It was a blackwater river flowing out of a forest. The pH of the water is about 4.5 and the river had sandy banks. We spotted some cryptocorynes growing submersed in the water, bingo!
We walked along the banks into the forest to explore the forest. I found some small cryptocorynes growing on the sandy banks as shown below.
We fished out a stalk of submersed cryptocoryne specimen to check, it had a long rizhome.