Perhaps river banks should be steeper , but it is not easy to do .
Try to get as close to the river banks before the landscape gets pulled up when using the height tool.
In case you can look underneath, like a lifted up carpet, heigthen the landscape a bit further away then.
And fill up the flat parts with trees. Just like it is in Mostar itself close to the river banks.
Molva's Mostar bridge looks great by the way.
(I wish I could create 3d models...)