A river.
The definition for river is "a large natural stream of water flowing in a channel to the sea, a lake, or another such stream."
However, in your question you asked for a 'land form', but you also asked for a 'stream of water'. This confused me slightly, so please let me know if I did not interpret the question you were asking correctly. Also, a river is a stream of water, however, is not a landform.
