Seagulls eating shit or something like

Into the underground Senne river, everything is very-very dirty. Is it a river or is it a sewer ? That's not very clear for me. This river is contaminated, it's smelly. Sludge are grey and sticky. It's disgusting. At the end of the tunnel, seagulls are eating some small things floating on the water (is it still water ?). Sorry for the quite patronizing tittle, but it was unfortunately something like that.