SMALL DAMS

M anagement of water quality
These results highlight the advantages to having precise studies like the microhabitat
method to serve as tools in decision-making when establishing compensation flow
levels in accordance with the spirit of the law on fishing; the objectives of that law
were recently taken up again in the water resources legislation of 3 January 1992.
In addition to characterising habitat reductions at various low flows, it makes it possible
to identify sensitive species and stages and therefore the periods that may be
critical if flow is maintained too low.
Modulation of compensation flow
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It may therefore be proposed in some cases that compensation flow be modulated to
limit effects during such critical periods and to recreate a certain hydrological rhythm.
A study by Lignon du Velay was the opportunity to test various scenarios of modulated
compensation flow management according to the season, with two or three different
levels of flow in the course of the year 1 . Those different flow scenarios were then
converted into records of useful weighted surface areas for brown trout in an average
year and a dry year.
The main conclusion of that study concerned the summer period, which is the most
critical for most of the stages in the existing compensation flow situation. Increasing
compensation flow in the summertime would improve habitat potential in this case,
especially for adult trout, and all the more so in dry years.
Optimal Q % reduction in WUA versus max.
WUA at optimal flow
Q=0.5m 3 /s Q =1.5 m 3 /s Q =3m 3 /s
Brown trout Adult 2.5 23 0.5 1
Juvenile 1.75 15 2 4
Alevin 1.75 13 2 7
Fry 3 79 40 0
Atlantic Salmon Juvenile 4 63 17 0.5
Alevin 2.5 54 9 0
Fry 3 67 27 0
Table 1 - Reduction in weighted usable areas for brown trout and atlantic salmon at various flows at
the Sapet station on the Allier river
1. See Bibliography, reference 15, p. 158.