ted or slushy in appearance. Deteriorated ice may
jams. Freeze-up ice jams generally occur during
often appear darker. Pools of water may form on
the early to middle part of winter, during periods
top of the ice, enhancing the deterioration pro-
of very cold weather and relatively steady dis-
cess. Water may also run over the top of the ice as
charge. Breakup jams occur from mid- to late win-
water levels increase. The shape and size of leads
ter and are usually associated with sudden large
and cracks change as ice melts because of contact
increases in discharge due to snowmelt or precip-
with warmer air and water. Changes in water lev-
itation or a combination of both. Breakup jams
el may also expose vertical sections of ice that can
commonly occur during periods of higher air tem-
provide information on the condition of the ice.
peratures and thaws. The dates and times of jam
Sheet ice which has begun to deteriorate has a
formation should be recorded.
characteristic appearance, termed candling. Can-
Locating the downstream end of the jam (the
dling will occur soon after exposure to sunlight
jam toe) is essential because knowledge of the
though, so it is best to check for candling soon
conditions at the jam initiation point (e.g. open
after an ice piece is exposed.
water, solid ice cover, bridge) can help in identify-
The ice cover may be monitored remotely by
ing the cause(s) of the jam and in evaluating pos-
satellite, although current technology is expen-
sible mitigation measures. Variability or lack of
sive, images are only occasionally available, and
variability of the jam toe location from year to
the resolution of the images may not be desirable.
year is also important to note, since some long-
Two major satellites that could be used are Land-
term mitigation measures may rely on jam initia-
sat (U.S.) and SPOT (European). The Landsat im-
tion at a particular location. Locating the upstream
agery covers an area roughly 100 100 miles, at
and downstream ends of an ice jam may be diffi-
an approximate cost of 00, while the SPOT
cult for a ground-based observer in the Lower
image covers an area roughly 40 40 miles at an
Platte River system, since the rivers cover a large
approximate cost of 00. The SPOT image of-
area and access may be limited. Aerial reconnais-
fers higher resolution (or in other words, the size
sance would be particularly useful in obtaining
of the smallest distinguishable object) at 3366 ft,
this information as well as information about the
while the Landsat satellite offers resolution of only
appearance of the jam and locations of flooded
100 ft. Cloud cover can obscure the target area,
areas.
rendering the collected image useless. The satel-
The rate of upstream progression of an ice jam
lites also only pass over a specific area once every
should be monitored, if possible. The location of
1618 days. A new radar system is being tested
the upstream end of the jam should be noted peri-
that should be able to detect areas of river ice
odically, both to estimate progression between
with flowing water underneath. This technology
reaches and to aid in emergency response deci-
may prove useful for detecting the location of the
sion-making. Again, aerial reconnaissance pro-
main channel for some mitigation techniques.
vides the most complete data on ice-jam progres-
However, the cost of this is unknown, and an
sion when access is limited. River geometry and
image may not be available in time to be of use.
hydraulic conditions affecting jam progression
should be noted. For example, it may take several
hours longer for the ice to progress upstream past
Jam formation
a riffle than through a relatively uniform reach.
and evolution
Observations of ice-jam formation processes
are important in evaluating the effectiveness and
Other useful data
timing of mitigation measures. Important param-
Knowledge of the potential upstream ice sup-
eters include the location of the downstream end
ply is useful in estimating the extent, thickness
of the jam; conditions at the jam initiation point,
and possible stages associated with an ice jam.
such as stage, discharge and condition of ice cov-
Important parameters include the length and av-
er, if any; any obvious cause for jam initiation,
erage width of the ice cover upstream from the
such as obstruction to ice transport by a compe-
jam, the extent of open water between the jam
tent ice cover; and the time of jam formation rela-
and the upstream ice cover, and the presence and
tive to the occurrence of other events such as pre-
stability of upstream ice jams which might be ad-
cipitation and increases in discharge.
ditional sources of ice to the jam. Estimates of the
It is important to identify the type of ice jam at
thickness and relative competence of any upstream
the different jam sites because different mitiga-
ice should be made, if possible. Again, actual ice
tion measures are suited to different types of ice
thickness measurements from the surface of an
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