Concept Checking

[Jkaen]

Hey all, been a while since I posted here, as I got seconded out to our Cairo office, and no longer have a computer capable of much more than word processing.

Anyway, I am knocking round a concept for a world (well continent) and wanted to sense check things with you guys first.

Basic premise goes along the lines of:

Mountain ring sitting in ocean, various races living on the rock.
Large slab of rock magiced in to act as dam across opening in ring
Now trapped water magiced out.
races now free to expand onto new formed plains

From this point on I want the world to make as much sense as possible with no magic. Potential issues I see which I am not sure will cause problem are:

I have a river coming from the dam and forming a lake. Given this lake is below the water table is it possible to drain or will my river eventually drown everybody on the newly formed plains?

Can I logically only have 1 river on my plains?

I have no indigenous flora or fauna for my plains, assuming I bring these in from elsewhere also, is approx 1000 years a reasonable amount of time for things to establish like they had always been there?

Given fairly free reign over rock types am I ok assuming no seepage from the ocean to my plain?