,,, I clicked on the line that reads "some German photographer" at the top of the thread and that photo shows an area similar to the Rakaia River, halfway down the east coast of New Zealand's South Island. The river stones there are rounder though.
Heather and I took the Morgan down to Kaiteriteri a couple of years ago and I don't recall an area like that photo. Kaiteriteri has beautiful white sandy beaches. It could be further along from where we went though.