If you accidentally save over your base grid once you've merged a dam into it, there is a very easy way to fix the grid without going and creating a new grid.
If you create a new grid, its likely that it won't have the same origin and dimensions as the older grid. This means that it can't be used for volume calculations with older grids since origin, spacing, and dimensions have to be exactly the same.
Using the command Drape grid on surface, the elevations of the grid can be reset to the original surface elevations and the grid will still be valid for volume calculations with derivative grids.
- Grid/Merge/Drape grid on surface
- Grid/Merge/Merge dam into grid