There are at least three ways to protect water quality in water supply systems:

• Use a relatively pristine natural source
• Manufacture a pristine source through storage reservoirs
• Filter or treat nonpristine water sources

Historically, water providers have tended to take a supply-side perspective, always seeking to solve quantitative or qualitative shortage issues by extending their supply systems deeper into remaining pristine sources (first strategy above), but over time they have been forced to enact the second and third strategies. As explored by other articles in this volume, strategy two dominated water management practices until the last quarter of the 20th century. Some commentators now believe that the time has long since passed when such supply-side strategies were appropriate and that demand-side management (forcing/encouraging water users to use water more ...