Private arrangements are framed by a set of institutional rules, either public policies or property rights that actors activate in order to defend their positions. This is particularly visible in the field of the environment where human pressure is increasing scarcity and generating rivalries between competing users. How do rules intervene in the resolution of rivalries? I suggest that users activate rules to assert their rights against their rivals and find out a solution to the rivalry. Three hypotheses follow: the owner uses his property rights; the non-owner activates a public policy that acknowledges him as the final beneficiary; and the owner activates a public policy if he cannot exclude the rival from his property. The empirical test, carried out on four cases of local water rivalries in Belgium and Switzerland, validates these hypotheses and shows that public policies are more frequently activated than is initially expected.