Priority, Probability and Proximate Cause: Lessons from Tort Law About Imposing ESA Responsibility for Wildlife Harm on Water Users and Other Joint Habitat Modifiers

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. 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 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.

This article raises questions about how the ESA should be applied where streamflow deficits cause harm to endangered wildlife. To that end, the article reflects upon two basic principles of tort law, namely causation and allocation of responsibility in cases of harm caused by multiple tortfeasors. Although the key Supreme Court decision on the enforcement of the ESA - Babbitt v. Sweet Home Chapter of Communities for a Greater Oregon - requires federal wildlife agencies to prove that a person's habitat modifying activity, such as diverting water, is the proximate cause of harm to an endangered species, the rich body of tort law on causation has largely been ignored by the agencies and the courts. Similarly, tort law has been disregarded when it comes to deciding how responsibility for harm should be allocated among multiple habitat modifiers. By overlaying the learning of tort law on the system of prior appropriation, the article hopes to illuminate alternative approaches to allocating responsibility for river harm that are not only more equitable but more efficient.

Keywords: Endangered Species Act, Proximate Causation, Instream Flows, Sweet Home

Accepted Paper Series