Welcome to the NorthWestern Energy website. Please choose a state/region below so we can deliver you the most personalized and relevant site experience possible.
Hebgen is a storage reservoir and provides many benefits, including recreation and fish habitat. It also supports irrigation and electric generation downstream.
Hebgen Dam does not produce energy. We use Hebgen Dam to manage flows downstream for the Madison River fishery and Ennis Lake elevations.
Ennis Lake elevations are controlled by Madison Dam, which produces power. Madison Dam was recently upgraded a 12-megawatt facility, up from 8 megawatts.
Low snowpack could make for challenging water year in 2025
Low snowpack could make for challenging water year in 2025
A snowpack that was looking good in late March was all for naught. We have not seen much precipitation, and above-normal temperatures in April and May have resulted in a downward trend in the snowpack.
May runoff forecasts from NRCS show May–July runoff to be 84% of normal or lower. These months are critical, as this water is the main source for filling the reservoir. The NRCS runoff forecast table shows a May–July 50% exceedance at 240 KAF (Thousand Acre-Feet), while NorthWestern Energy is forecasting these months closer to the 70% exceedance at 215 KAF. Inflows during this period could resemble recent low-runoff years such as 2021, 2016, and 2013.
Snowpack report
(As of Thursday, May 14, 2025)
Station Name
Snow Water Equivalent (in)
Median Snow Water Equivalent (in) (1991-2020)
Snow Water Equivalent % of median (1991-2020)
Black Bear
24.0
38.8
62
Madison Plateau
11.1
19.6
57
West Yellowstone
0.0
0.0
Whiskey Creek
5.3
8.7
61
Weather Forecast
The short-term weather forecast indicates potential for precipitation and below-normal temperatures. However, the long-term forecast is calling for below-normal precipitation and above-normal temperatures.
NorthWestern Energy will continue to monitor snowpack conditions throughout the remainder of winter and spring. If current trends continue, it could be another challenging water year.
Managing water temperatures to protect the fishery
Managing water temperatures to protect the fishery
In the summer, we provide pulse flows out of Hebgen Lake into the Madison River below Ennis Lake. Our pulse flow program is designed to maintain lower Madison River temperatures at or below 80 degrees in order to prevent fish mortality caused from elevated water temperatures.
The Madison Thermal Decision Support System (DSS) was developed to protect fisheries and other river resources.
Our Federal Energy Regulatory Commission (FERC) license outlines how we manage flows in the Madison River and lake elevations at Hebgen and Ennis reservoirs. It specifies flow requirements at several locations on the river and elevation requirements for both Hebgen and Ennis reservoirs. NorthWestern’s goal for managing water in the Madison is to balance the resources relying on the water provided from Hebgen Reservoir including recreation, fisheries, water temperatures and power production downstream all while operating within our FERC license requirements.
However, in drought years, such as 2021, there are times we cannot meet the flow requirements while also maintaining minimum lake elevations. There simply isn't enough water. During these low water times, we work FERC, Montana Fish Wildlife & Parks, the U.S. Forest Service, the U.S. Fish & Wildlife Service and other regulatory agencies to determine the best management for the limited amount of water we have.
In addition to consulting with FERC and the resource agencies, NorthWestern initiated the Madison Stakeholder Engagement Process in summer of 2022. The purpose of the engagement process is to gather and exchange information among those directly affected by river flows and reservoir levels, agencies responsible for fisheries and recreation management, and NorthWestern Energy. The benefit of the process will be a greater understanding of issues and challenges of operating during times of limited water availability.
Stakeholder Engagement Presentations and Documents
NorthWestern works diligently to meet its FERC obligations and is committed to fulfilling its responsibilities at the Hebgen Development. Those responsibilities at Hebgen are numerous and complex.
Forecasts and modeling are closely followed by NorthWestern Energy to manage releases to the Madison River downstream and to capture and fill Hebgen Reservoir in the spring.
Limited snowpack and spring precipitation create challenging conditions to fully fill Hebgen reservoir while providing required flows to the Madison River.
Water releases from Hebgen reservoir feed Ennis Reservoir to support pulsed flow releases out of Madison Dam.
The pulse flows are calculated using a model that helps determine the volume of water to be released, allowing for the conservation of water while maintaining safe water temperatures in the lower Madison River.
Low flows directly affect the amount of electricity generated at Madison Dam.
NorthWestern Energy’s stewardship responsibilities of the river resources include balancing the many interests of multiple stakeholders.
The cooperation and flexibility of all help to protect and maintain the long-term health of the river system, which is our priority.
