A process of removing semi-solid material from water, often referred to as waste water treatment, is used to treat waste water from manufacturing processes, sewage and the like. To that end, a variety of techniques have been developed and used in the treatment of waste water. One popular technique incorporates a settling tank (alternately referred to as a clarifier) that separates semi-solid material (popularly known as sludge) from the water via gravity. Using this technique, the waste water (influent) enters a clarifier, wherein the sludge is encouraged to settle to the bottom. As the sludge settles, density gradient layers form in the clarifier, with the densest layers disposed toward the bottom of the clarifier. Relatively clear water then exits from the top of the clarifier, while the sludgeclimate change in parks nationwide.

In some Alaska parks, the climate transformation is too gradual to be detected by casual visitors, Winfree said. But many experts see it.

"Those of us that go into these places over time can definitely notice the changes," said Jim Stratton, Alaska regional director for the National Parks and Conservation Association, an environmental organization.

Some changes are obvious in Kenai Fjords National Park, a popular destination south of Anchorage known for its ice-capped peaks, tidewater glaciers and abundant marine life.

The retreat of Exit Glacier, one of the park's best-known features, has forced park managers to reroute trails through areas that were under ice just a few years ago. The glacier's retreat also has left a sheltered pavilion that was built in the 1990s far from the