Depending on where you placed that drop in Colorado it also could've ended up in the Pacific. The continental divide is in Colorado and all water west of the divide will eventually drain into the Pacific and all water east will drain to the Atlantic.
Another fun, if more subtle divide is right around Hibbing, MN -- step west and water flows down the Mississippi to the Gulf of Mexico, step north and water flows up to Hudson Bay, step southeast and water flows through the Great Lakes to the Atlantic Ocean. https://www.google.com/maps/@47.4206243,-93.0430116,12.7z
The Great Basin is an endorheic basin (no outflow) located west of the Rockies, so not quite all the water.
From Wikipedia: “ The hydrographic Great Basin is a 209,162-square-mile (541,730 km2) area that drains internally. All precipitation in the region evaporates, sinks underground or flows into lakes (mostly saline). As observed by Fremont, creeks, streams, or rivers find no outlet to either the Gulf of Mexico or the Pacific Ocean. ”
I've been enjoying placing a drop right along the divide and seeing which way it goes. For example, up near Lenawee Mountain (aka A-basin). I've also noticed that up there, sometimes there is no route to either ocean. I guess it ends up in a high-alpine catchment and stays on the divide.
https://en.wikipedia.org/wiki/Continental_Divide_of_the_Amer...