in freeze up and a similar advance in break up
16
. These break ups occur when both forces are reduced to a mini-
mirror the longer term response rates found by Mag- mum and the ice cover simply melts away, similar to the
nuson and others
8
but caution is required in relying on way lake ice melts. By contrast, the largest floods are pro-
such simple temperature-based relationships because duced when the two opposing forces are greatest – a large
they can change over time
6,17
. flood wave colliding with a strong, intact ice cover
4
.
Large-scale, comprehensive records of river and lake-ice
thickness are relatively rare. One data set compiled for
Water level
(m above
Canada over the last 50 years
18
does not reveal any ob-
sea level)
vious trends over the latter part of the 20th century
19
,
although smaller-scale regional trends in Northern Eu- 220
rope and Asia have shown a tendency to thinner ice over

l
e
v
e
l
the same period
20
.
a
t
e
r
218

w
Due to the complex relationship between climate and
216

i
c
e
-
j
a
m
freshwater ice conditions
6,21
, future projections of river and
u
m
lake ice have largely relied on the temperature-based rela-
m
tionships described above. Projections generally indicate
214 a
x
i
O
p
e
n
-
w
a
t
e
r

le
v
e
ls
1990 open
water peak
flow
M
further delays in freeze up and further advances in break
up, with the amount of change depending on the degree
212
0 4 000 8 000 12 000
of warming that is forecast
10,22
. For accurate prediction of
many ice characteristics, such as composition, thickness,
Discharge (m
3
/s)
strength and even duration, however, the complicating ef-
Observed annual maximum water
levels during spring break-up,
fects of snow cover need to be considered
2,23–26
.
1962-1996
Figure 8.3: Example of enhanced water levels produced from
River flows, break up and flooding
river ice, Liard River (Canada). The lower curve shows the corre-
spondence between river flow and water levels under open-wa-
ter conditions. The much greater maximum water levels possi-
River-ice break up on cold-region rivers is often the most ble under ice-jam conditions are illustrated by the upper curve.
dramatic hydrologic event of the year and capable of pro- The transition in break-up severity from dynamic to thermal
ducing flood-level conditions exceeding those possible
break-up effects (see text) is depicted by the gradually shaded
under higher flows during the open-water period
1
(Fig-
area between the two curves. Dots are observed annual maxi-
ure 8.3). In temperate climates, river ice can go through
mum water levels during the spring break up. The 1990 dashed
line shows the maximum recorded flow for the Liard River – but
a series of freeze-up/break-up cycles, whereas in colder
note that the water level corresponding to this peak flow is lower
climates break up is typically a spring event. In either
than for many break-up events with much lower flows. Effects of
case, break up starts when the driving forces – primarily
climate on snowmelt runoff and ice characteristics will lead to
the flood wave from snowmelt, sometimes augmented by regional changes in break-up severity and associated frequency
rainfall – exceed the resisting forces operating to keep the
and magnitude of ice-induced flooding.
ice cover intact (ice thickness and strength). The mildest Source: Based on Prowse and others 2002a
27
CHAPTER 8 RIVER AND LAKE ICE 205