Abstract: Mississippi River flooding in 2013 and 2016 caused severe underseepage and development of several medium to large high-energy sand boils behind the landside levee toe at Kaskaskia Island, IL. This levee system is located between St. Louis and Cape Girardeau, MO, and is part of the Kaskaskia Island Drainage and Levee District on the Middle Mississippi River. Flooding on the Mississippi River in 2013 and 2016 was below the design flowline for this levee. This report documents a case history study into the causes of seepage, piping, and sand boil development at a levee reach at Kaskaskia. Site-specific geotechnical data were collected and evaluated to determine the causes for poor performance at the studied levee reach locations. Data collected involved design documents, geologic and geotechnical borings, closely spaced cone-penetrometer tests (CPTs), electrical resistivity surveys, laboratory soil testing of sand boil ejecta, CPT samples from targeted stratigraphic horizons in the subsurface, and both piezometer and river-stage data. These data indicate sand boils present within this levee reach involved a chronic seepage condition that became progressively worse through time. This condition was directly related to the underlying site geology, namely the top stratum thickness and the depositional environment in this levee reach.