This seems like a pretty straightforward question, but there are some interesting issues that come up in making a response.
The USGS operates and maintains a nationwide streamgaging network of about 7,000 gages.
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.
For streamflow records to reflect variations in climate there needs to be an absence of any other major causes that would radically alter streamflow patterns during that time.
Six criteria by which station records were examined for suitability for inclusion in the Hydro-Climatic Data Network (HCDN) were defined as follows:
There may be occasional equipment or database problems where erroneous data are reported for short periods of time until corrections can be made.