From the U.S. Geological Survey (USGS):
“A new report by the U.S. Geological Survey evaluates how well the USGS streamgage network meets needs for streamflow information by assessing the ability of the network to produce various streamflow statistics at locations that have streamgages (gaged) and that do not have streamgages (ungaged).
“The report analyzes where there are gaps in the network of gaged locations, how accurately useful statistics can be calculated with a given length of record, and whether the current network allows for estimation of these statistics at ungaged locations. …
“In general, the eastern United States has better coverage than the western U.S.”