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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
The SWE (Snow Water Equivalent) that has been forecasted over the past three months has not resulted in much runoff into the Hebgen Lake reservoir. In May, the NRCS May–July runoff forecast projected 240 KAF (thousand acre-feet) at its 50%e exceedance level and 215 KAF at its 70% exceedance level (see chart below). NorthWestern Energy’s internal forecast showed 217 KAF with a bias-adjusted value of 200 KAF. We use the bias-adjusted KAF as a buffer for forecast errors. Unfortunately, conditions have been even worse than those forecasts, and we simply aren’t seeing the inflows we expected. Currently, we are forecasting that number to be closer to 175 KAF for those 3 months, which would be around 61% of normal. We are currently seeing similar trends as 2021, 2015, 2007, and 2013.
NRCS is forecasting 106 KAF over the next two months at its 70% exceedance level, while NorthWestern’s forecast is closer to 88 KAF for the same period.
Currently, outflows are at 430 cfs below Hebgen and will stay lower to help fill the lake.
Snowpack
Snowpack
Not much to report on the snowpack, as we are pretty much done with runoff. The only SNOTEL site that still has snowpack is Black Bear (see chart), but it should be melted out by June 9.
Weather Forecast
Weather Forecast
The short-term forecast for the West Yellowstone area is not showing much precipitation until the latter part of the forecast period, and those projections can still change significantly.
Temperatures around West Yellowstone are currently high. The long-term forecast doesn’t look much better, with slightly above normal temperatures and below normal precipitation expected.
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.
