You can use Sandbox Tools, From Contours on what you have. Cut and paste in place the geometry of one into the other.
I don’t know if the result is a stream like structure or not ;^)
What would this actually be made of? Is it like a slide (a slide like structure ;^)?
If this were like a water feature, you’d start with excavation and then add a liner. Boulders, etc. are added after the fact. So maybe you make the basic structure onto, or into, which ‘rocks’ would go…
Instead of the free-hand line that has a gazillion line segments, start with a rectangles or a U shape with fewer edges. Copy that a number of times. You can stich the Us together and move, rotate, resize, faces and edges.