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Effects of ice on rivers
Users may see the "Ice" flag for the discharge for some sites periodically during the winter months for the latest or recent data, or as flagged on the discharge graph. The discharge is flagged and not available due to the problems with the stage data from ice which computes erroneous discharge data.
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.
A description of the various forms of ice on rivers is available from the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory in a River Ice Guide (PDF file). Photos of ice effects on Washington rivers are: Pend Oreille River (large photo), Wenatchee River (large photo), and Entiat River (anchor ice).
Photo credits: Entiat River photos courtesy of Jean Maynard (January 2004), other photos courtesy of Spokane Field Office personnel.