This year flooding has ravaged thousands of homes and businesses in communities across the U.S. And scientists say the prevalence of invasive weeds is one of the factors that may be contributing to the damage.

These foreign invaders are overrunning many vital "riparian" lands – the ecologically diverse natural habitats that run along the millions of miles of our nation's waterways and help to prevent or moderate flooding.

"Healthy riparian areas populated by native plants can store thousands of gallons of water per acre," says Linda Nelson, plant physiologist with the U.S. Army Corps of Engineers and former president of the Aquatic Plant Management Society. "They filter the water that flows into a stream after a storm or snow melt and can also mitigate the effects of river flooding. But unfortunately the protection capacity of many of our vital riparian areas is being degraded by invasive weeds."

The native plant species typical of a healthy riparian corridor prosper there and have dense root systems to hold soil in place and protect against erosion. As a result, the soil is highly permeable and can absorb water entering the river and overflowing the banks. Healthy riparian land can even improve water quality as dense, native vegetation absorbs potential contaminants and traps sediment.

But weeds that overrun native riparian vegetation can change everything and seriously degrade our nation's valuable water resources. Common invaders include reed canarygrass (Phalaris arundinacea), saltcedar (Tamarix aphylla) and leafy spurge (Euphorbia esula).

Another good example is knotweed, an aggressive species from Asia introduced here as an ornamental. The most common varieties of this troublemaker include Japanese knotweed (Polygonum cuspidatum), giant knotweed (P. sachalinense), Himalayan knotweed (P. polystachyum) and a Japanese and giant knotweed hybrid (P. X Bohemicum). Knotweeds have been spotted in 41 states and are becoming a real threat to riparian areas, particularly in the Pacific Northwest.