Water quality is an important indicator of ecosystem health. It is crucial to the maintenance of healthy habitat for aquatic organisms and a source of drinking water for terrestrial wildlife and humans. Water quality in a national park may reflect activities taking place upstream of the park’s surface waters as well as within the park itself. Road construction, recreational activities, livestock and agriculture, sewage leaks, deposition from atmospheric pollutants, and many other factors can impact water quality. The San Francisco Bay Area Network (SFAN) of national parks is particularly susceptible to these impacts because of its close proximity to a major urban center. With this in mind, the Inventory & Monitoring Program closely tracks water quality in the six of the eight SFAN units to determine range, variability, and trends - and to compare these with federal and state water quality objectives.