Not counting flooding, we sometimes apply too much water to our lawns with automatic sprinklers.
Too much water on the lawn can deprive the roots of oxygen and retard development, or possibly even kill the lawn. During the warm humid summer months, too much water can increase the likelihood of disease. If you are concerned that you may be applying too much water, reduce the watering time until you begin to see lawn wilt, than increase your time by a small amount.
On average a lawn only needs about 1" of water each week either from rainfall or supplemental watering.
Over watering increases the chances of water run-off, water that can't be absorbed through the soil, and trickles into the street.
This water is more likely to enter our storm sewers and then into our rivers. Storm sewers do not pass through the water treatment filtration plants but instead are dumped directly into the closest stream or river.
Over-watering a lawn increases the possibility of washing fertilizers, herbicides and insecticides into the river.