Rivers With Borders: Bi-National Water Management Model for the St. Mary-Milk Basins

Transboundary water management in semi-arid basins is increasingly challenged by climate variability, aging infrastructure, and rigid allocation frameworks. The St. Mary and Milk River Basin, shared by Canada and the United States, is governed by the 1909 Boundary Waters Treaty and the 1921 International Joint Commission Order, which allocate water based on reconstructed natural flow. Despite this framework, both countries routinely experience unmet entitlements due to hydrologic variability, operational constraints, and limitations in diversion and storage infrastructure. These challenges have gained urgency following recent droughts and infrastructure failures that have exposed vulnerabilities in the existing system. This thesis applies a bi-national water management model of the St. Mary and Milk River Basin using the Water Resources Management Model. A detailed Base Case configuration is constructed to replicate current infrastructure, operating rules, and naturalized inflows over the 1982 to 2014 period. The model is then used to evaluate structural and administrative scenarios designed to reduce entitlement shortfalls. Structural scenarios include increasing the diversion capacity of the St. Mary Canal from 600 to 850 cubic feet per second and the addition of an on-stream storage reservoir at the Forks location in the Canadian Milk River basin. Administrative scenarios examine alternative credit accounting frameworks that modify how surplus and deficit deliveries are tracked within a water year. Results show that increasing canal capacity improves United States entitlement receipt on the St. Mary River but does not eliminate shortfalls due to the timing and short duration of peak snowmelt flows. The Forks Reservoir primarily redistributes Milk River water within a year, improving Canadian irrigation reliability without increasing total natural flow available for apportionment. Administrative credit mechanisms increase operational flexibility but cannot fully compensate for physical constraints when implemented alone. Combined structural scenarios preserve the individual benefits of storage and conveyance improvements but also reveal persistent system limitations.