I have developed quite a few horizontal stretch marks across my lower back. what can i do? as they are very noticeable.

You have options: You see these mostly in tall guys in their rapid growth spurts. They'll be red at first and then fade some. A dermatologist with a laser can make them fainter but I suggest that you consider keeping them for now -- somebody special in your future may like them, and tall is fun. You have complete control over your physical fitness, the best gift you can give yourself. PhotoGuy isn't complaining.