For my rivers I normally start off with size 19 Constant Circle brush. [This is all for World Painter]. I drag along the rivers path and then, I just bring down the brush size to 9 [Constant Circle]. I continue making more river but for a shorter distance. Then I changed to 5, more river, two, more river and then finally I finish off with a really small trickle, or one. This will create a cool looking river.

Now, you set the smooth tool, set the intensity to 100% go to the right hand side and locate the "Only on" button. Go into that and set it to Land. After that smoothen out the river and then click the "Only on" button again and switch to Water. This one you have to be more careful with, but go over the river again.

That is how I usually make my rivers. For this map I done that around 3 times and then got the "Cracks" brush and used that over the entire map to make it marshy.

Hope this helps! But making rivers is a trial and error process so if this method doesn't appeal then try some other stuff. :)