Chapter 2 - § 2.5 • EQUITABLE APPORTIONMENT AND INTERSTATE COMPACTS

• Jurisdiction: Colorado

§ 2.5 • EQUITABLE APPORTIONMENT AND INTERSTATE COMPACTS

The use of water in Colorado did not occur in a vacuum. Because Colorado is a headwaters state and the rivers that begin within its borders cross into Colorado's neighboring states, conflicts among states over the waters of interstate streams led to the judicial allocation of water in interstate streams. In 1907, the U.S. Supreme Court decided Kansas v. Colorado⁴⁸ and laid out the doctrine of equitable apportionment.⁴⁹ Equitable apportionment "is the doctrine of federal common law that governs disputes between states concerning their rights to use the water of an interstate stream."⁵⁰ Equitable apportionment recognizes that interstate streams are "more than an amenity, [but rather] a treasure [that] offers a necessity of life that must be rationed among those who have power over it."⁵¹ Under this doctrine, while each state can choose its own water law system, it cannot impose its choice on any other state. Moreover, "[e]quitable apportionment . . . can determine times of delivery and sources of supply to satisfy that delivery without conflicting with state law, for state law applies only to the water which has not been committed to other states by the equitable apportionment."⁵²

The basis for equitable apportionment between states, as articulated by the Supreme Court is:

[I]f an allocation between two appropriation States is to be just and equitable, strict adherence to the priority rule may not be possible. . . . So far as possible those established uses should be protected though strict application of the priority rule might jeopardize them. Apportionment calls for the exercise of an informed judgment on a consideration of many factors. Priority of appropriation is the guiding principle. But physical and climatic conditions, the consumptive use of water in the several sections of the river, the character and rate of return flows, the extent of established uses, the availability of storage water, the practical effect of wasteful uses on downstream areas, the damage to upstream areas as compared to the benefits to downstream areas if a limitation is imposed on the former—these are all relevant factors.⁵³

Equitable apportionment of the waters of interstate streams may also be accomplished legislatively, by interstate compact or Congressional act.⁵⁴ The Supreme Court explained:

The compact—the legislative means—adapts to our Union of sovereign States the age-old treaty making power of independent

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