Surface Water Volume Calculation

I'm trying to calculate the volume of surface water resulting from the insertion of a dam or weir along a stream.

I have created a surface for the weir & water along the terrain mesh, but can't quite figure out how to isolate the resulting volume. Any help or suggestions appreciated!! Attached the rhino & GH files.

Eventually I'd like to optimize the amount of water that can be retained by a maximum number of weirs given a maximum weir height... but for now calculating the volume resulting from 1 weir is a start...