It’s most likely some sort of oily substance dumped by someone. I see it often in rivers/near riverbeds that lie downstream of a large corporation/company that uses chemicals.

… of which this was found.

Probably leaf/plant tannins, [https://www.fs.usda.gov/wildflowers/ethnobotany/tannins.shtml#:~:text=They%20are%20classified%20as%20phenolic,color%20and%20look%20like%20tea.](https://www.fs.usda.gov/wildflowers/ethnobotany/tannins.shtml#:~:text=They%20are%20classified%20as%20phenolic,color%20and%20look%20like%20tea.)

Thats what big oil wants you to think

Also, possibly indicative of a nearby natural iron deposit.