Good answers here, but thinking this way may help. The river was deep. Rivers on flat ground, left to their own devices, migrate laterally a lot. We don't see i

Good answers here, but thinking this way may help. The river was deep. Rivers on flat ground, left to their own devices, migrate laterally a lot. We don't see it much, but if you think of oxbow lakes https://www.google.com/search?tbm=isch&q=oxbow%20lake https://www.google.com/search?tbm=isch&q=oxbow%20lake and recognize that they sediment up & fill in over time, you'll get the idea.