The era of dams, it has been widely declared, is dead. So what comes next? In Common Waters, Diverging Streams, William Blomquist, Edella Schlager and Tanya Heikkila argue that the future may lie with "conjunctive management," or coordinating the use and storage of surface water with water in underground aquifers. When surface water is plentiful, excess water can be stored underground, for use later when surface water is not so plentiful. The concept offers one of the most promising means for increasing reliability, and, by providing water users with "banked" groundwater for dry years, it can help protect natural stream flows.

This is not just a theoretical concept: Blomquist, Schlager and Heikkila focus on Colorado, Arizona and California, where conjunctive use has been practiced for decades. Each state has widely differing strategies and goals for conjunctive management. In Colorado, for example, the practice is most frequently used to maintain flows in surface streams, both to meet water obligations to downstream states and to protect endangered species habitat. In Arizona, it has become an integral part of the state’s groundwater management act and helps developers shore up the 100-year "assured supplies" required for their housing projects. The authors examine the legal frameworks that make conjunctive management work, those that don’t, and the evolving effort to fine-tune the concept.

After decades of pedal-to-the-metal groundwater "mining" in the West, write the authors, "this rapidly developing region, which finds it nearly impossible to build additional above-ground water storage facilities, also rests atop a cavernous amount of underground storage capacity ... If ever need met opportunity, it has happened in the case of conjunctive management in the West."