You may not have known about it, but there was a map layer of just about every stream in the country that would show it's estimated streamflow in CFS. It looked like this:
Then you could click each stream and get a forecast of the flow. Yeah, it was really cool, though some of the flows seemed to be kinda off. That map layer was served up by NOAA's National Water Model. NOAA is still serving a map layer for the NWM and it can be viewed in this map. However, the NWM map layer that you see in that map is a png image. The NWM map layer that was displayed on the RP map was a from a "feature service" instead of the png "image service". The feature service streamed each stream feature to your browser as a series of coordinates that made up the stream line and some information about each stream like {name: North Fork Frenchbroad River, CurrentCFS: 236}. That allowed your browser to draw the deep blue line that was easy to see against the Google Terrain base map...and then label it with the current CFS. That isn't really feasible with the map image service that NOAA currently publishes. I just emailed them to ask them if they plan to publish the NWM output as a feature service again. I'll let you know what they say.