St. Mary's Reinvestment Act This bill authorizes the Bureau of Reclamation to use funds through FY2032 to rehabilitate the St. Mary Diversion Dam and Canal Headworks, which are part of the Milk River Project in Montana. In addition, the bill requires Reclamation to (1) coordinate with the Blackfeet Tribe with respect to the replacement activities that are carried out for the rehabilitation, (2) study the ability of project beneficiaries (i.e., water users) to pay for the costs of the rehabilitation, and (3) establish repayment terms based on the study. Finally, the bill requires the federal cost share to be at least 26.04%.