Rain gets the soil moving
Rivers often turn brown after rain because runoff brings in tiny particles of soil and moving water keeps many of them suspended. A swollen river can also stir up sediment already in its channel. The particles scatter light and make it harder to see through the water.
Watch what settles
Imagine two clear jars with a little soil mixed into water. Leave one still and keep gently moving the other. Larger particles may settle first in the quiet jar. Finer grains can remain suspended longer, so a lighter layer at the top does not mean all the sediment has gone. Rivers similarly drop some particles as the flow slows.
Brown can mean more than one thing
Some water stays coloured partly because of material dissolved in it. A jar that clears as sediment settles tells a useful story about particles. Colour by itself gives little information about what the dissolved material is. Follow the water back to how surface runoff reaches a river to see how material from a field or road can get into the channel.
Try the companion Academy exercise, or find your way through the water cycle from cloud to coast.
Read the source: US Geological Survey · Sediment and suspended sediment.