Gold is no longer the most precious treasure of the American West. Water is.

In the arid western half of the United States, the unquenchable thirsts of industry, agriculture, and growing urban areas have nearly drained the region dry. There is no longer enough water to satisfy the conflicting claims of the many groups fighting over it.

Among the claimants are American Indian tribes. They hold water rights dating back to treaty obligations of the U.S. government--rights that often conflict with state water-rights allocation doctrines. Currently they are locked in legal combat with...