The response of Escherichia coli and total coliform concentration to increases in river discharge was investigated. Artificial hydrographs were generated on eight occasions between 21 October 1979 and 3 March 1981 by releasing water from Thruscross Reservoir in North Yorkshire into Fewston Reservoir. The majority of the releases were made after rainless periods to isolate the effects of stream channel entrainment from those induced by rainfall on the land surface. In the absence of rainfall, bacterial concentrations are shown to increase more than 10-fold in response to stage increases. It is suggested that two stores of bacteria must exist on the catchment, the first being a land store and the second a channel or near-channel store. Movement from the land to the channel store must relate to hill slope hydrological processes, whereas movement between stores in the channel fluvial system may be closely allied to sedimentary processes. Some consideration is given to bacterial levels in relation to European Economic Communities guidelines for contact recreation.

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