with upstream ice booms (Perham 1983). At
ever, due to both the potential adverse effects on
breakup the reduced river width can improve the
river morphology and the possibility that a new
stability of unfrozen ice accumulations, making
ice jam location may be created downstream
them thinner and less of an impediment to flow,
where the artificially steepened cutoff reenters the
and it might even prevent breakup of the solid ice
natural channel.
cover (CummingCockburn and Associates
Another form of channel modification is a di-
1986). Such structures can also create a storage
version channel to divert excess flood waters
area for ice floes at breakup, reducing the volume
around a jam or beyond an area to be protected. A
of ice passing downstream. Opposing dikes have
diversion channel can also be used in conjunction
been used for ice control on the Burntwood River
with an ice control structure to improve its perfor-
in Manitoba and on the Pasvik River in Norway
mance. For example, if an ice control dam or weir
(Perham 1983, Janzen and Kulik 1979).
were used to control a breakup ice run, a high-lev-
Artificial islands of soil or rock or both have
el diversion could be used to limit the discharge
also been employed in many areas, including the
reaching the structure, reducing river stages to
St. Lawrence River near Montreal, to help hold
prevent local flooding and ensuring the stability of
ice in place, just as natural islands often do (Per-
the ice being retained. Such a diversion can be de-
ham 1983). A smaller variation on the use of artifi-
signed to remain dry except during flood events
cial islands is the construction of stone-filled tim-
so that it will be available to function as an open-
ber cribs to anchor an ice sheet. An example of
water channel and not add to the ice supply reach-
using timber cribs for controlling ice can be found
ing downstream ice jam sites.
on the Narraguagus River in Cherryfield, Maine
Instead of diverting excess flood waters, it is
(Perham 1983). Three cribs, oriented in a triangu-
also possible to divert or store ice during breakup
lar pattern, are located about 125 ft upstream of a
in areas where it will cause less damage. Even
7-ft-high dam and spillway.
without human intervention, significant volumes
A longitudinal dike or series of structures
of ice are often left behind during an ice run in side
aligned with the flow along the center of a chan-
channels, on floodplains or simply grounded on
nel can be employed to decrease the effective
the riverbanks. By developing low overbank areas
width of a river (CummingCockburn and Asso-
where ice can easily leave the channel during
ciates 1986). Since the river width that an ice cov-
breakup, perhaps supplemented by dikes or
er or accumulation must arch across is reduced,
booms to redirect ice movement, the volume of ice
stability is increased and a thinner cover with
passing downstream can be substantially reduced.
lower water levels may result. Channel divider
Ice anchors are an additional form of channel
structures can be used to promote early ice cover
modification that can be used to control ice. While
formation, delay ice cover breakup, and initiate
ice control dams and weirs are constructed to slow
and maintain stable breakup accumulations at
the flow of water by raising water levels, ice an-
desirable locations. A divided channel design
chors are essentially pools excavated in rivers to
was incorporated into the Burntwood River
help stabilize ice during formation or breakup.
diversion project in Manitoba (Cumming-Cock-
During freezeup the pool can help to form an ice
burn and Associates 1986).
cover early in the winter season and obstruct the
passage of ice floes to downstream areas. During
Channel modifications
the breakup period the pool can serve to moderate
Ice jams tend to form in areas of change in
stage fluctuations caused by rising discharge, thus
slope, riverbends, slow-moving pools and con-
maintaining a stable ice cover in a manner similar
strictions. Channel modifications can be used to
to an ice control weir or dam. An ice anchor con-
improve the passage of ice through such reaches.
structed on the Bow River near Calgary, Alberta,
involved the excavation of about 140,000 yd3 of
An example of such a channel modification can
bed material, resulting in a pool approximately
already be found in the BufordTrenton area in
650 ft long, with a top width of about 330 ft and an
the form of a channel cutoff constructed by the
average depth of about 11 ft (Andres and Fonstad
Corps of Engineers in the late 1950s between cross
1982). On rivers with significant transport of sedi-
sections 1569.24 and 1574.16. This cutoff isolated
ment, however, such dredged pools could be re-
a sharp riverbend, eliminating a long length of
filled and rendered useless in a short period of
channel and significantly increasing the local
time.
slope. Such cutoffs must be used with care, how-
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