Arcata Marsh, also known as "The Arcata Marsh and Wildlife Sanctuary", is a series of artificially restored wetlands in Arcata, California, on Humboldt Bay, which also function as a filtration system for wastewater treatment. The Arcata Marsh is around 320 acres (half a square mile), and includes 5 miles of trails. It is located about a half mile from the center of Arcata.
Humboldt Bay had been a rich resource for indigenous groups in the area for thousands of years, but European settlement of the area greatly damaged the native ecosystem, as filling in areas of Humboldt Bay for dairy pasture, construction of wharves and industrial buildings, and the presence of residential wastewater greatly degraded the marsh between the 1850s and the 1960s. Changing attitudes towards natural resources made the local populace want to do something about the marsh, especially as the area became a center of the environmental movement in the 1960s. The city also needed a way to treat its wastewater, so what happened is that after conventional water treatment, the water was let into a series of ponds, which were the Arcata Marsh.
Arcata Marsh is a popular place to visit, both for locals and for the many people who come to Northern California to see the natural sites. Humboldt Bay is one of the best places to watch birds in the United States, and the Arcata Marsh might be the most convenient place to do that. If there is a downside to the marsh, it is that many of the scars of the areas industrialization are still visible (although they also have their own aesthetic charms, with old pilings sitting in the middle of ponds), and that as a place with many visitors, the Marsh might be too loud for many more timid birds.
But the Marsh shows how municipalities can make environmental restoration work for them. The area around the water treatment plant would normally be unusable for residential or commercial purposes, so turning it into an environmental amenity is a way to "use" land that would otherwise be going to waste.