discharge at freezeup were significantly different
Aroostook River include the Tinker Dam pool in
or more unsteady, or, to a lesser degree, if the
Fort Fairfield, the flat, braided reach in
weather were either milder or more severe. We
Crouseville, the vicinity of the USGS Washburn
feel that these variations, however, would not be
gage, and above Donnelly Island in Wade.
significant.
All the major jamming sites on both rivers
were found to be at transitions from a steeper to
a milder river slope. These are classic ice jam-
Breakup
Although the 199192 documented breakup
ming locations. The river simply has insufficient
was relatively mild and uneventful, we feel the
energy to transport the ice through these sites so
timing of the running and jamming of the vari-
that the ice slows or stops and begins to jam. A
ous ice reaches to be typical. Another winter of
jam will remain stable for a longer period at
observations and measurements is required to
those sites where flow cross section is large and,
verify this.
therefore, flow velocities are low.
We identified three general ice reaches during
breakup within the study area on the St. John
ICE JAM MITIGATION MEASURES
River. In each reach, the breakup process is ini-
tially independent of the events in the other
Our review of past ice jams and the field ob-
reaches. The first reach is from Grand Falls, New
servations carried out during the 199192 winter
Brunswick, to Fort Kent or about 60 miles, the
season showed us that the ice jams on the St.
second is from near the St. John town line to the
John River and the Aroostook River are breakup
Big Black River confluence area (50 miles), and
jams. Ice jam flooding, therefore, can be alleviat-
the third covers the remaining upper reaches, up
ed in the St. John River basin by controlling ice
to the Northwest Branch, 53 miles upstream from
runs during breakup. The engineering methods
the Big Black River.
that are available to do this can be divided into
The St. John River reach from the base of Big
four main categories: ice-control structures,
Rapids in Dickey to the confluence with the Alla-
channel modifications, operational techniques
gash River sustains yearly ice jams. The severity
and early warning systems. Depending on loca-
of these jams mostly depends on how quickly the
tion and ice conditions, these methods can be
breakup takes place. The combination of rapid
used alone or in combination.
breakup and an earlier ice jam remaining in place
at Dickey caused a record flood on 9 April 1991.
Ice-control structures
The 1991 and 1992 breakups were close to the ex-
The general purpose of an Ice-Control Struc-
treme. A fast stage rise combined with relatively
ture (ICS) is to contain or delay ice runs until the
competent ice during a runoff event results in
ice-carrying capacity of the river downstream of
early ice runs, as happened in 1991. In 1992, how-
the ICS is sufficient to avoid ice jamming and re-
ever, the slow melt and rotting prior to breakup
sulting flooding at critical downstream loca-
provided for an easy ice-out and no flooding.
tions. An ICS can be located at an existing jam
More than 50% of the ice had melted in place be-
site or could create new jam sites at locations
fore any of the ice runs in 1992.
when no adverse effects will ensue. Ice-control
The reach above the ledge outcrops near the
structures include the following.
St. John and Fort Kent line and the Grand Falls
Dam pool are other major jam sites. Small, inter-
Weirs or small dams
mediate jams also occur in other reaches. The
Approximately 6 to 8 ft high, weirs either sta-
major jamming areas in the upper river include
bilize a naturally forming ice sheet or create a
the Big Black River confluence, PriestlyDead-
low velocity pool where an ice sheet will form.
water, Seven Islands and the confluence area of
They also provide a local storage area to tempo-
the Northwest and Southwest branches.
rarily hold incoming brash ice during an up-
The Aroostook River also has three major ice
stream breakup. They can be equipped with ice-
reaches that act independently of each other. One
holding piers to enhance their ice-holding capac-
reach extends about 12 miles from below the Car-
ity, and with bascule gates to allow recreational
ibou dam to Fort Fairfield, another from above
navigation during the ice free seasons. Examples
the dam to the Village of Crouseville in Wash-
are the ICS on the Oil Creek, near Oil City,
burn or about 20 miles, and the third reach ex-
Pennsylvania, and a proposed ICS for Cazen-
tends upstream from Crouseville.
ovia Creek, near Buffalo, New York (Fig. 6).
The major ice jam flooding reaches on the
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