Yes actually there are plenty of standalone lakes. For example it could have a spring underneath it or it's source could be just rainwater filling up a hole that had a source a long time ago but was weathered away, etc. For a real life example of a stand alone lake. Crater lakes, For example one famous one, Crater Lake, Oregon USA. What had happened was some time ago a large volcano exploded the top flew up, the middle pieces dissapated, and the bottom crater remained. The top fell back down inside the middle of the crater. Over many years of rain and snow collecting in it, it eventually formed a lake in the center of the crater, with the top of the mountain becoming an island. Has the clearest water of most lakes tis does.
yesh...sorry for the long history story, but there are lakes that stand alone!
I DO love the grass textures and the trees are nice too, I'm not to keen on the shadow of the left mountain on the grass, does make it look like it's floating on the land mass. Perhaps bluring it slightly or just making the grass itself darker and not having the shadow there will do the trick?