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Gulf Islands National Park Reserve of Canada

Anatomy of a Creek

The Headwaters | The Wetlands | Sediment | Brackish Water | The Estuary

The Headwaters

The headwaters of a river are the small streams or wetlands that are its source .

The headwaters combine to make larger streams. These in term, combine to become the river. Eventually, the lower course of the river (often wider and slower in velocity) will meet the ocean. This area where the river's fresh water and the ocean's salt water mix freely is called the estuary of the river.

The Wetlands
Picture of the Lyall Creek wetland The Lyall Creek wetland
© L. Sumi, Parks Canada / 2005

Wetlands are environments at the interface between truly terrestrial ecosystems and truly aquatic systems, making them different from each yet highly dependent on both. Because coastal wetlands absorb the force of strong winds and tides, they protect the adjoining terrestrial areas from storms, floods, and tidal damage. Similarly, wetlands found inland can moderate surface and groundwater flow providing more consistent moisture to surrounding ecosystems.

Wetlands support a wide variety of wildlife and therefore the conservation of wetlands is of prime importance for the preservation of critical habitat for many species.


An estuary is typically the tidal mouth of a river, and estuaries are often characterised by the deposition of suspended sediment. Sediment is the silt, mud, clay or small rocks deposited by a river or lake. As the water velocity slows, sediments fall out of the water column and are deposited on the bottom. In Lyall Creek, one of the problems that had to be addressed by the restoration effort was a disruption of sediment dynamics resulting in a buildup of sediment above the culvert and a deficit of sediment downstream of the culvert.

Brackish Water

Brackish water is sea water mixed with fresh water. In the Lyall Creek estuary, the brackish surface water covers a range of salinity .

Diagram of a typical estuary. The diagram illustrates a single fresh water stream dividing into several streams as it approaches the ocean. The arrow at the top of the diagram shows how the level of salinity of the water increases in proximity to the ocean. Diagram of a typical estuary
© F.Binard, Parks Canada / 2006

It is characteristic of many brackish surface waters that their salinity can vary considerably over space and/or time. There is a tendency for freshwater (less dense) to float as a lens above the salt water (more dense) although currents, tides and surficial features can promote mixing as part of the dynamic relationship between the two waters.

The Estuary

An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea. In an estuary, sea water mixes with fresh water.

Picture of The Lyall Creek Estuary The Saturna Island estuary
© T. Golumbia, Parks Canada / 2005

The sand and mud banks and marshes are relied upon as habitat by, for example, wading birds. Estuaries are more likely to occur on submerged coasts, where the sea level has risen in relation to the land, as this process floods valleys to form rias and fjords. These can become estuaries if there is a significant river flowing into them.

Background of Lyall Creek
Restoring the Habitat of Lyall Creek
Baseline Inventory and Monitoring
Salmon Species