Help needed about Adding a river to a terrain

I am new to this engine and editor so please be gentle.
I am going through some training and reached a part where a plane is to be added to define those parts of the map that cannot be walked on.
The plane would be set so that it marked out the river so it cannot be walked on.
So... my problem is I did not create a river when I built the terrain and so most of the land is at height 0.
I tried sculpting the terrain downwards but it only laid out the small rising I had setup so the plane shows up in the area I sculpted and most of the map.
I thought I could just adjust the terrain objects Y to make it higher and be able to continue sculpting downwards but that did not work so I am stuck.
Is there a quick way to solve this or am I looking at starting over.
Also when starting should I have set the height so I could dig downwards?

 

That's unfortunately the case. For a fresh terrain, it's good practise to always use the "set height" function and raise the terrain entirely by some units, then move the actual terrain object back down by the same amount. This is so you atleast have the option to dig into the terrain should you need to in the future.

 

Ok thankyou.
I have been looking at the learning options and started the Introduction to Terrain Editor but got stuck as it seems to use Terrain Tools 3.0.1 selected via the Package Manager Advanced options which doesn't appear in the 2020 Editor and therefore I am am stuck again.
Any hints as to which training I should take for me to learn Terrain editing?