Whenever we have a scenario like this, which is called a "dead valley", we build what is called a "cricket" in between the two slopes that create the dead valley. The cricket is essentially sloped so that the water sheds out of the valley you see in the photo. Most of the time you cannot see the cricket from the ground unless you are standing a good distance away from the house. Good observation! Here are a few examples of crickets that I was able to find online.

cricket 1

cricket 2