the emiquon national wildlife refuge and the emiquon project cover the historic beds of "flag lake" and "thompson lake", which were shallow, alluvial lakes created by the illinois river during the geological period that followed the last ice age. heavy loads of sand and silt carried southwest by the river created almost random, undulating topography along the river's bed. the river responded to these deposits by repeatedly shifting its course, leaving long, narrow sections of abandoned riverbed behind it. two of these sections became flag lake and thompson lake.