This tutorial not only shows you how to add interesting rivers and lakes to you map, it introduces ESSENTIAL techniques to accurately connect your maps objects, and how to select and unselect map objects.
In addition, you be introduced to the powerful Change Property command, and much more!
Rivers tend to combine to create bigger rivers.
Rivers tend to flow east to west.
Rivers never flow uphill - unless of course you're creating a magical river - so you should be sure to work them into the mountains and contours of your world.
Add the narrow river to your map, starting from a mountain, then...
Connect another narrow river to it, and then...
Attach a larger river to both, and draw it in a curving path all the way to the coast.