Justices Ketanji Brown Jackson, Elena Kagan, and Sonia Sotomayor reportedly joined Chief Justice John Roberts and Justice Brett Kavanaugh to form a 5-4 majority.
The court refused a move Friday by the two states and neighboring Colorado to join a consent decree on the matter, instead upholding the United States’ exception.
The consent decree stated how New Mexico and Texas would overcome previous disagreements regarding how the former supplies water to the latter.
However, the United States contended that their planned deal was incompatible with a historic compact and would oblige the federal government to follow protocols it had not approved.
The current case is based on the 1938 Rio Grande Compact, which established a debit-credit mechanism to manage river use in three US states while accounting for changing hydrological circumstances.
Historically, New Mexico has received 57% of the domestic Rio Grande flow, while 43% has been diverted to Texas.
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