Right, lets start on this...

First up, I have drawn the rough contours for the map. We need a peninsula that forces the water into it instead of dropping down the side of it. That would be an unusual relief to generate that but I reckon that two hills either side of the river and then the peninsula in front should do it. The thieves guild can then dam it and divert off to the right.

I have done this with contours for the cliff, hill and sub sea. They are not uniformly spaces. Since the cliff is 250 feet height I will shade with 2' per gray scale where mid gray is sea level. The top of the cliff, and the sub sea will need to be smoothed a lot to blend it together but the cliff will have to be done separately.