A count on moving water

We were counting floating litter from the bridge, but the current carried some pieces beneath an overhanging branch before we could identify them. Foam containers were easy to see; transparent wrappers were not. A single bottle turned in an eddy and passed our marker three times. We needed to avoid counting it as three different objects. Reflections from the afternoon sun hid part of the water, and the bridge's central support created an area neither observer could see. When our counts differed, the objects had already moved downstream.

Describe the difficulties of producing an accurate litter count. You may support your points with brief quotations.