Why a kingfisher needs clear water

A Common Kingfisher sits above the water and watches. When it sees a fish, it dives head-first and takes the fish with its bill.

This method of hunting sets a condition: the bird must be able to see into the water. In turbid, polluted water the fish are still there, but the bird cannot hunt them. Pollution therefore removes the bird’s food supply even where fish remain.

That is why this species is used on this website as a measure of the river: where a kingfisher can hunt, the water is clear and holds fish.