Variation 2: After the new uses contemplated in Variation 1 are implemented, User A wishes to expand use in the upper reach. The state could permit the consumptive use of 1 additional unit in the upper reach without adverse effects to salmon in the middle reach under average or normal conditions. When the upper-basin water supply is less than normal, however, users A and B will both continue their uses until the water available to salmon is exhausted, unless that water is afforded legal protection. Thereafter, User A’s junior rights will be curtailed in favor of User B’s senior downstream rights. Unless the water requirements of salmon in the middle reach have legal protection, however, the salmon will suffer adverse effects in below-average water years.

Variation 3: Now consider User C’s downstream uses that require 5 units (they could be consumptive or nonconsumptive). In a normal water year, User B must pass that much water through the middle reach. Since this “pass-through” water also benefits salmon in the middle reach, User B can still consumptively use 5 units in the Middle Reach (10-5 = 5 units “pass through” to downstream uses). An additional unit of development can occur in the headwaters or middle reach. Beyond that margin, the water needed for salmon will be reduced.

If the stream is wholly within the jurisdiction of one state, these variations can be successfully managed so long as salmon instream flow requirements have legal protection. Such protection can result from a water right or water reservation with its own priority date that is administered along with other priorities on the stream. Legal protection also can result from a regulatory program, perhaps under the Endangered Species Act or under a water quality statute that requires maintenance of a given streamflow. Without legal protection for the water necessary for salmon in the middle reach, however, increases in upstream water uses may eventually encroach upon flow levels required to sustain a salmon population(s).