After a big storm rivers are always muddy high and move really fast. I was just wondering how long it takes for that to clear up?  Im fishing a small shallow river but has lots of fast moving water. How long till I can fish it?

I think it probably depends on the water volume and the amount of water draining from different areas. I would look for reports for the river you have in mind. I know the Shenandoah River here in VA usually takes about 4-5 days to clear up after rain.