How do i thicken the base terrain?

I am trying to make a river, but my terrain is as thin as a piece of paper when i look at the side. How can i make this thicker so i can dig into it to make the river?

Computer world works differently than real life, everything you see in CG is “faked”.
Check out terrain editing tutorials: https://www.youtube.com/results?search_query=unity+terrain+editing

Would you have any videos in particular you would recommend? I have seen some of these and i don’t hear or see them say anything about rivers? Mainly basic mountains and textures. Thanks for any and all help.