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NW Fishletter #366, February 6, 2017

[13] NorthWestern Operators Restore Fish Flows In Severe Winter Conditions

Nearly a dozen NorthWestern Energy personnel braved difficult conditions to restore minimum flows on the Madison River in southwest Montana after high winds caused a 100-kV power line to break, which caused a trip at the 9-MW Madison Dam powerhouse.

The 110-year-old Madison Dam, which creates the 2.6 mile-long Ennis Lake, is one of the nine developments that comprise NorthWestern's 334-MW Missouri-Madison Hydroelectric Project.

"As area winter weather had been extreme for two weeks prior, NWE operations staff fought sub-zero temperatures, high winds, river icing and drifting snow" in their efforts to fix the line and restore the 1,100 cfs minimum flow below the powerhouse, NWE said in a Jan. 9 report to FERC [P-2188].

Under its FERC license, the investor-owned utility is required to restore interrupted minimum flows within 40 minutes, but "due to drifted snow, access to the faulted transmission line above Madison plant was very difficult," so the flow was not fully restored until 24 hours after the trip, which took place just before noon on Dec. 19.

Minimum flows fell below the prescribed 1,100 cfs threshold for a total of 12 hours during the 24-hour event, reaching an estimated low of 585 cfs in the initial moments of the event.

Once the gates were reopened, large ice sheets began to float down Ennis Lake, intermittently blocking spill gate openings until they could be manually broken up by a team of workers using spud bars.

In addition, flows released from Hebgen Dam upstream didn't fully reach the lake due to "freeze-up and ice jamming," compounding flow reductions. That forced NWE to slowly draft Ennis Lake.

"It was very difficult for Madison operations staff to manage this unique event, with these many external variables in play at the same time and with power to the plant and to flow sensing and other spill gate equipment compromised," the report noted.

"It was a perfect storm of events," Jon Jourdonnais, NorthWestern Energy lead hydropower license compliance officer, told NW Fishletter. Jourdonnais said he's never such a confluence of events compounded with so much ice coming down at once, in his 30 years at NorthWestern Energy.

NWE provided a list of actions it would take to prevent a recurrence. The utility said that while some short-term effects on downstream fisheries may have occurred, the extent of any impacts "may not be known for some time," but it will continue to monitor fish health and population. -B. T.

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