make boundaries alround drainage area in dwg-file

[Anonymous]

Hi

I have a DWG file with contour map. I attached several other DWG files to it to make it a complete  map with houses , streams, lakes , woods.. 

Im trying to make boundaries alround the drainage area, but im having a really hard time understanding how to do.  in all the guides they are just talking about other file formats then i have like TIN-files. I cant get it to work.

Somebody thats nows what to do if you have a DWG file with other DWG files attached to it with xref and u want to make boundaries alround the drainage area and cut off the rest?

thank u

/Per