Targets for St. Croix Water Flows and Lake Levels May Prove Difficult to Satisfy

Due to climate change, the Forest City Dam on the St. Croix River could see water levels on the nearby East Grand Lake exceed the maximum targets set by the IJC order up to four times a year. Credit: IJC

Changes in weather patterns due to climate change could cause reservoir lake levels to go beyond minimum and maximum mandated levels in the St. Croix River system, according to a climate “stress test” recently undertaken by the International St. Croix River Watershed Board.

The test is a part of the IJC’s Climate Change Framework, a set of tests and considerations for IJC boards coast-to-coast to develop plans that will allow them to successfully do their jobs in the future. All boards were required to consider how climate change could impact their specific duties. In 2017, as part of this initiative, the St. Croix board began a pilot project to apply the framework to the St. Croix River.

One of the St. Croix board’s primary roles is to make sure water flows and levels satisfy IJC orders of approval along the river, through a series of publicly and privately operated dams. In addition, to support board objectives for maintaining clean water and encouraging fish passage, the board monitors water quality and fish passage at dams. This information is made available to federal, state, provincial, and local fishery and water quality managers.

As a first step, the St. Croix board met with climate change experts from the University of Massachusetts in June 2017 for a workshop, said Bill Appleby, the board’s Canadian co-chair. Based on board concerns about future water flows and levels, university researchers used computer modeling to find out how different climate conditions might impact the region and the board’s work.

According to the university’s report, climate models for the St. Croix basin generally agree that the region will warm up as time goes on, with on average warming between 0.2-4 degrees Celsius (0.36-7.2 degrees Fahrenheit) by 2050. But there is still uncertainty on how precipitation patterns will change. Precipitation models predict an annual range from a 5 percent decrease to about a 25 percent increase by 2050.  

On the whole, this warmer and likely wetter future could increase the chances of an early season snowmelt, which in turn would shift the timing of the peak water flows in the spring. In an area where the snowpack historically has held most winter precipitation until the springtime, warming temperatures could bring more winter rain, diminishing the water storage the snowpack typically provides. This could exacerbate any drought trends in warmer months, with less water available to store in reservoirs or allowed to flow.

“(The report) highlighted where the sensitivities would be (in the watershed) under certain scenarios,” Appleby said. “If we have drought conditions, it pointed out where we’re vulnerable.”

Using a variety of climate change models, the analysis suggested that the higher predicted temperatures could result in ecological shifts that negatively impact aquatic habitat and fish passage, as increasing air temperatures raise water temperatures, promoting algal growth rates in reservoir lakes and potentially causing more evaporation. Changes to water quality and temperature can force fish to spend more energy to move upstream to spawn. And too much water going over a dam during spawning season can mean alewives and other species are unable to get to and climb the fish ladders, Appleby said – something that happened in 2014 when heavy rains prevented fish from using the ladders effectively for several weeks.

The board and its consultants combined each of the climate change models with a model of the St. Croix basin’s water system, which shows how water moves throughout the watershed. These were run together at major locations along the river system to determine the frequency of flow and water level violations (when the flows or water levels go outside of the limits the IJC orders lay out) projected to occur. Those specific locations were the three regulation dams covered by the board’s mandate: Forest City, Vanceboro and Grand Falls, as well as the reservoir lakes located at each of those.

Projected mean precipitation changes using a climate model and variety of carbon emission scenarios for the St. Croix River watershed. Results show changes between the mean of the years 2036-2065 (on the left) and 2070-2099 (on the right) relative to the historical period of 1971-2000. Each point represents the results from a single model run. As air temperatures increase, the likelihood of a wetter climate increases. Credit: IJC

The results from the climate stress test suggest that it may not always be possible to satisfy the board’s mandates on minimum flows and reservoir lake levels over the next 30 years, based on the plausible effects from the changing climate on the St. Croix system. For the three reservoir lakes, changes to weather and precipitation patterns saw an increasing chance of exceeding the board’s target range.

The final component of this pilot project is climate adaptation. At the moment, the board is limited in what it can do by IJC orders on the dams, which were last reviewed in 1994, Appleby said. The orders lay out goals for flows and water levels for different dams and reservoirs, and another review would likely be necessary for the board to plan around climate change. Appleby said members are already preparing for a new review of orders over the coming years, and the report’s findings will inform that.

Given that extreme weather events are considered more likely as air temperatures increase globally, the report said the board could begin to plan for intense, frequent storms and intense droughts by giving dam operators guidelines to deviate from normal procedures. For example, if a major precipitation event is forecast for the region (like a nor’easter or the remnants of a tropical storm) the board could allow operators to release water to below minimum lake levels to absorb the runoff inside the reservoirs, which could mitigate some downstream flooding.

The board could also allow operators to reduce water levels during long droughts to maintain minimal water flows in the river. While they would still need to balance that against recreational uses and water quality on the lakes, during the fish spawning season maintaining flows for a successful passage is vital.

“The risk analysis highlights the sensitive portions of the system for further consideration by the board,” said Barbara Blumeris, St. Croix board secretary and a project manager with the US Army Corps of Engineers.

Under the climate change framework, the board also will revisit the climate analysis periodically to see if new data or other changing conditions suggest opportunities to make their plans more flexible, depending on what revised orders would allow the board to do. The St. Croix board could identify a climate change point person to develop an adaptive management plan and perform periodic short reviews of flows and levels to see how the system is doing.

Thanks to this pilot project, the IJC can refine its own climate change framework, and the St. Croix board has an idea of what it has to prepare for and can share its experiences with other IJC boards across the transboundary region.

Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.