These are chick faecal pellets. The female collects the pellet directly from the chick, removes it from the nest then leaves it away from the nest, in this case into shallow running water. It appears this was done several times in a day and the same place on different days. The pellet has a very flimsy sac, about 4-5 cm overall. The white is presumably uric acid. Other faecal matter can be seen in the sac. I did not attempt to locate the nest, but it would be reasonably close by.