I've searched for this in the wiki but couldn't find anything. basically I want to make some rivers much deeper than what they are now.. what tool is used for that?

If you hit H while in a worldspace it opens the land editor. You can then lower the terrain. Though, for it to look good you would also then have to move all the objects in the rivers lower to match, then probably regenerate the LOD to match.

I found the place I want to edit, but I don't seem to be able to edit the terrain? I'm supposed to chose it from cell view right?

No, you hit H then click in the render window to edit the terrain. Set the options in the Landsacpe editor window to do what you want. You will have to screw around a bit to learn it, but it's simple.

got it thanks