are higher than forecasted, or if there are unforeseen unit outages, the LGS pool may not be drafted back to the MOP range during this time period.”


A summary of the 40 percent and 30 percent spill operations is at


Also see:


--CBB, June 9, 2017, With adult Spring Chinook Passage Stalled At Little Goose, River Managers Experiment With Spill,”


--CBB, June 2, 2017, “Water Supply Forecasts Still Going Up; Now Ninth Highest Since 1960, Lower Granite Twelfth Highest,”


-CBB, May 19, 2017, “Climate Scientists Explain Ins And Outs Of Idaho’s Wild Winter This Season; No Drought Areas In NW,”


--CBB, April 14, 2017, “Big Water Mainstem: Runoff Supply Forecasts Continue To Rise At Columbia, Snake River Dams,”


--CBB, April 7, 2017, “2017 Runoff: Central Idaho’s Deadwood Summit gets 147 Inches Snow; Sees Five Times Above Normal,”


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