Not only it can be washed perfectly but river sand (and gravel) tend to be much better for concrete when compared to "crushed" aggregates. More or less the rea

Not only it can be washed perfectly but river sand (and gravel) tend to be much better for concrete when compared to "crushed" aggregates. More or less the reason is that usually for a given size river ones are harder/more resistant, and have rounded edges (which also imply a bigger resistance) due to the cycles (wear, long time immersion, etc.) they were subjected to in the river. And, as a final advantage, concrete made with rounded aggregates is usually easier to pump.