Straddling the border between northeastern Texas and northwestern Louisiana lies a 32-kilometer- (20-mile-) long body of water known as Caddo Lake. Long brimming with fisheries and waterfowl habitats, the lake played host to something new in 2007: an invasive weed. Known as Salvinia molesta or Giant Salvinia, the weed threatened to smother the largest natural lake in the American South.
This pair of images, acquired by NASA’s Landsat 7 satellite, show the progress of the weed infestation on Caddo Lake. In the bottom image, dated April 18, 2001, the lake is clear of the invasive weed. In the top image, dated, April 19, 2007, floating weeds appear as mats of green near the shore—an extension of the verdant green surrounding the lake’s blue waters. The weed’s intrusion into the lake is especially apparent in the lake’s western extremity, where the spreading weed appears as a pale, almost unearthly green.
In the months following the acquisition of the April 19 image, the weed continued spreading. According to the U.S. Geological Survey, Salvinia molesta can double in size in as little as two days, making it one of the world’s most noxious aquatic weeds. According to The New York Times in July 2007, Texas residents living along the west side of the lake spent $35,000 installing a 2-mile (3.2-kilometer) net to try to keep the weed from spreading. Holes left in the net to allow boats to pass, however, allowed the weed’s leaves to float through in some places, sometimes clinging to the boats themselves.
As reported by The New York Times, the Giant Salvinia mixed with water hyacinth, another invasive aquatic plant. Water hyacinth also has the ability to spread rapidly, evidenced in Lake Victoria and the Rio Grande.
Lake Jänisjärvi is a roughly oval-shaped lake, some 13 by 17 kilometers (8 by 11 miles) across, in northwestern Russia, near the Finnish border. The basin for this lake was formed hundreds of millions of years ago by a meteorite impact.