I struggled with how to control widths of streams imported from shapefile.
I actually am modeling trails, not streams, but that category works well for anything linear.
If you do this, use the order and magnitude at very bottom. I believe the magnitude is a multiplier, and I leave it at 1. Change Order to make things wider or narrower.

I saw another way old post on using buffer prop of water area type to do this but I tried that and the linework did not drape. Water areas seem to be one elevation only.
In the past, we made narrow closed shapes and colored them as coverage areas.
Using the stream type solves that so I'm surprised this width issue did not have several posts on it. We have one now 
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I'm just here for the Shelties