Backflow is the result of water flowing in the reverse or unwanted direction. This generally happens when the normal supply or upstream pressure in a pipe drops below the downstream pressure, allowing contaminated water to get drawn in the opposite direction of the usual water flow. Not only is it inconvenient, but it can also be dangerous. Backflow—caused by backpressure or back siphonage—can be a serious health risk because it contaminates potable water supplies with foul water.