According to years-long US satellite images the rivers start off blue and turn to yellow then green in many places. Good/not-so-hot?

Probably not so hot.  However, it depends on what's turning them color (minerals, algae, sewage, industrial pollution, etc.).

If the colour has been steady, depending on flow, for as long as anyone can remember, then provided it's not actually from a 100-year-old factory still pouring trade effluent into it somewhere upstream, it's probably natural.

The time to worry is when the river takes on unfamiliar colours for itself, for either drought or flood conditions.

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A mature river's natural colour in steady flow is often a drab greyish-brown, depending a lot on the rock residue forming the bulk of its sediment.  Some of the rest of the sediment is actually fish-droppings! ("Mature" here meaning the slower-moving stretch across the lowlands approaching its estuary, usually downstream of most of its tributaries.)

Green usually means a heavy algae load, but unless normal for that river, it could indicate a lot of agricultural fertiliser in the water. 

Blue is typical of clear water reflecting the sky, but can also result from suspended glacial till or clay particles, and heavy loads of them can give the water a cloudy, blue or grey tinted, slightly milky, appearance.

One clue to the normal state of a river in a country like the USA can be its name, if indigenous and ancient, because ancient indigenous names are usually just dryly descriptive when translated. (I live near a famous bay-bar called Chesil Beach, a huge bank of shingle. Why Chesil? Turns out Cisel is Old English for... shingle.)   


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I used to work with a  model-railway enthusiast, and he told me making a river look natural was one of the most difficult aspects of making model scenery.