I wish I had taken more pics along the way, but I'll try to explain anything that doesn't have a picture. So, I started with 3 pressure treated 2x12's 16 feet long. Because of the height of the sides of the stream, I needed more clearance than a 2x12 would allow, so to get more clearance I cut the bottom arch out and fastened it to the top. I'm well aware that laminating the pieces together would have been better, but lacking a way to properly joint the mating surfaces, I went with lots of screws and 2x4's screwed to the inside for added support. 

Just another angle of the same thing. I'll expand on how I drew the arches a little further down. 

Dry fit with one of the runners before I cut all three. Here I was planning on using steps to keep as much wood intact as I could for structural integrity, but decided to make it somewhat ADA compliant and have a smooth transition from ground to bridge. Also thought the top looked kinda clunky, so I narrowed that a bit as well. 

And here is how I drew my curves. Three screws and a stick of pvc. May not be perfect, but better than freehand. 

Here is the basic bones of the bridge. I did add more cross braces, but this gives you the idea. 

I know I skipped a few steps here, but basically just bolted on cedar 4x4's to define the edge for safety and to mimic the look of piles on a pier. 

Used cedar decking for the top. Almost there!

Done! Lightly oiled because I couldn't decide in a finish that would be more durable, so I get to oil it up every few months now. 

Stress test! I holds the weight of a toddler. That's a good sign. 

Another angle 

And another 

Final resting place. 

Slightly different angle

Close up. 

And the money shot! Hope you enjoyed!