A “parallel Evolution” of Flood Risk Management along the Rhine and the Sacramento Rivers

The Sacramento River in California, USA, and the Rhine River in Europe both have histories of major flooding events and great efforts to manage flood risk. We compare these two watersheds with an interdisciplinary lens to explore the goals, approaches, outcomes, and “parallel evolution” of differing flood risk management paradigms. The two basins share hydrologic similarities, but each approach to managing floods reflects the basin’s unique historical, environmental, and governance context. The Sacramento basin is entirely within the state of California, whereas the Rhine is a transnational river that drains nine European countries. The Rhine basin is larger and has a much larger population compared with the Sacramento basin.  The Sacramento basin has high interannual precipitation variability and receives most of its precipitation in the winter with significant mountain snowfall. The hydrology of the Rhine is also strongly influenced by mountain snowpack, but has precipitation that is more evenly distributed throughout the year. Flood-risk management on both the Sacramento and Rhine Rivers has evolved from ad hoc and local approaches, towards more systematic planning, culminating in significant state-level control in California, and state, federal, and transnational management on the Rhine. This transition was driven in recent years by the Central Valley Flood Protection Act and the European Floods Directive. Management of each basin has been shaped by an event-based evolution, in which disasters have driven management responses, tools, and approaches. Flood-risk paradigms in both basins include significant investment in engineering protection and, increasingly, soft-policy adaptations. Over time, flood management methods and objectives in each basin have become more diverse. For example, single-objective approaches have evolved towards multi-benefit projects. Both basins are expanding consideration of floodplain ecosystem importance and both now consider climate change to some in flood risk management.  Flood-protection levels are higher on the Rhine than on the Sacramento. Some areas of the Rhine have 1000-year or better protection whereas a  200-year-level protection for urban areas is now required in the Sacramento basin. The Sacramento River and the Rhine River are geographically and hydrologically similar in surprisingly many ways, including in the flood risk they pose.  But the flood-risk management paradigms in the two basins have evolved differently.  We argue that the differences are a form of “parallel evolution,” reflecting historical and political contrasts between the two systems.  Such contrasts present opportunities for alternative tools and lessons that can be explored and perhaps imported in both directions.