Evaluation of the June 2005 flood in the Bow River on trout and mountain whitefish population
How BIG was the Flood in 2005?
Average monthly discharges for June are shown below. Historic data obtained from the Water Survey of Canada, indicates higher average discharges have occurred, particularly the flooding in 1932, coinciding with the construction of the Glenmore Dam. In 2005, average discharge in June was approximately 298 m3/s, peaking at nearly 813 m3/s. This is a large amount of water despite the dams at Glenmore and Bearspaw.
Published studies looking at fish communities after catastrophic flood events, indicate these communities typically respond to levels of size and growth exceeding pre-flood conditions. S uch events as a 1:100 year flood are often less catastrophic than activities that lead to or result in chronic stream degradation, such as sedimentation from road ditches or improper maintenance and installation of culverts.
Impacts to the Bow River Fishery
Fish and Wildlife Division staff evaluated the entire length of the Lower Bow River (Bearspaw to Cluny) in 2001. During this assessment, population sizes were calculated based on the catch rates of trout from electrofishing. In 2005, we re-evaluated the population sizes at 3 locations, after the June floods, allowing for a comparison to the 2001 study.
The three locations we looked at were:
Index Site (4 km) located downstream of Policeman’s Flats
Confluence with the Highwood River near McKinnon’s
Downstream of the Carseland Weir
Trout per Km
Index Site (4 km) located downstream of Policeman’s Flats |
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| The index site assessment is located above the confluence with the Highwood River. No significant changes were noted between assessments completed in 2001 and 2005. There was an increase in the small brown trout (less than 10 inches) in 2005, which is likely a result of increased efficiency of electrofishing. After the flood, less aquatic vegetation and improved water clarity allowed the boat to get closer to shore and improve catches. |
Confluence with the Highwood River near McKinnon’s |
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| Several kilometres were electrofished near the confluence below the Highwood River. A strong increase in smaller (10 inch and less) trout, especially rainbows, was evident in this section of the Bow River. This results in a significant increase in rainbow trout numbers per kilometre, but this is largely attributed to those small rainbows, likely displaced downstream from the Highwood and Sheep Rivers during the flood. Although the numbers of brown trout in the intermediate and adult size ranges indicate a decrease, there is no significant change in population numbers (i.e. error bars) |
Downstream of the Carseland Weir |
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The area below Carseland again shows an increase in overall abundance of both Rainbow and Brown trout. We suspect this is the influence of the smaller fish that may have been displaced downstream and from the Highwood River systems. At this location, there is an increase in the adult sizes of brown trout (i.e. >38 cm) that was not as evident in the two other upper locations. Overall, no significant changes in the population size below Carseland, except for the increase in the smaller (less than 10 inch) trout. The impacts the June 2005 flood may have had on the recently spawned rainbow trout in the Bow, Highwood and Sheep rivers is unknown. We will continue to monitor these rivers and assess the impacts over time. We do not expect to see any distinct impacts to the health of the Bow River Fishery! |
