The treaty-making power applies to all matters which may properly be the subject of negotiations between the two governments. This power has importance, especially on international streams. Important functions with respect to international streams have been vested in international agencies created pursuant to the provisions of treaties. This power is also the basis for treaties with Indian Tribes through which certain rights to use of water have been reserved. However, the federal government through the treaty-making power cannot restrict the police power of a sovereign.
In Asakura v. Seattle, 265 U.S. 332 (U.S. 1924), the court observed that “The treaty-making power of the United States is not limited by any express provision of the Constitution, and, though it does not extend so far as to authorize what the Constitution forbids, it does extend to all proper subjects of negotiation between our government and other nations. Treaties for the protection of citizens of one country residing in the territory of another are numerous, and make for good understanding between nations. Such a treaty is binding within a state. A rule established by it cannot be rendered nugatory in any part of the United States by municipal ordinances or state laws. It stands on the same footing of supremacy as do the provisions of the Constitution and laws of the United States. It operates of itself without the aid of any legislation, state or national; and it will be applied and given authoritative effect by the courts.”