The first flush of rain after a dry period washes accumulated dust, bird droppings, pollen, and other contaminants from the roof surface. This first flush is the most contaminated water in any rain event and should not enter the cistern. A first flush diverter is a simple device plumbed into the downspout that captures and diverts the first volume of water (typically 0.5 to 1.0 liters per square meter of roof) to a location away from the cistern, then automatically switches to direct subsequent, cleaner water into storage. First flush diverters are inexpensive, easy to install, and significantly improve the quality of water entering the cistern.