Leonardo studied water relentlessly. The most visible and dynamic of all the elements, water provided a means of understanding the forces that drove all the elements into action. He compared the flow of water to the flow of air and quite accurately, water waves to sound and light.

Leonardo was particularly interested in vortices and the erosive power of water, both of which he considers on Fol 18v. In the upper drawing, flowing water encounters an obstacle in its path. He observes how the heavier water lying at the bottom of the stream encounters the obstacle first and rises up, crashing into the lighter water on top and forcing it outwards to form vortices. In the lower drawing, water gushes forth through the gates of a lock and Leonardo notes how it erodes the bottom more on one side than the other.

The vortices in Leonardo’s drawing are like the spiralling curls of hair or the leaves of plants that often appear in his paintings and drawings. This illustrates his belief in the interconnectedness of natural forms and the all-governing creative power of divine natural force.

In Leonardo's words
Moving water strives to maintain the course pursuant to the power which occasions it, and if it finds an obstacle in its path it completes the span of the course it has commenced by circular and revolving motion.