Users of the USGS streamflow maps may notice a decrease in the number of colored dots appearing on the map during cold season months. This decrease can occur in any area where ice affects the flow of a river or stream.
When ice forms, the river cross-section is constricted and produces a higher river stage for any given amount of discharge that exists during ice-free periods. This condition is known as backwater. Although accurate themselves, the river stages can no longer be used to accurately and directly compute discharge from the rating curve (which relates stage to discharge) because the amount of backwater varies with time, temperature, and other factors.
More complex procedures that require the use of meteorological and hydrological data from other stations in the area must be used to determine discharge at the affected stations. Unfortunately, these more complex procedures cannot yet be applied in real time.
As a result, stations with ratings both sensitive and subject to the effects of backwater are not generally depicted on the national map. This condition is currently most obvious in, although not restricted to, the states of Montana, Wyoming, South Dakota, Minnesota, and Maine, where a relatively large number of colored dots have disappeared from the map.
It should also be noted that real-time data reporting conventions vary from state to state. Some USGS offices, for example, continue to use the rating curve to compute and report discharge in real-time during ice conditions. Where they do, however, these offices also include on their real-time data web pages a cautionary note about the effects of ice.
Such differences in real-time data reporting practices explain why, therefore, North Dakota continues to have a relatively large number of colored dots on the national map while South Dakota and Minnesota do not.