bassonman;
Now I have a better idea of what you trying to depict. Easiest solution I can think of short of remasking etc. etc. is this. Cut a piece of card stock paper (back of a pad of paper will do) to the width of the outside border. Insert it to the edge of the frame and re-spray the area. The card stock will mask the border and you can remove it immediately.
The other suggestion is you may want to tone the water down. The colour as shown on your post is too stark for a river. A slight greenish tinge to the water, especially if it is still. For a "colour sample" of this type of diorama, rent the film "A Bridge Too Far".
In the opening scenes the Germans are shown in retreat on a bridge over a calm river and you can see a full length shot of the river, the bridge and the colours of each in full sunlight. Just my
Cheers;
Gregory