Because water is a renewable resource that moves so quickly and easily through the hydrological cycle and across national boundaries, it has proved the most difficult natural resource for States to understand and control. As a result, it is difficult to estimate the changes in the system value of water associated with investments in new water resources infrastructure and other policy interventions. In part, due to the inherent variability of hydrological systems, hydrological data were more difficult to collect and analyze than information about most other natural resources. In some developing countries hydrological data are still treated as State secrets and so States often end up leaving themselves in the dark about basic hydrological facts and development opportunities. Hydrological data categorized as national security information might be compartmentalized, and thus States must make decisions without the full range of available information. In such situations it can be difficult to distinguish myths from realities about how a river basin actually behaves, and the risks and opportunities associated with alternative infrastructure investments (Sadoff et al 2013). For example, the Ganges is the world’s most populous river basin with over half a billion inhabitants, yet even in the 21st century remarkably little is known about the way in which the river functions and how its waters are used.