Terraces
Over the life of a river, the channel within which it flows is slowly eroded away through gradual downcutting into the valley. Terraces, steps, ancestral flood plains, are a series of flat platforms elevated above the modern floodplain of a given river. They can form both through aggradation and degradation, but downcutting leaves older terraces inaccessible even by modern floods.
Terraces are first created through the natural movement of a meandering channel over its floodplain, creating a wide, flat valley floor. Then some environmental trigger (e.g. tectonic uplift, increased sediment supply, increased water supply, or a change in base level) causes the stream to downcut more rapidly, leaving the old floodplain elevated and out of reach of the channel even during flood events. As this happens over and over again, a series of terraces is left behind.