AutoCAD 2013/2014 DWG Format

[#1]

I made a lake using lines set at different elevations to show depth, is there a way to calculate the volume of the lake in AutoCAD 2013 from just the line elevations and area without having to make the lines a solid surface

[#2]

For rough estimate, loft bottom and top, MASSPROP command

or use C3D

[#3]

The volume is roughly 17,250 cubic units.  This was calculated inside plain AutoCAD directly from the drawing geometry using the MapWorks Surfaces add-on.