One of the unintended but inevitable consequences of federal constitutions is the creation of transboundary waters, river basins that are shared among multiple states/provinces, and, in some cases, other countries, as well. The existence of transboundary waters creates difficulties for the riparian governments sharing them because they are locked in a common pool situation in which each government faces simultaneous and contradictory incentives: in their own interests, they are motivated to secure access to resource flows, but, in the collective interest, they must also conserve the resource stock. Managing such a situation requires collective action, and much has been written about the factors that are important in facilitating such collective action, such as the importance of shared interests, mutual trust, and the enforceability of intergovernmental commitments. However, in many basins there is a long history of intergovernmental cooperation in water management, and this history, in itself, has become an important factor shaping more recent water management efforts. Early efforts at transboundary water management focused mostly on either water apportionment or dispute resolution, and these early water management regimes have proven strongly path dependent, shaping more recent management efforts that have focused on water conservation. In other words, early institutional choices in transboundary water management, when water conservation was not a priority, have shaped transboundary conservation regimes now that water conservation is a priority, pointing to the importance of institutional path dependency as a factor in shaping intergovernmental cooperation. This argument is explored through a comparison of four distinct transboundary river basins in Australia, the United States and Canada. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]