"Echoes of Eden" Dayle King Searle

You may have seen these bushes flowering the past few weeks. With their round white flower clusters, it would be easy to assume these plants are related to snowball bushes, but they are not. So what are they?

We spent last weekend in western  and northern Idaho. As we drove along the Payette River, I kept seeing bushes covered with large white flowers. They were growing everywhere--along the river, on the hillsides, even clinging to rocky cliffs. This is what I saw...     Do you know what it is?