Where does the rain go once it hits the ground? Some of it soaks into the ground, and some of it evaporates. The rest flows into creeks, streams, lakes, and rivers, eventually making its way to an ocean. The route that this water takes is described by thousands of regions called watersheds. Visualizing how these regions connect to each other involves finding and reformatting some public data, a bit of fun graph algorithm coding, and some challenging graphics performance issues associated with the display of 100,000 polygons in your browser! Come see a demonstration of an interactive open source web application that you can use to see everything that is upstream or downstream from any location!