Picture if you can the stratified face of a towering southwestern mesa, showing the numerous geological layers of which it is composed piled one on the other, and you will have in your mind’s eye a perfect portrait of the long sequence of legal regimes composing New Mexico’s current water law. Sovereign after sover eign has washed over the scant waters of desert New Mexico, each bringing its own version of water law with it, each leaving behind the distinct imprint of its version as it was overlain by the next layer. Current New Mexico water law is thus the special sum of its many distinct his tor ical parts. In the desert Southwest, intensive water use by indi gen ous Indians began before the Spanish arrived in 1540, but polit ical conquest put Spain as a succeeding sover eign in a peculiar position. In the case of Spain, medi eval Spanish law gov erned new water rights acquired after the change of sover eignty. But the often ac know ledged, frequently breached obli ga tions of Medieval inter na tional law also required that Spain as the succeeding sover eign honor water rights previously acquired under its antecedent sover eigns, the Native Amer icans themselves. In a final, ironic twist, it fell to Spain as succeeding sover eign to define exactly what the pre existing rights of Native Amer icans were (Hall, 1991). The pattern would repeat itself time and time again over the next centuries as one New Mexico sover eign succeeded to another. In 1540, Spain and its water institu tions succeeded to Native Amer ican ones. In 1821 Mexico succeeded to Spain. In 1848 the United States succeeded to Spain and gov erned New Mexico through the Federal territory of New Mexico. In 1912, the state of New Mexico arrived and, within assigned Constitutional lim ita tions, succeeded to the United States. In every instance each new sover eign imposed its own law, recog nized water rights acquired under the sover eigns that had come before, and relegated to itself the job of defining those acquired rights (Simmons, 1968). The complication of current New Mexico water law is the sum of all of these pro cesses and functions.