Based on 'Rock glazes of NZ' Minna Bondy
Sample 76. Rakaia River Sand
Minerals similar to 75 but larger grains are accompanied by much more extremely fine-grained material this almost certainly rock flour from the glaciers at the head of the valley
Apologies, I knew that with all theses samples I was bound to get my rivers muddled somewhere. The tests themselves are well labelled with GPS locations of all sites but the photos, especially those under bridges all start to look the same.
This image is actually the Rakaia river, not the Rakitata.
It's interesting that Bondy states that rock flour is present in the sample. The river is a lot bluer than that of the Opihi and Rakitata and has the milky quality of the glacier lakes. This comes from light reflecting on the fine-grained particles, even though it was a miserable cloudy day. She compares this sample to Sample 75 which is the Waitaki River sand over 200km away but also glacial feed.
I have been careful to avoid mud and sediment that gathers on the banks of the rivers, it is easily distinguishable from the sand by colour and grain.