Isotope Monitoring and Water Balance Assessment of the St. Mary’s/Milk River basins

This project examined changing water balance conditions and source water contributions in the Milk River by applying water-mass (stable isotope) balance approaches. The project also demonstrated the utility of stable water isotopes for understanding the St. Mary and Milk Rivers water balance and continued the development of a long-term stable isotope monitoring network for the basins that supports source water separation and identification. A coupled isotope-hydrologic model of the basins was applied to simulate isotopic separations and source water signatures. The model was also used to assess the potential impacts to the water balance and source water separations under various climatic and diversion scenarios.  The study found that isotopes can be used to apportion diversion flow at the eastern crossing; however, there is increased uncertainty due to unknown baseflow (groundwater) fluxes along this reach. Physically based tracer-aided modelling would allow for more accurate separation of baseflow from diversion water, using isotope tracers. This project provided recommendations for a low-cost analysis and monitoring protocol that may assist the AOSMMRs in interpreting the natural flow and channel losses of the Milk River for improved calculation of natural flow estimation.