Residents of Chicago, Illinois, have the collective efforts of the Illinois-Michigan Canal, the Chicago Sanitary and Ship Canal, the Cal-Sag Canal, and the phenomenal effort of reversing the flow of the Chicago river in 1892, to thank for their regular supply of clean water. 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 to keep the city well hydrated.

However, there have been some less predictable – and less desirable – consequences of this hydrological merging. Asian carp, also known as silver or flying carp, are threatened in their native habitat in the Chinese Heilongjiang, Yangtze, and Pearl River basins, but have become increasingly successful at populating the Mississippi river network since first being introduced into Arkansas catfish farms in the 1970s to control algae. Through latching onto the canals as a way of spreading their domain further across the country, there are now substantial fears that this invasive species may be able to access Lake Michigan and the wider Great Lakes – which store an incredible 21 per cent of the entire world’s fresh water.

‘The actual impact of Asian carp on the Great Lakes has yet to be defined clearly, and we think this uncertainty is part of the reason why they are such a prominent issue,’ says Dr Cory Suski, Associate Professor in the Department of Natural Resources and Environmental Sciences at the University of Illinois. He points out that adult Asian carp are too large to have any local predators, that they have a ‘prolific’ reproduction rate, and they can outcompete native species for food. ‘Could they survive in the Great Lakes or is there not enough food for them?’ he asks.

‘Would they move into and take over tributaries? No one is predicting positive benefits should Asian carp enter the Great Lakes, hence the impetus on keeping them contained.’

Halting the exploratory activities of the fish has therefore become a key concern for local lawmakers, who have undertaken efforts such as installing electric fences in the Chicago Sanitary and Ship Canal. However, any large-scale efforts to create obstacles for 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 substantial problem with another.

This was published in the May 2017 edition of Geographical magazine.