I came across this link to a PDF on Indiana Dams:
http://www.state.in.us/dnr/water/files/damguidelines.pdf

This is a cut and paste on the part in question:

Prior IDNR approval (permit) should be obtained by anyone desiring to construct
or significantly modify (work that is not routine maintenance) a dam before work
begins, if the dam meets any one of the following criteria:
- The drainage area above the dam is one (1) square mile or greater.
- The height of the dam, measured from the natural streambed below the crest of
the dam to the lowest point on the crest, is equal to or greater than 20 feet.
- The volume of water impounded by the dam at the emergency spillway elevation,
is 100 acre-feet or more.


I am a little confused on the way this is written (about the height). So would the height of the dam be measured from A to C, B to C or A to Spillway elevation?
A is the orginal dirt grade before construction.