Boundary Waters Treaty

Boundary Waters Treaty governs water sharing for the St. Mary and Milk rivers between the United States and Canada.


The 1909 Boundary Waters Treaty governs the sharing of waters in international streams between Canada and the United States and established the International Joint Commission (IJC) to monitor compliance and resolve disputes.

The 1921 Order of the IJC set the St. Mary River at the international boundary, and the Milk River at the eastern crossing of the international boundary as the points where apportionment occurs and each country’s share is determined. It also outlined the roles for Accredited Officers (AOs) for the St. Mary and Milk Rivers. One officer is appointed by each of Canada and the U.S. to measure and apportion the water.

The AOs calculate the natural flow volumes, and the apportioned shares for each country, twice per month. The AOs signed a Letter of Intent (LOI) in 2001. It is a mutually beneficial agreement allowing each country to use more of its allotted share during seasonal low flow periods.


The St. Mary and Milk River basins: Canadian and American entitlements infographic explains the apportionment procedures, including the LOI.


The 1921 Order has been challenged at least 3 times since 1921. The most recent challenge was in 2003 when Montana requested an evaluation of whether the 1921 Order is compliant with the Boundary Waters Treaty.

In response to Montana's challenge, the IJC held a series of public meetings in the basins in 2004. The Province of Alberta attended the meetings and made several submissions to the IJC, providing background information and arguing there was no need to review the 1921 Order:

The IJC subsequently (December 2004) set up the International St. Mary Milk Rivers Administrative Measures Task Force to investigate measures for improving the administrative procedures for the apportionment. The Task Force produced a report in April 2006 for public comment, and Alberta submitted detailed comments:

The submissions from the State of Montana and the Province of Alberta continued to provide differing views, so the IJC requested further exploration. The 2 governments were specifically asked to explore collaboration on water management, cooperation on the rehabilitation of the St. Mary Canal. This included future arrangements for increasing each country’s ability to better access the full amount of water available to it under the current apportionment.

The Montana – Alberta St. Mary and Milk Rivers Water Management Initiative (the Joint Initiative) was established to fulfill the IJC’s request. From 2008 to 2011 did extensive collaborative work to gather background information and model potential options. From 2011 to 2018 staff from the 2 governments made progress on, but were unable to complete, a final report summarizing the work and recommendations for the most promising options.

With the 2021 announcement of the IJC’s ‘International St. Mary and Milk Rivers Study’, Montana and Alberta agreed to officially close the Joint Initiative and turn their attention toward participation in the IJC study. The Initiative produced 3 reports prior to ceasing their work:

  1. Background Information Report
  2. Modeling Report
  3. Process Report



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Email: [email protected]