Plummer's Law is a principle in the field of geology that describes the relationship between the size of a rock fragment and its ability to be transported by water. It states that larger particles require a greater flow velocity to be moved compared to smaller particles. This concept helps in understanding sediment transport in rivers and streams. The law is named after William Plummer, who contributed to the study of sedimentology. It is important for predicting how different sizes of sediment will behave in various water conditions, which is crucial for fields like civil engineering and environmental science.