1 . The people of Chicago, Illinois, US, can thank three canals every time they drink a glass of clean water today. The building of the Illinois-Michigan Canal, the Chicago Sanitary and Ship Canal and the Cal-Sag Canal all helped the Chicago River start flowing in a different direction in 1892.

Since their construction over a century ago, these waterways have connected Lake Michigan and the Mississippi river network and ensured a regular flow of fresh water of Chicago.

However, some unusual and unfortunate side effects have also been created because of their construction. Asian carp(鲤鱼), also known as silver or flying carp, are threatened in their native habitat in the Chinese Heilongjiang, Yangtze, and Pearl River basins. But they have become increasingly successful at filling the Mississippi river network since first being introduced into Arkansas catfish farms in the 1970s to control algae(水藻). By swimming into the canals and spreading further across the country, there are now major fears that this introduced species may be able to access Lake Michigan and the wider Great Lakes - which store an incredible 21 percent of the entire world’s fresh water.

“The actual influence of Asian carp on the Great Lakes has yet to be explained clearly, and we think this uncertainty is part of the reason why they are such an obvious issue,” says Dr Cory Suski, Associate Professor in the Department of Natural Resources and Environment Sciences at the University of Illinois. He points out that adult Asian carp are too large to be hunted by anything locally, that they have a prolific(多产的) reproduction rate, and they can steal food from the other native species in the rivers. “Could they survive in the Great Lakes or is there not enough food for them?” he asks.

Stopping the fish from spreading has therefore become a key concern for local lawmakers, who have made efforts such as building electric fences in the Chicago Sanitary and Ship Canal. However, any big efforts to stop the carp - such as allowing the Chicago River to once again flow in its original direction - would in turn threaten the security of the city’s water supply, replacing one big problem with another.