The structure on the Des Plaines River is a choke point between the Illinois River, which is infested with the invasive carp, and Lake Michigan. The next step is developing design and engineering specifications, expected to take three to four years and cost about $28.8 million. Under the new agreement, the Corps will pay $18 million and Michigan $8 million. Both states will collaborate with the Corps as it designs the complex mechanism, which will require thousands of pages of drawings. Government agencies, advocacy groups and others have long debated how to prevent them from reaching the Great Lakes, where scientists say they could out-compete native species for food and habitat.