Water and the civil-common law divide

The next article from the special issue of Western Legal History on Southern California water is Peter Reich's own "The All-American Canal and the Civil-Common Law Divide". Reich writes:

The wide divergence in legal doctrine between civil law in Mexico and common law in the United States illustrates why resolution of water disputes... has been so difficult. Mexican water rights, descended from Roman and medieval Spanish concepts and applied in the northern territories that became the U.S. Southwest after 1848, have traditionally been shared among various users, especially during times of drought....
U.S. water law... differs significantly from Mexican law, particularly in the western states, which developed the doctrine of prior appropriation granting an absolute, exclusive legal right in surface waters and underground streams to the first beneficial user....

These conflicting water doctrines have affected not only internal jurisprudence in Mexico and the United States, but their relations with each other as well. In 1928, the two countries' representatives to the International Water Commission were negotiating the status of existing diversions from the Colorado and Rio Grande, and the American section proposed that, as a matter of comity, such uses be recognized and confirmed as prior appropriations. The Mexican section rejected the proposal, stating that it could not agree to any restriction on its sovereignty over river tributaries within its own territory, and so could not recognize even established uses of this water on the other side of the border....

Due to these doctrinal disparities, Mexico-U.S. water disputes have been addressed largely through diplomacy rather than the courts....

Notwithstanding divergent legal regimes and diplomatic limits, a few examples of Mexico-U.S. legal integration suggest opportunities for water law harmonization. The traditional Roman distinction between gradual accretion and rapid avulsion of watercourses became the basis for the International Boundary Commission's settling ownership of the Rio Bravo/Rio Grande bancos (sandbar islands). Bancos were allocated in the Colorado River on the same basis.... [T]he commission used a legal historical source, in this instance an ancestor of Mexican civil law, to fill a gap and reconcile two divergent regimes.

Colorado River Dry Delta, terminus of the Colorado River
 in the Sonoran Desert of Baja California and Sonora, Mexico