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

Types of Ice Jams - Classified by Season of Occurrence
  • Freeze-up Jams
    • Form early to mid-winter
    • Air temperature below freezing
    • Steady water flow
    • Composed of frazil and broken border ice
    • Smooth to moderate surface roughness
    • Unlikely to release suddenly
  • Break-up Jams
    • Can occur any time after ice cover formation
    • Generally form mid to late-winter
    • Can form more than oce per season
    • Air temperatures near freezing
    • Unsteady water flow (surges)
    • Composed of broken sheet and border ice
    • Moderate to extreme surface roughness
    • Highly unstable! Can release suddenly
  • Mid-winter Jams
    • Form with mid-winter thaw
    • Characteristics of break-up jams
    • Refreeze in place forming significant blockage
    • May result in additional problems later in the season

Types of Ice Jams - Classified by Formation Characteristics
  • Anchor Ice Accumulation
    • Active frazil deposited on bed
    • Grows upward from stream bed, raising water level
    • In turbulent, shallow areas
    • Occurs during freeze-up
    • Referred to as an 'ice dam'
  • Surface Congestion
    • Layer of frazil pans and rafts
    • Forms at constriction or floating obstacles
    • In slow-moving water
    • Occurs at freeze-up or break-up
    • Referred to as a 'surface jam'
  • Undercover transport/deposition
    • Frazil/small blocks carried beneath ice cover come to rest in slow-moving areas
    • Accumulations build until velocity too great to allow deposition
    • Tends to fill deep sections with accumulation following topography of streambed
    • Referred to as a 'hanging dam'
  • Submergence/Frontal Progression
    • Pans/blocks submerge at upstream edge of jam or obstacle
    • Jam progresses upstream at thickness greater than individual pieces
    • Little transport beneath jam
    • Thickness great enough to withstand increased external forces
    • Referred to as 'narrow channel jam'
  • Failure and Thickening
    • Existing jam fails when downstream forces surpass jam strength
    • Cover transported downstream until resisting force increases
    • Cover thickens when it comes to rest, increasing jam strength
    • Commonly occurs with break-up jams
    • Referred to as 'wide channel jam'
  • Pressure Ridge Ice Jam
    • Line or wall of broken ice forced up by pressure.

Ice Jam Affected Hydrograph

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