Apocorophium lacustre abundance in rock bag samples collected during 2020–2021 benthic amphipod survey across the Illinois Waterway (n = 308). The x-axis indicates a sample’s location in river kilometers along the length of the Illinois Waterway. See Figure 1 for a detailed description of the Illinois Waterway but note that river kilometers are identical for portions of the Chicago Sanitary and Ship Canal and the Calumet-Sag Channel/Calumet River. The value x = 0 represents the confluence of the Illinois and Mississippi rivers; x = 526 in the Sanitary and Ship Canal represents the Chicago Lock and Dam where Lake Michigan drains into the Chicago River, and x = 536 is where Lake Michigan drains into the Calumet River (located south of the Chicago River).