R iver pollution represents a classic negative externality that spills across political boundaries. The source region generates emissions that pollute the river, but social costs are borne downstream. In cases ranging from Western Europe to China, empirical studies have documented significant free riding along interjurisdictional river boundaries (Cai, Chen, and Qing 2013;Sigman 2002Sigman , 2005and Sandler 2006). 1 Sigman (2002) compares pollution in domestic and international rivers. She finds that stations immediately upstream of international borders have higher levels of biochemical oxygen demand (BOD) than similar stations elsewhere. Gray and Shadbegian (2004) find that near the Canadian-United States border BOD discharges are higher and that fewer inspections take place. Using Toxic Release Inventory data from 1987to 1996, Helland and Whitford (2003 show that facilities' water emissions are higher in counties that border other states. Sigman (2005) finds that free riding gives rise to a 4 percent degradation of water quality downstream of authorized US states. Lipscomb and Mobarak (2013) exploit a Brazilian natural experiment in which county borders are redrawn frequently and consequently changes strategic polluting behavior around borders. They conclude that solutions to cross-boundary spillovers require active regulatory involvement by upper-level governmental officials.