River morphology

thumb|upright=1.1|Nam Khan flows into the [[Mekong at Luang Prabang in Laos.]] A tributary, or an affluent, is a stream or river that flows into a larger stream (main stem or "parent"), river, or a lake. A tributary does not flow directly into a sea or ocean. Tributaries, and the main stem river into which they flow, drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean, another river, or into an endorheic basin.

end of a river or stream

start of a river or stream

thumb|Confluence of the Bhagirathi and [[Alaknanda Rivers at the Ganges in Devprayag, India]] thumb|The same confluence viewed from upstream at a different time; note the swirl of sediment from the Alaknanda River

thumb|Rhine–Meuse–Scheldt delta, with the flow from right to left, showing several streams branching off from their main streams

landform of an entrenched transverse valley

change in shape and direction of river channels over time

big circular ice floe (ca 1 m ... 50 m diameter) rotating on a river