Groundwater flows across many scales in space and time. The oldest, deepest and longest flow paths – regional flow paths, can span 1000’s of kilometers (many surface water drainage basins,), and hundreds of thousands of years. Where, when and how much do these deep currents interact with the shallow subsurface systems? How old can stream water be? How far away and how deep in the earth should we be thinking about co-managing these resources? We’ve been using special tracers that tag very old water to show that they can been identified in many river systems around the world.