You need a lot Water to get it. very rare.
I got in ~330 Water 22 pieces.
That was not bad

100 water and only 3 ;(

Artisan 1 Processing, +1 Embroidered Craftsman Clothes, 391 river water: 35 alluvial gold (11:1), 967 purified water (1:2.5). Was averaging closer to 10% gold rate until the end.

I think I remember getting a few at Skilled level, but wasn't really very reliable until at least Professional.

1565 water = 128 Gold.
Master 3

Kaki Crafting Pearl outfit and +2 crafting Armor + alchemystone +11% better crafting
I got also 3840 new fresh water out of the bottles
8.4Million silver worth gold+water from the 1.5K bottles after processing.

Heating river water doesn't give any Alluvial Gold

Filtering does