(1) Opposite of degradation!
(2) When sediment deposits in a watercourse, and more of it accumulates than gets washed away, it builds up. This is aggradation.
Aggradation causes a stream bed to rise and its slope to flatten out.
Sometimes also seen where a stream flows into or out of a lake, where a natural dam forms at the point where the flow of water loses its force.
(3) The cause of flood plains and alluvial fans, often trending from large rocks to gravel to sand, out into the finest silt that gets mucky and treacherous when wet.
If you go there, and see eyes watching you, don't go there, OK?