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North Central River Forecast Center
Glossary of Ice Terminology &
Information About the River Ice Network

The River Ice Network is a cooperative effort between Federal, State, and local government agencies to collect river ice observations and store them in a centralized database. Volunteer observers periodically send reports describing river ice conditions in the North Central U.S. to a North Central River Forecast Center (NCRFC) database which can be queried from the NCRFC website.

The North Central River Forecast Center is the lead agency in this endeavor and has been working closely with the St. Paul District of the USACE, CRREL, the Minnesota Division of Emergency Management, the USGS, and the Minnesota State DNR to develop the network in Minnesota. During each ice season we hope to expand the Network to the rest of the North Central U.S.

If you know of a location where ice jams form and cause flooding or structural damage in or near the river please notify us via e-mail.

During the late fall, winter and early spring, the NCRFC issues weekly River Ice Statements pertaining to the channel conditions and ice coverage for Locks and Dams along the mainstem of the Mississippi River and the Illinois River. Most of the information comes from U.S. Army Corps of Engineers offices located in Chicago, IL, Rock Island, IL, St Louis, MO and St. Paul, MN.

These reports are usually issued every Tuesday, and the most recent message contains the future issuance date for the next River Ice Statement. The product files contain weekly summaries of river ice coverage and channel conditions within the NCRFC region as well as a brief discussion on future river ice conditions based on short-term climate predictions.


    River Ice Terminology

  1. Anchor Ice:

    Submerged ice attached or anchored to the river bed, irrespective of the nature of its formation.

  2. Border Ice:

    Ice formed along and fastened to the shore. Border ice does not extend the entire width across the river. Also called Shore Ice.

  3. Breakup Jam:

    Accumulation of broken ice pieces that restricts the flow of water; may contain frazil ice or remnants of freezeup ice jam.

  4. Candled Ice:

    Decayed sheet ice which takes the appearance of thin vertical crystals shaped like candles.

  5. Frazil Ice:

    Fine, small needlelike structures or thin, flat, circular plates of ice suspended in water. In rivers and lakes it is formed in supercooled, turbulent water.

  6. Freezeup Jam:

    Accumulation of frazil that restricts the flow of water; may contain some broken border ice pieces.

  7. Pancake Ice:

    Circular flat pieces of ice with a raised rim; the shape and rim are due to repeated collisions, composed of frazil and slush.

  8. Shear Walls:

    Ice left along shoreline when a freezeup or breakup jam fails and moves downstream.

  9. Sheet Ice:

    A smooth, continuous ice cover formed by freezing (lake ice) or by the arrest and juxtaposition of ice floes in a single layer.

  10. Slope Change:

    A change in the slope of the river. Typical examples are where two rivers meet, and upstream of a dam or reservoir.

  11. Slush Ice:

    A floating agglomerate of loosely packed frazil ice which remains separate or only slightly frozen together.

National Weather Service
North Central River Forecast Center
1733 Lake Drive West
Chanhassen, Minnesota 55317
w-ncrfc.webmaster@noaa.gov
Page last modified: December 21, 2003
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