If a drop of rain fell anywhere in the US you choose, what path would it take to the ocean? River Runner will show you and it does it with a really awesome 3D view. Try it out here:

https://river-runner.samlearner.com/

The javascript source lives here and uses USGS NHDPlus data and their NLDI API to visualize the path of a rain droplet from any point in the contiguous United States to its end point (usually the ocean, sometimes the Great Lakes, Canada/Mexico, or another inland water feature). It’ll find the closest river/stream flowline coordinate to a click/search and then animate along that flowline’s downstream path.