Watercourses are a quintessential common-pool resource. In arid regions of the United States, use of the available is privatized by the adoption of prior appropriation. Prior appropriation has admirably done its job in eliminating unauthorized, illegal, self-help redistributions of the commons that threatened a tragic free-for-all competition. Over time, prior appropriation proved inadequate at solving other commons problems. Prior appropriation creates strong incentives to over-rapid development of water-dependent uses and wastefully excessive uses designed to obtain the right to use (in perpetuity) larger than needed amounts of water. Checks in prior appropriation have failed to do their job and a major environmental problem looms in the form of ever-increasing instances of total stream dewatering. Again, prior appropriation could, in theory, address de-watering, but has failed to do so. Additional coercive law is needed, the most effective and direct of which is rigorous implementation and enforcement of minimum flow and level regimes.