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River Ice and River Ice Processes
 MIJAD Home   Ice Types and Processes   Jam Types   Monitoring   Ice Jams West of the Divide   Ice Jams East of the Divide

Types of River Ice - Two basic types of ice found in rivers
  • Columnar
    • Thermally grown
    • "Black ice" or "blue ice"
    • Tends to occur in slower flow
    • Very clear, allows solar penetration
    • Ice consisting of columnar shaped grain. Ice growth continues downward in long, vertically oriented crystals after a cover has been established.
Columnar1 Columnar2
  • Fine-grained
    • Frazil or snow
    • "White ice"
    • Tends to form in open, super-cooled water that is fast flowing or turbulent
    • Resists solar penetration
    • Fine crystals of ice suspended in turbulent, super-cooled water.
    • Water turbulence inhibits crystal coagulation into an ice sheet.
    • May accumulate on water bed surface or submerged objects forming anchor ice.
    • Also called 'needle ice'.
Frazil1 Frazil2

River Ice Formation
Thermal Ice Formation
  • Thermal ice tends to occur in slower flow.
  • Very clear, allowing for solar penetration
  • Border Ice
    • Forms along, and is attached to, river banks.
    • May also be attached to other obstacles such as bridge piers.
    • Calm water flow provides best conditions for growth.
    • Water level changes or surface waves can fracture existing border ice or detach it from the shore.
Frazil Ice Formation
  • Frazil ice tends to deposit...
    • Where the stream slope changes from steep to mild
    • Downstream from location that are turbulent enough to remain open most of the winter (e.g. rapids)
  • The implications of frazil deposition...
    • The thicker the ice, the longer it will take to break up
    • Potential jam location
    • Increases ice volume
  • When to be concerned about thicker than normal frazil deposition
    • Sudden period of intense cold when there is little to no ice cover to insulate the water surface
  • Frazil Pans / Pancake Ice
    • Rounded, flat accumulation of frazil ice with a rough, raised rim.
    • Shape and rim are due to repeated collisions with other frazil pans.
  • Frazil Rafts
    • Collections of frazil pans that have collided and frozen together.
  • Ice Bridge
    • Continuous ice cover of limited size extending from shore to shore like a bridge.
    • Tends to occur at natural flow constrictions.
    • Also referred to as 'arching'.
    • Can occur as frazil pans accumulate and nearly cover water surface.
  • Juxtaposed Ice
    • An edge to edge accumulation of frazil rafts and pans that appears similar to a jigsaw puzzle.
    • As the spaces between the pans and rafts freeze, the ice cover is strengthened.
  • Hanging Ice
    • In areas where an ice cover does not form, frazil is continuously produced.
    • As the slush moves into a flatter, slower moving portion of the stream, the frazil will accumulate under the ice cover. Can result in a hanging dam.
  • Anchor Ice
    • Ice attached to the channel bed.
    • Forms when turbulent flow transports frazil crystals or supercooled surface water deeper into the flow, allowing ice to build on underwater objects.
    • Can develop into extensive ice blankets.
    • Remains bonded to surface until ice buoyancy sufficient to lift attached material or until water warms, weakening the bond between the bed and ice.
  • Aufeis Ice
    • Ice on ice.
    • Forms when water from a spring, stream or culvert runs on top of existing ice and freezes.
    • This occurs wherever there are continuous sources of water and freezing temperatures.
    • Stream aufeis can fill the channel with ice that forces streamflow out of the banks to cause localized flooding.
  • Snow Ice
    • Ice on ice.
    • Weight of snow on ice cover enough to cause submergence, saturation and freezing.

Period of initial formation of a continuous ice cover on a body of water.
Predominated by border and frazil ice.
  • What to look for as river starts to freeze up
    • Is there border ice? How far from the shore has it grown?
    • Is there moving ice? What types are present?
      • Frazil slush?
      • Frazil pans?
      • Fragmented sheet ice?
      • Large sheets?
    • How much of the open channel does moving ice cover?
  • What to look for after ice cover forms
    • How smooth (or rough) is the surface?
    • How high are the surface variations?
    • Is there anything to indicate the ice is starting to decay? Is ice snow covered?
    • Is there melting snow?
    • Is the ice beginning to melt or rot?
    • Do you see 'candled' (or honeycombed) ice?
    • Are there cracks in the ice?
    • Are they parallel to shore? How far out?
    • Are they perpendicular to shore?
    • Is the ice fractured (parallel and through the ice) near the banks?
    • How far from shore?
  • Sheet Ice
    • Smooth, continuous ice cover.
  • Hummocked Ice
    • Flow under the ice collapses the juxtaposed ice cover which then refreezes.
    • Result is a thicker ice cover.

Period of disintegration of ice cover.
  • Thermal break-up
    • Weather factors most important
      • Warmer air temperature
      • Warmer water temperature
      • Reflectance of the ice surface; surface color influences absorption of sunlight
    • Direct sunlight plays a large role
    • Ice 'rots' or melts in place, much like on a pond or lake
    • Water on ice decreases reflection, may promote melting
    • Development of open water allows more heat to enter the flow, melting ice from below.
  • Mechanical break-up
    • Physical factors most important.
    • Ice cover is pushed higher or lower, causing it to break into pieces
      • Hydrodynamic forces acting on ice cover exceed cover strength
      • Results from an increase in stream discharge... increased energy to the system
        • Precipitation event
        • Snowmelt event
        • Dam operation... large, sudden increase in releases
    • Rule-of-Thumb...A stage (water level) increase of 1.5 to 3 times the thickness of the ice is needed to lift, break and transport the ice cover.
  • What to look for to indicate ice is breaking up
    • Are there cracks in the ice?
    • Are they parallel to shore? How far from shore?
    • Are they perpendicular to shore?
    • How far apart are the cracks on average?
    • When did the ice begin to move?
    • How long did it take for the ice to clear out?
  • Decay - Candle Ice
    • Rotten, columnar-grained ice.
    • Melt occurs at the edges of individual ice crystals.
    • Eventually, these crystals begin breaking away from the main ice body.
  • Rotten Ice
    • Ice in an advanced stage of disintegration.
  • Hinge Crack
    • Typically, hinge cracks form along both banks.
    • In narrow channels, a single crack may form down the middle of the channel.
  • Transverse Crack
    • Cracks across a streambed formed as the ice cover lifts with the rising water level.
  • Crack
    • Fracture formed in an ice cover or floe that does not divide it into two or more pieces.
    Ice Clearing
    • As the water level continues to increase, ice will be lifted until at some point it will be freed from the geometry of the channel and move downstream.
  • Ice Run
    • Flow of ice with the current of a stream or river, particularly with break-up.
    • May be light or heavy, and may consist of frazil, anchor, slush or sheet ice.
  • Ice Ledge
    • Narrow fringe of ice that remains along the shores of a river after break-up.
Many thanks to Dr. K. White with the Cold Regions Research and Engineering Lab and
Dr. F. Hicks with the University of Alberta, Edmonton for their photo contributions.

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