All rivers flow on a constant downward path. Originating atop mountains or bubbling up from deep underground, rivers and streams can cross great distances over land before their journey ends at the ocean.

Estuaries form where fresh water and salt water meet. In an estuary, all of the nutrients, sediment, and chemicals carried by the river mix and mingle with those carried by the sea. Because of this mixing and mingling, estuaries are some of the most productive ecosystems in the world.

Estuaries support a huge abundance and diversity of species, including shellfish, lobsters, worms, crabs, algae, fish, birds, and even humans! Estuaries have long been favourite sites for human settlement as they usually offer an abundance of food and water.

In Dunedin, littleneck clams (known locally as cockles or tuaki) thrive in estuaries. Long a favourite food source for Māori, they are now the basis of a major export industry for the city. A carefully balanced ecosystem is crucial for these bivalves to thrive. Too many nutrients and pollutants carried by waterways from human activity upstream can result in dead-zones and a build-up of contamination. Salinity levels are also important. The littleneck clam is primarily a marine animal and can only tolerate a moderate fresh water influx from small sources such as streams. The estuarine environments around Dunedin provide a perfect ecosystem for this valuable resource.

Everything that enters a stream or river winds up in the ocean, thus affecting the estuarine environment and all the creatures and critters that depend on that environment, including ourselves!