In the coming years we will see wars over rivers between nation's that have greater demand. Disagreements are already occurring worldwide. As dams get built to control water flow, downstream nation's have little say over their water supply. Think Egypt and the River Nile. In game nation's upstream will have more control. Once a city builds a dam and reaches a certain population number say 15-20 pop the river dries from the dam on to rest of it's flow. Cities relying on this flow for housing will receive major housing problems. Also I think they should receive major amenity issues as well as farms along former river will go to base yields.