Twain's riverboat captain experience meant that he spent much time just looking at every nook and cranny of the river's edges. So, in chapter 8 his references,
"mostly it was big trees all about, and gloomy in there amongst them," and
"a deep sound of "boom!" away up the river," and
"a bunch of smoke laying on the water a long ways up -- about abreast the ferry" and
"the ferryboat was floating with the current"
are all sensory details consistent with what a person very familiar with the river would know.
For those of us who haven't ever been alongside a river's edge, we get to experience it through his visual images, sounds, and feelings.
As a reader, I can see the currents he refers to, I can feel the breeze and imagine what it visually does to the trees. I can imagine dark places in and among trees and the sun peeking through in some spots.
His awareness of what happens on the river's edges also brings us closer to the reality of that world. He knew there were some pockets of homes along there. He wrote about some of the plunder that would brush up against shores. Logs would float out into the center sometimes, and the expanse of the river (sometimes 3 miles wide) is much different that the rivers many of us can relate to.
Without this descriptive writing, we would not be able to imagine how life on a river that large would really be.