Runoff water (which contains nutrients, metals and organic compounds) add to the volume/discharge of the water system while also increasing the amount of suspen

Runoff water (which contains nutrients, metals and organic compounds) add to the volume/discharge of the water system while also increasing the amount of suspended material in the water (turbidity). The increase in water discharge also causes sediment to lift from the bed where e coli bacteria live happily in the sediment (think of shaking a bottle with solids that have fallen to the bottom). The potentially harmful bacteria is now suspended in the water for a few days, also growing in number as a result of the runoff nutrients until the increase in the water volume drops and the water can no longer suspend the sediment. This cycle is the largest reason why, it's not just the runoff (which can also contain animial feces with e coli, etc), it's the runoff with nutrients stirring up the water and lifting e coli which then feasts on the nutrients. https://www.usgs.gov/publications/escherichia-coli-bacteria-density-relation-turbidity-streamflow-characteristics-and https://www.usgs.gov/publications/escherichia-coli-bacteria-...