1 texture, Water reflecting effect, but the landscape should
stay the

Hi all...

I have a terrain with an aerial photo mapped over it, there is some sea near the
top and a river running through.  I would like to have a little bit of
reflection on the water surfaces.  A reflection map would be fine as accurate
reflections in this case probably wont look to great (the camera path is high
above the terrain). What would be the best way to do this?

I thought i could somehow create a masking map to show which bits are to
reflect, then use that as an opacity map, with a material which has a reflection
map applied.  This method unfortunately reflects across the whole terrain, not
just the rivers...  

any ideas..??? thanks