The effects of irrigation effluent on a western Canadian prairie river

Abstract

Field studies were conducted during the peak irrigation period(July–August) at the mouth, 50 m upstream and 50 m downstreamoffour irrigation return flows draining a 50 km stretch of theOldmanRiver in the short grass prairie zone of southern Alberta.Watersamples were analyzed weekly. Physico-chemical variableschangesonly slightly at each site but salinity and conductivity weresignificantly higher at the downstream site (Expanse Coulee)andall sites showed an increase over 1978 data. Conductivityalong theleft bank of the river was measured at 50 m intervalsdownstream ofExpanse Coulee until, after 1.5 km, it declined to theupstreamlevel which in the laboratory was shown to require a 660%dilutionof the effluent. In field experiments conducted at ExpanseCouleeit was shown that macroinvertebrate biomass was significantlylowerdownstream of the mouth and significantly higher upstream.Chlorophyll-a was not significantly different betweenthemouth and upstream with both significantly higher thandownstreamwhere it was not detectable. Inorganic carbon assimilation byepilithic algae and heterotrophic potential of the bacteriawereboth significantly higher at the mouth and upstream andsignificantly lower downstream.Organic matter in the sediment was not significantly differentupstream or downstream of the mouth but there was a higherpercentage of large particles (125–250 μm) at the mouthanddownstream of EC a higher percentage of medium(60–125 μm) andsmall (0–60 μm) particles were found 50 m upstream. It isconcluded that at present differences in invertebrate biomass,chlorophyll-a, inorganic carbon assimilation byepilithicalgae and heterotrophic potential of epilithic bacteria atExpanseCoulee are not directly attributable to increase in salinitybutare more probably the result of increased suspended solids andsaltation.