Positive correlations in logarithmic scale were found between catchment areas and densities of bacteria in the upper reaches of the Ishite River in Ehime Prefecture, Japan. The constants, i.e. the intercepts and the slopes, given in the regression lines were analyzed to examine the factors affecting the bacterial counts. For this purpose we introduced the term “effective precipitation ” which gives weight to the rainfall recorded near the day of sampling. It was suggested that total bacterial counts were affected by effective precipitation and water temperature. On the other hand, fecal coliform counts appeared to relate only to water temperature.

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