The Clean Water Act designates several levels of protection for surface waters such as rivers, lakes, and wetlands. The highest level of protection comes from a designation as an Outstanding National Resource Water (ONRW). ONRW status means that no activity is permissible if it will result in lower water quality than already exists in the affected water.
Examples
Amigos Bravos website about ONRW designations in New Mexico
Koktuli River Outstanding Natural Resource Water Nomination. A coalition of groups nominated the Koktuli River in Alaska as an ONRW.
Kentucky Waterways Alliance webpage about ONRW designations in Kentucky.
Alaskans for Responsible Mining 