Either by reduced stream velocity, changes in the channel cross-section, or change to drier climatic conditions. Might occur when water flows from a stream into a lake or ocean. The water velocity drops abruptly. If sea level rises, then the stream's gradient decreases. This reduces the stream's capacity to transport sediment.
Valley fills in at the mouth.
With time, this aggradational process is tranferred upstream, so that sediments accumulate throughout the stream profile. After Pleistocene glaciation, rising sea level drowned the mouth of many river valleys and triggered aggradation along stream channels. D. Degradational Landforms 1. Cause and effect relationships in degradational processes. In general, the cause of degradation is any event the increased the capacity of a stream relative to its sediment load.