This indicator measures the percent of surface water with impaired water quality, when compared with beneficial uses defined under the Clean Water Act. Surface water includes the length of small, medium, and large streams and rivers, and the area of lakes and reservoirs. Water quality criteria are defined that protect beneficial uses and include ambient concentration of an important parameter such as dissolved oxygen, pH, temperature, or heavy metals. An LD50 or other metric of these parameters is used to ensure that the designated use or uses for a given water segment are not impaired. Impaired water quality means that one or more of the criteria adopted to protect the designated use or uses of an individual water body segment are not being met. Leading causes of water quality impairment in our nation are excess nutrients (nitrogen and phosphorus), sediment/siltation, pathogens, and metals.