Study ’Weighs‘ St. Mary and Milk River Waters to Gauge Evaporation

A map of survey sites where isotope samples were taken along the Milk River in 2020. Credit: Tricia Stadnyk

A recent study has found the downstream stretches of the Milk River in Montana and Alberta are especially prone to evaporation.

The study measured the isotopes of water molecules to gauge how much water is lost through evaporation as the rivers flow along their courses. It was funded by the IJC’s International Watersheds Initiative (IWI) and led by the University of Calgary.

“Think of isotopes as the water’s DNA—it tells you where the water is coming from,” said Tricia Stadnyk, study lead and the university’s Canada research chair for hydraulic modeling. “That has to do with the molecules containing hydrogen and oxygen. Water has a different weight depending on if it’s heavier or lighter on hydrogen and oxygen (the two elements that make up water).

“If you held 1 liter (33.8 ounces) of heavy water, you could actually feel the difference between it and 1 liter of ‘normal’ water,” Stadnyk said. As water moves through its natural cycle, certain processes such as evaporation favor lighter water, she said.

“This allows researchers to track the flow path of the water based on the weight. So, the theory is that without St. Mary River water, the Milk River samples should weigh differently because of evaporation,” Stadnyk added.

The two rivers are linked through the St. Mary Canal, a structure built more than 100 years ago by the US Bureau of Reclamation that allows water from the St. Mary River, which flows year-round, into the naturally ephemeral Milk River. Without this extra water, the Milk River can dry up during periods of low or no runoff or precipitation. In 2020, part of the canal system collapsed, cutting off the Milk River from the St. Mary’s flows for the first time in a century and spurring the study, Stadnyk said.

A couple weeks before the canal was repaired, funding came through to measure how the isotopes in the Milk River differed without the addition of water from the St. Mary River. This work was funded as a one-year pilot project to determine if isotopic monitoring is a viable means of estimating water flows, hydrologic losses (evaporation and to groundwater) and diversion amounts.

Canadian rivers are regularly sampled by Water Survey of Canada for isotopes. Stadnyk said she and her partners were able to use those existing efforts to collect water and streamline the study. Additionally, one of her partners, Innotech Alberta, provided water isotope sample data going back to 2016, allowing for comparisons to when the canal is fully operational. Some sampling was done in the St. Mary River for comparison and to label it as a ”source” within the Milk River.

The study found that the isotopic signatures (source waters and flow paths) of both the St. Mary and Milk rivers are distinct, albeit similar to the signatures of local groundwater and precipitation. Samples were only pulled from the Canadian reaches of both rivers, though the Milk River’s course flows northeast from Montana into Alberta for a stretch before turning back southward into Montana.

As the Milk River extended downstream in 2020 following the collapse, it saw heavier evaporation than in previous years. The Milk River’s eastern tributaries each see their fair share of evaporation, and the report indicates this heavily influences the “weight” of the water in the Milk downriver.

The main takeaway from this study is that since the isotope signatures of the two rivers are unique, it should be possible to use that information to track the different contributions from each river. Stadnyk said. Another IWI project is now underway to develop a monitoring routine for isotopic frequencies in the rivers.