Hello.
i found river that is created from few lines/parts.
How to join these parts in one?
For which purpose do you want to join them ? Where do you want to join them ?
Please do not attempt to join the data of the river on OSM.org. The previous mappers might have had a good reason to split the river. Perhaps the name changes, or they mapped the width, or …
When you use the data locally feel free to merge the different parts. This topic has been discussed on help.openstreetmap.org. Search there for merge and road (which is asked more often). You might find an answer that provides a ready-to-go solution for your environment.
You should use relation to combine different parts of a river
http://wiki.openstreetmap.org/wiki/Relation:waterway
Here is Volga river from the very beginning to the end
https://www.openstreetmap.org/relation/1730417
There is a limit on the length of each line (way) - 2000 nodes. Therefore it is just not possible to map in detail any longer rivers in one way. I normally think of splitting a way when it gets to a few hundred nodes. The relation allows you to map at different widths, allow mapping of main and side streams, et.c. as can be seen in the Volga relation shown above.
Could You describe how to create that relations?
In the editor.
The only editor with good support for creating relations is JOSM.
Nonsense.