You may not have to build a dam because of your specific situation. If you did, remember it's not quite the same thing as for your pond. It just needs to hold up enough to stay put and slow down the water movement. Notice even in catmandoos pics, how it can be sticks, stones ,etc. It does not have to be impermeable like your pond's dam. It can seep water thru.
From your drawing, it looks like you have a perfect spot for one. Right where the red lines come to a point. The water flow that is coming down to your pond wants to go straight, and that brings it into the red area. That current will bring the sediment in there to. Once in there, the water will escape back out and to your pond. I bet you could trap a lot of sediment in that spot.
