Environmentalists are cheering new federal rules limiting development on wetlands, which were announced by the U.S. Army Corps of Engineers yesterday. The regulations, aimed at slowing the steady disappearance of the ecologically sensitive areas, would make it much harder to build homes, roads, or anything else near small streams or creeks. In most cases, the rules would require developers and public agencies to secure specific permits for projects affecting a half-acre of wetlands or more, down from the current minimum of three acres. Each year, the U.S. loses an estimated 70,000 to 100,000 acres of wetlands, which provide habitat for a number of endangered species, reduce the risk of floods, and help purify drinking water. The Corps developed the new rules in response to a lawsuit filed by the Natural Resources Defense Council.