What happens to the 1944 Water Treaty with Mexico?

It survives, and Texas steps into it as a party in its own right. Boundary and water treaties are precisely the kind that carry over when sovereignty changes, and the 1944 treaty is an asset for Texas, not a problem.

What the treaty actually is

The water relationship along the Rio Grande is governed by the treaty signed in 1944 on the waters of the Colorado and Tijuana Rivers and the Rio Grande. It is administered by the International Boundary and Water Commission, a binational body with a United States Section headquartered in El Paso and a Mexican Section in Ciudad Juarez. Under it, Mexico delivers Rio Grande water to the United States on the order of 350,000 acre-feet a year, accounted for as 1,750,000 acre-feet over each five-year cycle. Texas, and Texas farmers in the Rio Grande Valley in particular, are the principal beneficiaries on the U.S. side.

Treaties about borders and water do not vanish at independence

This is settled ground in international law. Under the Vienna Convention on Succession of States in Respect of Treaties, opened in 1978, treaties that establish boundary and territorial regimes pass to a successor state rather than lapsing. The International Court of Justice confirmed in its 1997 Gabcikovo-Nagymaros judgment, when Slovakia inherited a Danube water treaty from the former Czechoslovakia, that this principle reflects general international law. A change of flag does not erase a boundary-water treaty. It transfers it.

An independent Texas gains a direct seat for the first time

Right now, the people with the most at stake in this treaty, Texans, do not sit at the table. The U.S. Section of the IBWC answers to the federal State Department, and Texas water is one priority among fifty states' worth of priorities in Washington. An independent Texas would deal with Mexico on Rio Grande water directly, as the neighbor that actually shares the river. When Mexico has fallen behind on deliveries in the past and Valley farmers have paid the price, Texans would be negotiating their own water rather than waiting on federal negotiators to make it a priority.

The arrangement is stable because both sides need it

This treaty has governed the river for more than eighty years through every kind of political weather because both countries depend on it. Texas supplies water to Mexico from the Colorado side of the ledger, and Mexico delivers from the Rio Grande side. That mutual dependence is exactly what makes the framework durable. An independent Texas keeps the structure and gains a direct voice inside it.

The bottom line

The 1944 treaty does not disappear. It carries over under well-established international law, and an independent Texas inherits it with a direct seat at the table for the first time, instead of relying on Washington to defend Texas water from a distance.