The most effective methods of keeping Asian carp from invading the Great Lakes via Chicago’s web of waterways could cost up to $18.4 billion and take 25 years to put in place, the federal Army Corps of Engineers concluded in a study released Monday.

But a corps official cautioned in a telephone briefing for journalists that there was no guarantee that the carp or other unwanted species would not get into the lakes by then.

The agency’s 210-page study, first ordered by Congress in 2007, laid out eight options to prevent the carp and other unwanted species from entering Lake Michigan, ranging from continuing existing efforts to building barriers that would seal the lake from the five Chicago-area streams that are linked to it.

Either blocking the lakefront waterways or blocking their two sources further inland would offer the greatest protection from invading species, the report said. But both options would prevent barges and other boats from using those routes, and would increase pollution in the lake and the waterways.