Mission area wrap around

I made an area for my level with a river running through it. But at the edges of the mission area, the terrain just wraps around to the other side. So if my river is diagonal in anyway, the river looks like is "jumps" over a bit at the end of the area. I've increased the area as big as it will go(as far as dragging the resize handles bigger). Is there anyway to increase the size or posibly restrict the player as to not see the "tileing" effect of the world.?