One of the most important events in the history of life in the Americas was when North and South America became connected by a land bridge—the Isthmus of Panama—which occurred between 15 million years ago and the present day. This connection allowed plants and animals to move between the two continents. Scientists have studied land plants to better understand when this event happened, but we still do not know how rivers played a role in how the land bridge formed. This project will study a special group of plants that live in tropical and subtropical rivers rapids and waterfalls, called riverweeds. Using information from both plant fossils and the DNA of living plants, we will study how these plants moved and evolved as the land bridge. Combining information about plants with geologic data, we hope to understand when rivers began to connect, and how these changes affected the plants living in them. This project will help science grow by training American scientists to work together with scientists in other countries and in various languages. The project will also help train new partners, strengthening the pathways for future American scientific research. This project will provide a novel lens on biotic migration during the rise of the Isthmus of Panama by leveraging the tight link between Podostemaceae plants and river evolution, adding a new element to the story of the Isthmus closure, and shifting the focus from terrestrial to unexplored freshwater systems. The tradition