Known in Siksik��powahsin as both Kin�ksisahtai and Poy��tahtai, the Milk River is a crucial water source for over 18,000 people, and its waters help generate food for over 1 million people annually. Mounting water shortages across the region present an ever-intensifying and compounding threat to the many communities that call the watershed home. In an average year, up to 90% of observed flows originate from the St. Mary River via the St. Mary Canal located in northwestern Montana. In May 2020, a drop structure along the diversion collapsed, and the Milk River experienced natural flows for the first time in over a century, revealing a unique opportunity to examine the state of the watershed without the influence of the diversion. In collaboration with the International Joint Commission, this research is constructing a comprehensive stable water isotope enabled model of the western portion of the watershed that will help inform crucial water governance and climate action decision making. This isoWATFLOOD model utilizes geospatial and hydrological data to simulate the ?18O and ?2H composition across the study region�s various hydrological storages and fluxes. This isotope simulation will then allow for a separation of streamflow components and an examination of water loss dynamics, sources, and sinks throughout the watershed. The development of an isotope modelling framework for the Milk River will serve as a vital tool in ensuring the persistence, protection, and vitality of its waters, and the betterment of climate mitigation efforts and water justice for all inhabitants of this precious watershed.