The streaming video looks at the unique environments, called estuaries, created when freshwater returns to the sea. Estuaries are capable of supporting enormous populations of living things and are richer than the richest farm lands in producing food. Two typical estuaries are presented: a temperate climate and a tropical mangrove lagoon. Viewers learn how tides are produced and how they affect the unusual dwelling creatures found in estuaries. Biological adaptation to constantly changing environments is also examined.