Thanks to the lower-than-usual snowfall over most of the US this past winter, there's low risk of major flooding as the snow melts this Spring (for the first time in four years!). Nonetheless, being able to forecast river flood events is of critical importance to local emergency managers, water & electric utilities, river navigation companies, and the US Army Corps of Engineers -- not to mention residents in flood-prone areas. That's why the National Weather Service (a division of NOAA) is charged with forecasting flood risk throughout the United States.
As noted by Tom Adams, Development & Operations Hydrologist for the NWS's Ohio River Forecast Center, R is currently used at 4 out of 13 River Forecast Centers to generate graphics representing real-time hydrologic ensemble (probabilistic) forecasts. (Five additional regions are expected to also use R later this year.) For example, here's a recent analysis showing that Clay City on Illinois' Little Wabash River has a small risk of minor flooding in the next couple of weeks:
R is also used for research and development at NWS for forecast verification and analyses for the calibration of distributed hydrologic models. You can see these models in action for the NWS's Eastern Region by exploring the interactive maps at the link below.
National Weather Service: NWS Experimental Short-term Hydrologic Ensembles (MMEFS)