Retrogradation's the term for the landward change in position of the front of a river delta with time. This occurs when the mass balance of sediment into the delta's such that the volume of incoming sediment's less than the volume of the delta that's lost through subsidence, sea-level rise, and/or erosion. As a result, progradation's most common: during periods of sea-level rise which results in marine transgression. This can occur during major periods of global warming and the melting of conti… (More on Retrogradation)