I've seen quite a few rivers from California to Florida, and there is one common thing about them - they are almost all (except a few rivers up North) muddy looking. Sometimes, water looks like it's about to rot, it's so yellow grinnish/greyish.... It's quite different from Europe, where river waters, generally speaking, are more or less clear, extremely abundant in water plants.

Is river muddyness natural or acquired state of the American rivers?

I suspect that much of the difference is one of climate. Hotter summers mean more algae. Also, soil type may play a part. Finer partical type soils, like clay which covers a large portion of the eastern mid west.

It's called pollution. Go to China and you'll see nothing but grey-colored rivers. Go up North to Canada and you'll see rivers with more different shades of crystal clear blue than could ever be in a rainbow. You must live near a large city?

I think that industrial pollution is the cause in only a small percentage of American rivers.

As mentioned in a previous post, algae & debris from plants in or on the river banks will color the water.

Note that the source of water in rivers is not primarily due to rain falling directly into the river. The water picks up various materials on the way to the river from where it originally fell as rain. I remember seeing muddy looking streams (not rivers) in Montana and Wyoming, far from any industrial plants or mining operations.

Some rivers are naturally muddy, such as the Mississippi.