Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 406

Ward and Short (1978) grouped macroinvertebrates into four types, on the basis of observed responses
to river impoundment: (1) Tolerant organisms having a ubiquitous distribution but forming large populations
under certain types of river regulation; (2) Organisms present in unregulated streams, but favored by
certain types of regulation; (3) Intolerant organisms present in unregulated streams, but reduced in, or
eliminated from, regulated streams; (4) Indicator species not normally present in unregulated streams, but
favored by regulation.
Considerable reorganization of the invertebrate community existing during and shortly after dam
construction may, however, occur as the water quality, and thermal and discharge regimes, stabilize. For
example, the impoundment of Mill Creek, Wisconsin, USA, had a dramatic effect upon the benthos
(Table 7.6). Many species were eliminated, and the fauna became dominated by chironomid larvae and
amphipods, while other benthos changed rapidly during, and after, impoundment (Hilsenhoff, 1971).
Four general response-groups may be recognized: species that were common prior to impoundment but
became reduced or eliminated after it (e.g., Hydropsyche and Elmidae ); species that were common under
pre-dam conditions, eliminated during dam construction or shortly after, but returned subsequently in
limited numbers (e.g., Baetis brunneicolor); species that seemed largely unaffected by impoundment but
experienced population explosions during, and immediately after, dam closure (e.g., Chironomus spp.);
and species that showed an increase in abundance after dam completion (e.g., Simulium vittatum ).

Table 7.6 Short-term macroinvertebrate changes during and shortly after dam construction at a riffle 300 m below
t he dam, Mill Creek, U.S. (after Hilsenhoff, 1971)

Before During 7-15 months after 19-27 months after

Ephemeroptera, Baetidae

Baetis brunneicolor 21 0 0 10

Diptera, Chironomidae

Chironomus spp. 0 508 345 3

Micropsectra spp. 5 80 239 41

Orthocladius spp. 74 54 66 210

Stictochironomus spp. 0 6 25 0

Diptera , Simuliidae

Simulium vittatum 173 336 901 742

Diptera , Tipulidae

Dicranota spp. 1 20 24 4

Amphipioda, Gammaridae

Gammarus 34 - 50 132

Pseudolimnaeus

Trichoptera , Hydropsychidae

Hydropsyche betteni 177 81 0 2

Coleoptera , Elmidae 142 17 0 0

Summer total phosphorous (ppm) 0.08 - 1.24 0.58

Summer total nitrogen (ppm) 0.77 - 3.16 1.37

(Values = number per standard sample; - = Not measured)

7.1.5.4 Fish and Fisheries
Dam construction appears to have had a greater impact upon riverine fish than any other human activity.
In 2 years after the completion of Lake Kainji, for example, catches of fish from the River Niger, Nigeria,
were reduced by 30% (Lelek and El-Zarka, 1973). One immediate consequence of river impoundment is the
conversion of naturally lotic environments to lentic habitats. The impoundment of relatively fast-flowing