Figure 5. Cross section of a freezeup jam.
ment play important parts in the development of nearly all the jam types in the
IAHR classification.
Season of occurrence
Ice jams are typically called freezeup or breakup jams, in accordance with the
season in which they form. This classification suitably represents the hydrological
and meteorological conditions prevailing during jam formation.
Freezeup jams form during periods of subfreezing air temperatures when frazil
ice production is great. Their composition is mainly frazil and broken pieces of
shore ice, as depicted in Figure 5. Frazil ice forms in areas of high water velocity
and turbulence, where heat loss from the water surface is greatly increased. Some
areas may remain open and produce frazil throughout the winter. Subfreezing air
temperatures reduce basin runoff, resulting in fairly steady water discharge from
base flow. The ice discharge varies as frazil production increases or freezeup jams
form and cut off the supply of frazil to downstream reaches.
When frazil travels downstream, it agglomerates as slush, which rises to the
water surface and forms ice pans. The pans may break upon passing through very
Figure 6. Surface jam resulting from juxtaposition of frazil pans.
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