Complex sentence: Here, the Colorado River is also much smoother and calmer than the stretches that lie above and below, with the exception of the former Glen Canyon reach that is now flooded under Lake Powell, formed by Glen Canyon Dam upstream.
Split into simpler sentences: Here, the Colorado River is also much smoother and calmer than the stretches that lie above and below. In the past, another crossing was the former Glen Canyon reach, but it is now flooded under Lake Powell, formed by Glen Canyon Dam upstream.