Yellow flower

Yellow flower

Observation - Yellow flower. Description: I am really sorry, I cannot work out what this is. I was on a river margin, multi-flowering heads on long (maybe 50-70cm) stalks. I didn't get a picture of the leaves. Help.

I am really sorry, I cannot work out what this is. I was on a river margin, multi-flowering heads on long (maybe 50-70cm) stalks. I didn't get a picture of the leaves. Help.