The New Zealand mudsnail, Potamopyrgus antipodarum was first discovered in the Snake River, Idaho in the 1980's. It is now rapidly spreading throughout the western US and has become established in rivers in seven western states and three national parks. It was discovered in the tailwaters below Glen Canyon Dam in March 2002. Subsequent searches in Grand Canyon found it distributed more than 225 miles downstream from the dam, so it was likely introduced several years earlier. Mudsnail populations often reaches densities greater than 100,000 per square meter in suitable habitat, and the species is associated with alterations in primary production and decreases in native invertebrate populations in rivers that it has invaded. Biologists are concerned about potential impacts it may have on native species, fisheries, and aquatic ecosystems in the western US.