From OpenStreetMap Wiki
|waterway||river, canal, stream...||Really needed ? All part of waterway may have different type; but this can help distinguish main aspect of this waterway.|
|name||Waterway name||Optional but recommended|
|main_stream (or none)||one or more||waterway=river, waterway=canal, waterway=stream...|
|side_stream||optional|| Same relation kind as this, a branch of main stream than return to it.
Ordered by end of side stream (integrated in tributary sort).
|side_stream||optional||waterway=river, waterway=canal, waterway=stream... a branch of main stream than return to it.|
|tributary||optional|| Same relation kind as this, for direct tributary.
Ordered from upstream to downstream (integrated in side stream sort).
Ordered from upstream to downstream.
|riverbank||optional||Multipolygone of waterway like in Relations/Proposed/Rivers. Relation can have waterway=riverbank, but all way part of this relation already have it.|