pH is a measure of how acidic or basic water is and ranges from 0 to 14, with 7 being neutral. A pH < 7 is indicative of slightly acidic conditions, whereas a pH of > 7 indicates alkaline conditions. Surface water pH may range from 4 to 11, and this is largely dependent on the relative source of major ions (charged particles) within the river. There are multiple factors altering aquatic pH levels, including seasonality, atmospheric deposition, underlying geology and inputs from residential, industrial and/or agricultural runoff. Temperature and biological activity within the water body may also influence pH.

The pH of a river is a strong driver of aquatic ecosystem functionality. A below neutral river pH can increase the availability of toxic metals and ions through chemical processes, while a pH above 8 can convert non-toxic ammonium to toxic ammonia. A healthy South African river should not have a pH that varies more than 0.5 of a pH unit or > 5% of background pH values of all times of the day.

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Sources:

(1) https://www.usgs.gov/special-topic/water-science-school/science/ph-and-water?qtscience_center_objects=0#qt-science_center_objects
(2) http://www.dwa.gov.za/iwqs/wq_guide/Pol_saWQguideFRESHAquaticecosystemsvol7.pdf