Sometimes chickens drink poopy water rather then the fresh water we give them. You could test the creek water over a couple of different days and different times to see if anything sticks out, but it's hard to say.
Side note, when we were looking at homes, when we had a septic inspection, we...
I'm so sorry! How old is she? Have you checked their crops at night and in the morning? Can they access anything poisonous?
@Wyorp Rock @azygous @Eggcessive please help?