I was expecting small particles jamming up a valve somewhere - I don't know exactky how all that junk gets into my water, but my pipes are of varied composition and age and the water is pretty hard - but this was more like gravel.

Much of the mineral stuff in water will continue "building" on itself. So it's not gravels in the water when it's flowing so much as building stalactites, stalagmites or whatever they are called in plumbing. It starts little, gets bigger. And if you can flush it out regularly, so much the better.