No rivers showing in render

[mrodgers 1]

So, been working on creating quite a large region that my computer is sure not capable of handling, but for now it's all I have until the tax return comes.  I didn't get any response from my last thread.  Perhaps I won't for this either, I don't know.....

 

I just need to confirm the following is I am guessing a RAM problem.  I created a 20x28 city tile region.  Been working on it forever now in Paint.net with this slow computer.  Finally got it working but when I created it in SC4Mapper, there were areas of the map shooting into the sky.  It rendered fine, I found it was artifacts in the JPEG conversion.  

 

I cleaned the JPEG conversion up and now it doesn't show any of the rivers when I load the region.  It is showing mostly all flatland rendered with sections unrendered but the rivers are there.  

 

Here's a screenshot.  There should be rivers running all through this.  The rendered parts are all flatland.  When I open that single tile, the river does show up.  This river should be running all through the region.

 

 

As I said, when I did the first render I had artifacts that created super high blotches all over, but all rivers did show up (I don't have a screenshot of that...)

 

I can't see how this is a computer spec or RAM problem because I have previously created the region from the same size image file and had all the water areas.  Can anyone shed some light for me?