Actually, before you rework the idea...

Of course my thoughts turn to a fantasy setting, and may not suit your needs.....but here goes....

In the real world, and unless there are major engineering works at play, I have to agree with bartmoss. Erosion would kill this idea. However....in fanasy setting where there be MAGIC, of a geological scale, I can see this as a battle between two elemental gods/kings....The water one opening the flood gates to the plane of water in an effort to destroy this plain, and the earth one doing its best to shore up the land and suppress the water. Call it a war between the two who's beginnings is lost in time......Some where in the middle of this lake is the gateway, and possibly an answer to the riddle...

Just some thoughts that may or may not help you make up your mind