Step 1: Create the first terrain

Create a terrain and then click the E to edit the default terrain. Don't worry about the material now. You can change it later to suit your needs.

This first terrain will be your river bed and river banks. You will want to create the shores for the river (the land). I usually place a rough outline with a few paint brush circles and then use the fractal option to give the shore line more personality. If you want an ocean shore, you would only create one area of land. After playing with the fractal option (I think I used round hills), I added some erosion to the coastline.

When you are done, click the check mark and see what you have created.

Once you have your first terrain, it helps to align the camera before things get too complicated. I like to keep the camera low to give a sort of "view from the boat."

The next step will be to select your newly created terrain and to duplicate it.