I totally agree with you; making realistic looking water is very difficult. One of the first things you'll need to do is master layer-masks. Jfrazierjr has an excellent tutorial here (http://www.cartographersguild.com/tu...s-gimp-ps.html) and if you read through that he has a specific tutorial for making photo-realistic river banks mid-way through the thread.

Here's one of my own river attempts from an unfinished map; there are no easy answers: you just need to use trial and error.

Click image for larger version. 

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