However, the streams in the headwaters are small compared to the volume in the rivers near their mouth, further downstream. Dilution really is one of the solutions to pollution, but small streams have a limited natural capacity to clean up excess nitrates and other pollution from concentrations of people, factories, roads, and farms.
Even a small community can severely pollute a small river. Bassett is a factory town that was "encouraged" by the state to clean up the wastewater dumped into the Smith River. Henry County created a Public Service Authority in part to finance the cleanup. The communities downstream of Bassett are the beneficiaries of the pollution control - as are the fish, wildlife, and recreationists who use that stretch of the river.
Martinsville gets its drinking water from the headwaters of Jones Creek, while Henry County draws its water from Smith River three miles below Philpott Lake. Both avoid extracting water from below Bassett. Philpott Lake serves as a giant settling basin, so it too enhances water quality downstream of the dam. (The Smith River below Philpott Lake is a top fishing destination for brown trout.)