Here is a tool for artist to make their river or spline like mesh with in unity editor with a really easy tool !
watch its video here:
and get it here :
I tried this some time ago. Very awkward to use and it constantly went weird on me and I had to start over. I finally just gave up. Just another asset that is difficult to use with sparse documentation.
If you were to improve it, make it easier to use and more stable especially for longer rivers, it might work better. It is a great concept, the ability to build a river mesh in the game. I already had my terrain with the valley for the river so something like this should have been perfect. But every so often, the mesh would just bend on itself and stick out oddly. Very stressful to use.
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thanks, for the report, please tell me where did you have difficulty, if you could send me an screen shot with your problem it would be more help to me to make it all better !
I will see if I have it still but I probably just abandoned and deleted it. It was a while ago. Honestly, you need to make it work smoother. Try taking a terrain with an indented river in it and laying down a river mesh with your tool. Make it a large terrain, 2000 x 2000 or even smaller multi-tiles. Try ti get a river from one side to the other, step by step. It is hard to do and frustrating. Make sure the river has some curves, like a real river and some vertical change, from slightly higher on one side to lower at the other. If you can get that to work, then your tool will be great. 
Thanks for your replies ,
this game “Parvaneh” had use my river tool in their game and had not any problem,
http://www.parvaneh-game.com/en/galleryhttp://www.parvaneh-game.com/en/gallery