Because he knew and loved the river, some of Twain's most descriptive and realistic writing is his description of the Mississippi River and the areas around it. He creates pictures of the scenes including every detail that would be noted by any character of the story who happened to be in the area along with Huck. It is easy for the reader to place him or herself in the setting.
I could see the sun out at one or two holes, but mostly it was bit trees all about, and gloomy in there amongst them. There was freckled places on the ground where the light sifted down through the leaves, and the freckled places swapped about a little, showing there was a little breeze up there.
Twain also took care when introducing new characters into the story. He provided enough detail to make it very clear and easy to understand the type of person and personality the new individual possessed. Huck provides information about Col. Grangerford's complexion, appearance, attire, mannerisms, and way of relating to those around him.