Just watch the video and all will be revealed:-

Or watch this one for a much quicker method.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2208705936
In summary:-

1) Use the terrain sculpt tool to make a dent.
2)Use the Set Height tool to carve your river at the depth of your dent.
3) Select the large plinth from the nomad DCI, place one and paint with the metallic colour of your choice.
4) use shift+click to fill your river bed with plinths.
5) use the terrain smooth tool to soften the edges.


Job Done