Fish and other aquatic biota that live in rivers and
streams have to contend with the variability of the
flow regime.
How variable is runoff/discharge?
From year to year?
From month to month
From day to day
Crowsnest River –4th order, around 3-4 m3/sec—10-20 m wide DA around 400 km2
Annual mean discharge of the Crowsnest River at Frank
20
m3/s
14
12
10
8
6
4
2
0
1982
1987
1992
1997
2002
2007
Exceedence probability
Annual mean discharge of the Crowsnest River at Frank
0.10
0.25
0.50
0.75
0.90
1995
12
1991
9.8
2005
8.7
1993
6.3
2002
6.2
1996
5.9
1992
5.5
2004
4.6
1999
4.4
1994
4.2
1986
4.1
1990
3.8
1988
3.7
1989
3.5
1998
3.1
1987
2.5
1997
1.7
2003
1.5
2000
1.4
2001
1.2
m3/s
14
12
10
90 percentile
8
6
75 percentile
4
50 percentile
2
25 percentile
10 percentile
0
1982
1987
1992
1997
2002
2007
We can now plot these data on cumulative probability paper
Exceedance probability (percentiles)
99.9 99
90
70
50
30 20 10
1
o
0.1
100
10.0
Discharge
m3/sec
o
o
o
o
1.0
Non-exceedence probability (percentiles)
Recurrence interval is the reciprocal of the probability
Based on this plot we can estimate that 1 year in 100 there will be a year wetter
than 20m3/s and a year drier than 0.8 m3/s, if there is no-longterm climate change
1 yr out of 4 discharge is above this line
Annual average 6.0 m3/s
2 yr out of 4 discharge above this line
Annual average 4.1 m3/s
3 yr out of 4 discharge above this line
Annual average 2.7 m3/s
Before Ottawa R dams
After Ottawa R dams
River Management can significantly
alter the seasonal hydrograph
St.Lawrence River Flood Plain at Lac St. Pierre—has rarely been flooded since the
late fifties—What effect might this have on the river?