An agency that handles river forecasts is the National Weather Service (NWS) - River Forecast Centers. There are several centers around the country, each covering its own unique area.
This seems like a pretty straightforward question, but there are some interesting issues that come up in making a response.
There are two basic kinds of floods, flash floods and the more widespread river flooding. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.
The USGS operates and maintains a nationwide streamgaging network of about 7,000 gages.
Several kinds of flood maps are available from USGS and other sources.Online USGS map showing real-time flood conditions
USGS real-time streamflow data typically are recorded at 15- to 60-minute intervals, stored onsite, and then transmitted to USGS offices every 1 to 4 hours, depending on the data relay technique used.
No one knows for sure what would happen if the snow and ice in the polar regions all melted. Sea level would rise, which would flood coastal regions. Climate would be affected worldwide.
Occasionally, a piece of equipment may malfunction or there may be physical problems at a station. USGS tries to correct a station or equipment problem within several days of its first occurrence, and is generally successful in meeting this goal.
The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring. The end of a drought can occur as gradually as it began. Dry periods can last for 10 years or more.
Go to the current water resources conditions site at WaterWatch for a map of real-time streamflow in the United States for the day of the year.