Question on the creation of cards (Cities: Skylines)

How to create a river with normal flow?
I only see this option: making a ditch from one end to the other, on one edge put 6 water sources. Only it is not flowing from one end to the other, spread out from source in all directions, and the farther the flow is weaker on the other side all the water is.
And the finished cards are all fine rivers flowing without any sources.