Following a 15-year period of consistently below average levels of water in Lakes Superior and Michigan-Huron—the planet’s two largest lakes by surface area—their shorelines rose dramatically during 2013–2014. In 24 months, the water level in Superior rose almost two thirds of a meter, and the Michigan-Huron level rose 1 meter, the fastest rates ever recorded there during a period of 2 calendar years.

Despite speculation that these changes occurred in response to anomalous weather patterns, including below-average air temperatures across the region, no studies to date have confirmed this link, and previous attempts to understand the basins’ water budgets have been stymied by the lack of consistent historical data. Now Gronewold et al. have developed a new water balance model to overcome this deficiency.

Using a statistical modeling framework, the researchers pooled all available data to generate estimates of the major monthly inputs and outputs to Lakes Superior and Michigan-Huron from 2005 to 2014. The estimates were then used to determine each system’s main hydrological drivers. In contrast to earlier studies, this approach can combine data from multiple sources and incorporate multiple estimates of each component while also quantifying the uncertainty associated with each source of information.

Their analysis indicates that the rapid rise in water levels in both lakes in 2013 was largely driven by increased spring runoff and precipitation over the lakes. By contrast, in 2014 the rise in Superior was predominantly due to reduced evaporation above it, whereas the increase in the Michigan-Huron system was due to a combination of all three of these factors, plus high inflow from Lake Superior via the St. Marys River, which links the two water bodies.

In the future, the authors hope to apply this new modeling framework to other large, freshwater lakes around the globe to better understand how their water levels may respond to both short- and long-term climate trends, information that will be crucial for guiding future water resource management decisions. (Water Resources Research, doi:10.1002/2015WR018209, 2016)

—Terri Cook, Freelance Writer