The adjustable hydraulic structure (AHS) is a uniquely designed barrier (see below) that will replace the 6th Street Dam as the primary sea lamprey barrier on the river. The AHS structure will be constructed approximately one mile upstream of the 6th Street Dam. Because of the significant sea lamprey production potential within the Grand River, sea lamprey escapement upstream of the project site would have a significant impact on the Lake Michigan fishery. The AHS is designed to ensure that sea lampreys do not migrate upstream of the City of Grand Rapids. This structure will deny sea lampreys access to more than 1,900 miles of new stream habitat, which would otherwise cost the Great Lakes Fishery Commission between $1.2 million and $1.8 million annually to treat with lampricides. The AHS structure is critical given just one sea lamprey escapement event upstream of Grand Rapids could inflict an estimated $60 million in economic loss.