“Over the last decade or so, a huge range of interests — from environmental groups to fishermen to shipping experts to politicians — have raised the alarm over just how much this artificial connection has created an opening for invasive species such as the Asian carp to make their way through North America’s waterways. And the costs associated with the damage caused by these species have been high enough to prompt serious consideration of closing off the link between the Mississippi and the Great Lakes.

How high? First, consider the figure $18 billion. That’s the estimate the U.S. Army Corps of Engineers released last week to re-insert a physical separation between the Great Lakes and the Mississippi system. The full report, the Great Lakes and Interbasin Mississippi River Study, was commissioned by Congress to address the growing threat of invasive species in the area known as the Chicago Area Waterway System. The final report details a wide spectrum of actions — ranging from essentially maintaining the status quo to engineering a complete separation over a 25-year period — but doesn’t offer recommendations on which course to take.”