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NorthWestern Energy Hebgen Operations Update

Jun 25, 2015

June 24, 2015
Current Conditions:

NorthWestern Energy (NWE) 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 June has averaged 51 percent of normal while May flows were 65 percent of normal. The snowpack has diminished and future forecasts will be precipitation dependent. Hebgen Lake elevation is at 6,534.57 feet as of June 24th which is 0.30 feet from full pool. Currently, Hebgen outflow is at 663 cubic feet per second (cfs), and at Kirby, the USGS gauge below Hebgen, flow is at 839 cfs while the Madison plant outflow is 1,150 cfs.

Inflow increased to 2,359 cfs on June 1, and Hebgen filled to full pool.  Due to the high inflow, outflow was increased in an attempt to keep the lake from overfilling. Flow reductions from that point were gradual to prevent fish stranding which caused the lake to draft about half a foot. Wind conditions moving the lake have also made it difficult to get accurate gauge readings for flow adjustments.

Forecast:

This weekend into next week looks to be particularly warm. Hebgen outflow has been increased in anticipation of the warm weather and potential pulse flow requirement. The current and extended forecasts indicate a high probability of above average temperatures out as far as 90 days. The precipitation outlook has more variability. It indicates a below average amount for the next few weeks, slightly above average for July, and has no indication of above or below for the months of August and September. Outflow continues to be managed conservatively to preserve water that may be needed during the summer.

Refill of Hebgen Lake is important for several reasons. In addition to maintaining the summer recreational level of the lake, our FERC license requires Hebgen Lake to provide adequate flow to meet downstream minimum flow requirements, and additional flow for potential pulse flows in the lower Madison River. Pulse flows reduce water temperature for fisheries protection. The pulse flow protocol has been developed and updated 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. The intake rehabilitation is currently scheduled for completion at the end of the summer. Upon substantial completion of the work on the intake, river flow will be transitioned from the spillway to the new intake gates.  This change is expected to return downstream water temperatures to what they were previously and aquatic insect populations will gradually return to those observed before the intake repairs began in 2008.

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, (866) 622-8081.

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