Pondweed

The Potomac has been called the wildest urban landscape in the world. In some places it is flat and broad and so clear that you can see all the way down to the rocks and sand on the bottom. In others it is thick with gardens of aquatic vegetation with native species like wild celery, curly pondweed and stargrass, and exotic plants like hydrilla.

If you have ever had a pond you will know Azolla, a.k.a. pondweed, duckweed fern or fairy moss.