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In addition to the answer by @Pastychomper‭ which seems to be the most likely reason in this specific case, crows often seem to leave dry food into puddles/small ponds in order to make it easier to chew and swallow later on. Things like small dead animals (birds, frogs, mice etc), bread or meat stolen from humans etc. 

I've had a crow doing this for many years in a birdbath, particularly in spring when they are feeding their chicks. It leaves the food in the birdbath then comes to pick up a couple of hours later, when it's all soggy.