Forage some riverweed.... I've been foraging for hours and no luck. Is there a minimum forage skill maybe? Mine is only at 75 (SK).
I don't remember this quest specifically, but there is one quest (it might be this one) that tells you you will have much better results if you are in the water when foraging, or something to that effect. I would suggest reading the quest text to see if there is such a reference, and following it if so.