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Hebgen Operations Update

Jul 24, 2015

Current Conditions:

NorthWestern Energy (NWE) manages the Hebgen Project taking into account current and previous month’s stream flow, precipitation and required pulse flow requirements downstream. Inflow for the month of July to date has averaged 67 percent of normal while June flows were 48 percent of normal. The precipitation in the Madison basin averaged 130 percent to date in July while June precipitation was only 44 percent of average. Hebgen Lake elevation is at 6,533.72 feet as of July 24th, which is 1.15 feet from full pool. With the hot weather during the first part of the month, pulse flows were required 11 days out of the first 12 days in July. Hebgen outflow averaged about 1,020 cubic feet per second (cfs) for the first week of July followed by approximately 915 cfs during the second week. Cooler weather, which reduced the pulse flow requirements, allowed reductions to the current outflow level of 822 cfs at Hebgen. At Kirby, the USGS gauge below Hebgen, flow is at 937 cfs.


For the remainder of July, the Climate Prediction Center indicates normal temperatures and a 40 percent probability of below-average precipitation. The 90-day extended forecast indicates a high probability of above average temperatures, but shows no indication of above or below average for precipitation.

Managing Hebgen outflow to provide downstream minimum flows, refill Ennis Lake from thermal pulse flow and irrigation withdrawal drafts is challenging this time of year.  Pulse flow requirements typically reduce as the days become shorter and nights become longer (mid-August). Pulse flows reduce lower Madison River water temperature for fisheries protection while maximizing water conservation.  Taking into account our FERC license requirements, the daily operation of the Hebgen and Madison hydro projects are subject to constant monitoring, re-evaluation, and collaboration with applicable agencies. Appropriate operational changes will be made, as necessary.

Construction Update:

NorthWestern Energy’s work at Hebgen Dam on the intake rehabilitation project has continued this year since early January. Work from now through the summer is directed towards completing the installation of all of the gates and the operating equipment, electrical installation, controls and automation, the operations building, and starting up and testing all operations. At the point in the intake rehabilitation project when gates are secured, work will begin to disassemble and remove the large cellular cofferdam in front of the intake structure. In early November when the coffer dam removal is estimated to be complete, the river flow will be immediately moved to the new deeper intake gates. Following discussion with Montana Fish Wildlife and Parks, the sequence of construction and lake temperatures at that time of year will have no significant downstream water temperature effects as a result of this change.

Concurrently with the removal of the cofferdam in front of the intake, construction of a new cofferdam will take place in front of the spillway at the other side of Hebgen Dam.  Much of the materials from the intake cofferdam will be re-used to construct this new cofferdam. The spillway cofferdam is needed to isolate the spillway from the reservoir for 2016. Beginning early in 2016, NorthWestern Energy will demolish and completely replace the spillway. The spillway replacement project is expected to be completed by the end of 2016.

About NorthWestern Energy
NorthWestern Energy provides electricity and natural gas in the Upper Midwest and Northwest, serving approximately 692,600 customers in Montana, South Dakota and Nebraska.  More information on NorthWestern Energy is available on the company's website at

Media Contact:
Butch Larcombe
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