FSX Water effects showing through blend mask

I am attempting to create a blend mask for some photo scenery. I'm testing with a small segment of the area that I want to have photo scenery on. I made a blend mask using Gimp but instead of getting a fade of the photo image into the default landclass I'm getting water effects at the edge of the imagery. Does someone know what I'm doing wrong?

Thanks for your suggestion. This is however an area of dry land except for the dirty pond under the airplane. Attached is a picture from Google Earth showing the area. I don't understand why the default landclass is being blocked as well as the photo scenery.

After working on this some more it appears that the problem was caused by the way I created the blend mask in Gimp. It seems that Gimp handles alpha channels differently that Photo Shop so one of the techniques in Gimp to add an alpha channel is to add a layer mask which you can edit. Then you merge visible layers which makes the layer mask become the alpha channel. So when I edited the layer mask it turns out that I was also (unwittingly) changing the image layer at the same time. Apparently resample interpreted this part of the image as being transparent and I ended up seeing water effects. Live and learn.