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A HISTORICAL PERSPECTIVE OF ICE JAMS ON THE LOWER MOHAWK RIVER
John I. Garver
Jaclyn M.H. Cockburn
Geology Department
Union College
Schenectady NY 12308
Ice jams are an annual occurrence on the
Mohawk River. As a northern temperate river,
ice jams are expected, but it is clear from the
occurrence and relative frequency of ice jams,
that the Mohawk is particularly vulnerable to
ice jams and the hazards associated with them.
Here we briefly review the history of
significant ice jams, we highlight research on
reconstructing ice jams, and then we propose
an active monitoring system that could be used
by emergency personnel to better respond to
active jams during breakup.
Ice jams occur when the frozen river breaks up
during events that result in rapid increase in
discharge. Ice out and ice jams always occur
on the rising limb of the hydrograph, when the
floodwaters are building. When flow starts to
rise it is not uncommon for unimpeded ice
runs to develop, but invariably the ice gets
blocked or impeded along the way by
constrictions in the river, especially where the
flood plain is reduced in size.
In a survey of the past ice jamming episodes,
we have come to the conclusion that any
restriction or narrowing of the flood plain and
constriction of the channel is a possible jam
point (Johnston and Garver, 2001). An
important point worth keeping in mind is that
deep sections of rivers move more slowly than
shallow ones, and therefore surface flow and
therefore ice movement is reduced. So, a
transition from a shallow to deep channel may
generate a point where ice can backs may
occur up, regardless of floodplain geometry.
The lower part of the Mohawk River has
chronic ice jam problems and the historic
record indicates that the section between the
Stockade and the Rexford Knolls is the most
jam-prone in the entire watershed (Figure 1).
As such the empirical evidence of ice jam
locations are relatively well known to local
emergency management authorities. However,
there is a general lack of information as to the
significance of individual jam points, and how
often jams occur in different areas. In addition,
many jam sites are inferred based on little or
no data.
Commonly, ice jams will build to sufficient
thickness to dam the river and this can result
in spectacularly rapid rates of water rise
behind the dam. In March 1964, the USGS
Cohoes Monitoring Station recorded the
greatest hourly flow ever recorded on the
Mohawk River when discharge peaked at 143
k cfs (1000 cubic feet per second), although
the mean discharge for the day was only about
half this level. In comparison to other floods
on the Mohawk River this was not a big event,
but the ice jam that formed resulted in very
high water levels for a short time: the high
discharge was due to an ice jam that formed
and subsequently burst forming an ice-jam-
release wave that surged downstream (Jesek,
1999). News reports from this event suggest
that the elevation of the backed up water was
about 25 feet, although as far as we know this
is unverified.
History. One of the worst ice jams in
Schenectady history occurred on 13 February
1886 when a spectacular ice gorge formed and
lodged in and around the islands near
Schenectady. In this event, one-foot-diameter
trees on the flood plain were reportedly
snapped in half, and when the water receded,
the remaining ice was piled 30 to 40 feet high
(see Scheller et al., 2002).
In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2009 Mohawk Watershed Symposium, Union College,
Schenectady NY, 27 March 2009
25
Figure 1: Map showing the elevation of measured ice scars on bank-lining trees along the Mohawk River
in the Schenectady area. Scars on trees indicate the elevation of a slow-moving jam that caused damage
along the riverbanks. The highest levels of tree scarring occur upstream from the Rexford Bridge and
upstream of the Burr Bridge abutments. This area has chronic ice jams (from Lederer and Garver, 2000).
During this event, ice jammed at the Scotia
Bridge, which linked downtown Schenectady
with the Village of Scotia. Our analysis of the
historic records indicates that this is a chronic
jam point (same as the Burr Bridge abutments
at the end of Washington St.).
The January 1996 flood is the worst recent
flood and it is fairly well documented. This
mid-winter thaw event (19-20 January 1996)
resulted in the breakup of the Mohawk River
and significant flooding, especially on the
Schoharie Creek. As recorded at the USGS
station at Cohoes, the event resulted in a mean
discharge for the day on the Mohawk of 92 k
cfs with a peak discharge of 132 k cfs
resulting in extensive flooding of the Stockade
area in Schenectady. Elevation of ice scars on
trees lining the river banks (Figure 2) allow
reconstruction of ice elevations and from these
data (Smith and Reynolds, 1983), jam points
may be inferred (Lederer and Garver, 2001).
In the 1996 event, the highest ice-scar
elevations occur between Lock 8 and the
Stockade area in Schenectady, and almost no
abrasion occurs below the Rexford Bridge.
Two possible jam points are inferred from the
data based on abrupt downstream elevation
changes of the highest ice damage on bank-
lining trees. One sharp elevation increase
occurs between the Freeman’s Bridge and the
D&H railroad bridge where ice scar elevation
increases from ~224 feet to ~226 feet (Figure
1).
Another sharp elevation drop occurs upstream
of the still-standing abutments of the old Burr
Bridge (a.k.a. “Scotia Bridge” after
reconstruction) where maximum ice-scar
elevations increases from ~226 feet to ~230
feet. We infer that the ice dam at the old Burr
Bridge broke shortly before flood crest based
on the maximum elevation of ice scaring just
downstream in the Schenectady Stockade
(228.4 feet), which falls just short of height of
the river at crest (229.5 feet). Both jam points
occur where abutments and berms (i.e. those
associated with bridges) have dramatically
restricted the flood plain thereby causing a
severe restriction in flow.
In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2009 Mohawk Watershed Symposium, Union College,
Schenectady NY, 27 March 2009
26
Figure 2: The tree-lined park in Schenectady’s
Stockade still bears ice scars from the 1996 ice
jam. Here the scar is about 14-15 feet above river
level. Photo taken in the Summer of 2000, five
growing seasons after the event, so it is well on its
way to healing itself (Photo: J.R. Lederer).
The 15 March 2007 flooding in the Stockade
was entirely related to ice jamming
downstream from the city of Schenectady
(Figure 3). During this event, discharge in the
Schenectady reach of the Mohawk River never
surpassed 45 to 50 k cfs, which makes this an
insignificant event with respect to expected
high water. However, the formation of the ice
jam and the resulting backup of water was
entirely responsible for the inundation that
occurred in the Stockade. This reinforces
earlier findings that the key component in
these events is the evolution of stage
elevation, which is not directly related to
discharge. Back up of water behind the 15
March 2007 ice jam resulted in a ~13 feet
elevation change. Breakage of the ice dam at
about 6:45 PM resulted in a downstream rush
of water referred to as an ice jam release wave
that was recorded at the USGS station at
Cohoes. Peak discharge at Cohoes occurred at
8:00 PM and then total discharge was 51.6 k
cfs. It is possible that that was an ice jam
release wave, but the measurements are too
coarse (every 15 minutes) to determine this
with certainty.
Figure 3: Flooding in the Stockade that resulted
from the 2007 Ice jam on the lower Mohawk River.
Picture taken in the late afternoon (~18:00) at
nearly peak stage elevation. Peak discharge
during this event was c. 50k cfs, but ice jamming
resulted in back up of water that caused flooding
(Photo: J.I. Garver).
The 2009 Ice Jam was, by historical
standards, an insignificant event. The ice out
event that occurred between 8 Mar and 10
March 2009 resulted in bank full conditions,
and an ice jam occurred, but there was not
significant flooding during this event. During
this event, we collected data on the elevation
of the river using two strategically placed
pressure transducers during ice out which
provides unique insight into how ice
movement progresses (Figure 4).
Following a relatively cold winter with heavy
precipitation, a moderate thaw accompanied
by moderate rainfall increased runoff and
subsequent breakup of river ice. At about
10:40 AM 8 March the water level rose
rapidly in the Stockade of Schenectady. At
about noon the R.A.C.E.S notes indicated that
the ice had jammed and stopped in place. The
toe of the ice jam was situated between the
Stockade and the Freeman’s Bridge (in,
essentially, Schenectady). The ice floe that
was jammed in place extended from the toe to
a point slightly upstream from Lock 8, so it
was about 4-4.5 miles long (~7 km).
In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2009 Mohawk Watershed Symposium, Union College,
Schenectady NY, 27 March 2009
27
Figure 4: Difference in river elevation between the Stockade and Lock 7 measured by pressure transducers
at 300 s intervals for the March 2009 ice out event. In this graph in situ measurements were made as a ~7
km ice jam lodged and then worked through the narrow channel in Schenectady. This plot shows the
differential between the Stockade where water backs up due to ice jamming. High values in this plot
indicate that the Stockade water level is higher than downstream sections of the river, and this backup is
inferred to be cause by ice damming. The effect of a surge from breakup appears minor in this event (i.e.
Jasek, 1999). (All times Daylight Savings time).
Downstream the peak flow at the Cohoes gage
was recorded at 13:00 of that same afternoon
(8 March) when 27.4 k cfs was recorded (all
times are Daylight Savings Time).
Historically, this is relatively low flow for an
ice out event. At the highest point the
differential between the Stockade and the
Lock 7 occurred at 2:00 PM (14:00) when the
difference was recorded as being 1.69 m.
This means that a 1.69 m rise occurred in 200
minutes (3.3 hr) or a rise of about 0.5 m per
hour during this interval. The jam stayed in
place with little apparent movement, until the
next afternoon, 9 March, when the ice floe
became dislodged and worked its way
downstream at about 16:20. Ice continued to
pass through the system through that evening
and the river was ice-free soon after.
Ice Jamming in Schenectady. Our analysis of
the historical records suggests that the Rexford
knolls, a bedrock-incised part of the Mohawk
channel, is a distinct and chronic jam point for
ice floes. This is because it is narrow,
confined and there is no floodplain that allows
the water and ice to spread out. Our research
shows that over the several hundred years, it is
typical for ice jams to form on the Mohawk
between the Old Burr Bridge abutments and
the Rexford Knolls - the most common jam
points on this entire stretch of the Mohawk
(between Schenectady and Lock 7).
As such, these ice jams pose a unique and
serious hazard for the city of Schenectady (and
to a lesser extent Scotia). We’d note that this
part of the river channel is unique because it
lacks a floodplain and because it is bedrock-
bound.
This part of the Mohawk is relatively young
having captured the main flow from the Paleo-
In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2009 Mohawk Watershed Symposium, Union College,
Schenectady NY, 27 March 2009
28
Mohawk at about 10 Ka (see Wall, 1995;
Toney et al., 2003). Prior to this time, it is
inferred that the Mohawk flowed north up the
Alplaus channel and through what is now an
abandoned channel occupied by Ballston Lake
and adjacent lowlands in the paleo-channel.
Although this is ancient history in the
evolution of a river, it is relevant here because
it provides a framework as to why this part of
the Mohawk River has such a special hazard.
Since capture and readjustment of the course
of the Mohawk, the river has had to rapidly
incise into the bedrock high that now forms
the Rexford Knolls. Even since settlement,
this stretch of the river has been treacherous,
and today we see that large ice floes have
trouble getting through this narrow incised
part of the channel. This is a natural feature,
and the reduction in the effective width of the
floodplain by abutments and berms – the
Burr/Scotia Bridge being a major one – has
exacerbated the hazard.
We suggest that the best mitigation strategy
for this situation is a real-time monitoring
network of pressure transducers that can
provide fast reliable data on the condition of
the ice movement through this key reach of
the river (Robichaud and Hicks, 2001; White
et al., 2007). These data could provide
emergency personnel insight into ice dynamics
(i.e. Figure 4) and a predictive tool that they
have not enjoyed in the past.
References
Jasek, M., 1999, Analysis of ice jam surge and
ice velocity data, Proceedings of the 10
th
Workshop on the Hydraulics of Ice Covered
Rivers, Winnipeg, pp. 174-184.
Johnston, S.A., and Garver, J.I., 2001, Record
of flooding on the Mohawk River from 1634
to 2000 based on historical archives,
Geological Society of America, Abstracts with
Programs v. 33, n. 1, p.73.
Lederer, J.R., and Garver, J.I., 2001, Ice jams
on the lower Mohawk River, New York:
Lessons from recent breakup events.
Geological Society of America, Abstracts with
Programs v. 33, n. 1, p. 73.
Robichaud and F. Hicks, 2001, Remote
monitoring of river ice jam dynamics.
Proceedings of the 11th Workshop on the
Hydraulics of Ice Covered Rivers.
Scheller, M., Luey, K., and Garver, J.I., 2002.
Major Floods on the Mohawk River (NY):
1832-2000. Retrieved March 2009 from
http://minerva.union.edu/garverj/mohawk/170
_yr.html
Smith, D.G. and Reynolds, D.M., 1983, Tree
scars to determine the frequency and stage of
high magnitude river ice drives and jams, Red
Deer, Alberta. Canadian Water Resources
Journal, v. 8, no. 3. p. 77-94.
Toney, J.L., Rodbell, D.T., Miller, N.G., 2003,
Sedimentological and palynological records of
the last deglaciation and Holocene from
Ballston Lake, New York, Quaternay
Research, v. 60, p. 189-199.
Wall, G.R., 1995, Postglacial drainage in the
Mohawk River Valley with emphasis on
paleodischarge and paleochannel
development. PhD dissertation, Rensselaer
Polytechnic Institute, p. 1-352.
White, K.D., Hicks, F.E., Belatos, S., Loss, G,
2007, Ice Jam Response and Mitigation: The
need for cooperative succession planning and
knowledge transfer, Proceedings of the 14th
Workshop on the Hydraulics of Ice Covered
Rivers.
In: Cockburn, J.M.H. and Garver, J.I., Proceedings from the 2009 Mohawk Watershed Symposium, Union College,
Schenectady NY, 27 March 2009
29