The Glen Canyon Dam has cut off 94 percent of the river’s sand supply?
It means that 6% still gets through.
That isn't a question.
some derives from erosion downstream of the dam as well. I don't know how much sand gets through the barrier, but I imagine 6% "sand" would be a lot higher than should be expected given the size of the dam and its settling pond (the reservoir). Silt and clay, finer-grained sediments, can remain suspended and pass through, but silt and clay are not sand and gravel, so if they mean "sand" as in true sand (and not as a pseudo-term for any sediment), then I don't see much getting past the dam.