Priority Matters: Lessons from Tort Law for Proving Causation of Wildlife Harm and Allocating ESA Responsibility Between Water Users

Abstract

The backbone of western water law is the basic notion of first-in-time-is-first-in-right. Beginning water law students have long been taught that under the law of prior appropriation, if there is not enough water in a stream to satisfy the reasonable uses of all diverters, junior users are obligated to close their headgates and pray for rain. Such occurrences have been rare because historically western states have built so much water storage (read dams) that short-term drought can be covered for even the most junior diverter. Although the assertion of priority is rare, it would be hard to imagine a more fixed principle in water law. It is increasingly evident, however, that this fixed principle of priority is being ignored when the Endangered Species Act (ESA) is used to curtail diversions to assure sufficient instream flow for threatened and endangered species. Rather than impose the regulatory burden on junior appropriators, the federal wildlife agencies charged with enforcing the ESA—the Fish and Wildlife Service (FWS) within the Department of Interior and the recently renamed NOAA Fisheries, formerly and still more commonly called the National Marine Fisheries Service (NMFS), within the Department of Commerce—have exercised discretion to pursue whichever appropriator they prefer. In several agency enforcement efforts, senior water rights holders have borne the brunt of obligations to provide more water. Although such efforts have been relatively infrequent, in part because the ESA has not been vigorously applied to harms caused by instream flow