A cleaning system for an agricultural combine includes a cleaning shoe including a chaffer having an upstream extremity and a downstream extremity, a cleaning fan for establishing a flow of air upwardly through the chaffer, and a transverse chaffer dam mounted proximate to the downstream extremity transversely obstructing a crop layer flow path extending over the chaffer from the upstream extremity to the downstream extremity while permitting air and threshed crop material to flow thereover and thereunder through a transverse opening between the chaffer dam and the downstream extremity.