4h Edition Terrain Modification Rule?

[Blackcloud6]

I'm setting up An Army at Dawn Scenario 6. It calls for a Wadi marker to be placed in Board 77 Hex 1002 and a Wadi placed in Board 78 hex 1011.  Both of these hexes are slope hexes of 20 meters elevations.  So, what becomes the wadi?  Is it just the hex the marker is placed in or does the entire hill encompassed by the contour line become a wadi?  Both seem strange.   The 4th Editon RB is silent on how to do this and there are no instructions in AAaD Special Rules.