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NorthWestern Energy Hebgen Operations Update

May 05, 2015

Current Conditions:

NorthWestern Energy manages the Hebgen Project taking into account current and previous month’s stream flow, snow-pack conditions and volume runoff projections. Inflow for the month of April has averaged 82 percent of normal while March flows were 97 percent of normal. Snowpack has decreased from 80 percent to 59 percent of average from March 1st to current. The NRCS volume runoff forecast for the April to July period that was issued for April 1st was 64 percent of average which is down 16 percent from the March 1st forecast.  Hebgen Lake elevation is at 6530.57 feet as of April 28th which is 4.30 feet from full pool. The reservoir is over 4 feet higher than the 20 and 30-year averages for the same time of year because of the below-normal conditions. The elevation is 2 feet higher than last year at the same time. Currently, Hebgen outflow is at 606 cubic feet per second (cfs), and at Kirby, the USGS gauge below Hebgen, flow is at 839cfs.

Forecast:

The recent moisture will benefit the near-term operation; however, the operation will be conservatively managed over the next few weeks.  The outflows will be reduced to 550cfs on April 29th to continue filling the reservoir. A new volume runoff forecast will be issued around May 7. The current operations target is that the lake will fill to full pool by the first part of July. The operation of the project is subject to constant monitoring and re-evaluation. Appropriate operational changes will be made, if necessary. With the lack of snowpack, this summer’s lake and river conditions will be heavily dependent upon precipitation.

Construction Update:

NorthWestern Energy’s work at Hebgen Dam on the intake rehabilitation project has continued this year since early January. This spring, the final concrete was placed behind the intake to bring the surface up to the planned elevations for the operations building and gate mechanical equipment.  Since diverting river flow to the spillway in mid-April crews have been replacing the stair access tower and worked to ready the intake for the installation of gate guides and the trash rack. 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. The intake rehabilitation is currently scheduled for completion at the end of the summer. Concurrently with the intake rehabilitation project this year, NorthWestern Energy is proceeding with construction work to remove the cofferdam in front of the intake and construct a new cofferdam in front of the spillway at the other end of Hebgen Dam.  At the point in the intake rehabilitation project when gates are secured and dam safety concerns are mitigated, work will begin to disassemble and remove the large cellular cofferdam in front of the intake.  Much of the materials from the intake cofferdam will be re-used to construct a new cofferdam in front of the spillway.  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.

NorthWestern Energy will provide a Hebgen operations update to interested stakeholders and the local media on a monthly basis throughout the summer months. For more information, contact Claudia Rapkoch, Director Corporate Communications, NorthWestern Energy, 406-497-2841.

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