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The most extensive damage to the system has been caused by dam construction. River organisms cannot live in the lakes created by dams. Also, dams alter downstream thermal regimes, especially hypolimnion drain dams This is because the dams release cold (4 degree C) water into the downstream river in the spring and summer, thus removing spawning and diapause-breaking signals to organisms in the spring, and removing the warm (Thermal Reservoir Research) summer temperatures required for growth. Also, dams release relatively warm, 4 degree C, water in winter; the combination flattens the river temperature curve; river organisms do not receive vital signals (Lehmkuhl, 1972) and species are eliminated from the system.
Saskatoon sewage once seriously damaged the system (Lehmkuhl, 1970) . Effects on organisms are shown in the above link. The effect from sewage was still severe in the summer of 2000; numbers of species and numbers of individuals show at least 80% reduction from what would be expected.
Organic Enrichment and Caustic Materials
In the past, untreated urban sewage from Saskatoon added nutrients and organic enrichment to the river, and there is evidence that caustic material entered the river as well. The delicate physical structure of the river habitat was overwhelmed, and organisms were adversely affected.
Rooted vegetation and algae grew in large mats providing substrates for filter feeders such as blackflies, and a fme organic mud filled the spaces in gravel and rubble substrate, making these areas unsuitable for a large community that lives on the undersides of rubble and boulders (see for example, Lehmkuhl ----. These conditions were found in the summer of 2000, as well as 20 to 30 years ago. The South River is most affected; there is little abnormal macrophyte or algal growth in the North River.
Photographs on this websiete show that the South Saskatchewan River, from Saskatoon to Birch Hills Ferry, has been modified by sewage (Lehmkuhl 1970, Lehmkuhl 1981, Lehmkuhl unpublished). Nutrients from sewage fostered plant growth, and added much fine organic mud to the system (see photos, Home).
Lehmkuhl 1970 shows that from the sewage impacted area, the gills appear to be burned and blackened in the mid-region of the Hydropsychid Caddis Fly (Trichoptera). Fungal masses grow on the head and antenna of the Heptageniid mayfly (Ephemeroptera). The stonefly (Plecoptera), probably Isoperla sp. has partially moulted, but the cast skin has not been shed from the body, and the insect is pulled into a curved position by the unshed cast skin. Specimens such as these are evidence of extreme conditions of pollution. Sewage treatment has since been improved, but the areas