issue with loweing the terrain

I am new to unity so I went and watched a youtube video on how to make water. In the video the guy used the raise/lower terrain tool to make a hole. when I try to use the raise/lower terrain tool all it does it flatten the hill/mountain it doesn’t do anything else. Am I doing something wrong? If so what can I do to fix it

Use the tool to raise parts, then use the tools to lower a riverbed through those parts.