Bankside reservoirs are man-made reservoirs generally sited close to a river from which water is pumped to be used for potable water supply. The use of a reservoir in this way provides a significant degree of bioloigcal and physical treatment for the water. Bacterial levels typically fall by several orders of magnitude and level of suspended matter als falls significantly. This is particularly advantageous in enabling low-lamd river waters to be used which may be carrying pathogens from upstream agricultural use or sewage disposal. Bank-side reservoirs also provide a substantial reserve of water in the event that the riverine source becomes unacceptably polluted. In such cases the inlet is closed until the pollutant has passed. There are duisdantages to the use of bank-side reservoirs in that they take a very large area of land adjacent to the river and in summer they can support explosive growths of blue-green algae which can reach bloom proportions and cause unacceptable levels of toxins to be released into the water.