The simple answer to your question is, when over the river, I was at about 300 feet above ground level. And, you’re right, there’s a lot more detail… excellent proof of that are the still photos I took, and especially the one of the fishermen on the bank of the river.

  In general, the rules say you need to be 1000 feet higher and 2000 feet horizontally from any “congested areas”, and in uncongested areas, 500 feet from any structure or people, so I always try to follow the rules. But in open country, away from airports, and with few people and structures, I can fly as low as I like. (The rules say, “An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.” [FAR 91.119] But if there aren’t any people or structures, then I don’t have a minimum. However, I do try not to scare the cows!) That should explain why, when I saw the bridge while flying over the river, I knew I had to ascend. But even if you have to fly at the minimums, you can still see a lot!