An estuary is a partially enclosed body of water along a coastline where two different bodies of water, meet and mix. They can be a place where freshwater mixes with ocean water, or where rivers meet chemically distinct lake water. Estuaries are unique places that are valuable to the environment and to society. This “estuary” area, washed daily by the salty tides, provides a home to marsh-plants, which in turn provide shelter to a myriad of shore and song birds. Estuaries provide homes for many species of wildlife, are important nursery areas for a variety of marine life, and help to filter pollutants from the water. For example, most fish and shellfish including salmon and oysters complete a portion of their life cycle in an estuary.
Cowichan Bay Estuary: Gerry Thomasen
Estuaries can be classified by their existing geology or their geologic origins. The five major geology types of estuaries are bar-built, delta system, tectonic, fjords, and coastal plain.
Bar-built estuaries are formed when ocean currents and waves build up sandbars or barrier islands. A protected area fed by small streams or rivers may develop behind them creating a bar-built estuary.
Delta system estuaries form at the mouths of rivers, where the silt and sediments washed downstream are deposited faster than they are washed away by ocean currents and waves. In the delta, where the river meets the ocean, sediment will be deposited in a fan-like shape out into the ocean.
Tectonic estuaries occur along fault lines. They were created when a major crack or a large land sink in the Earth, often caused by earthquakes, produced a basin below sea level that fills with water.
Fjords are long narrow valleys with steep sides that were ground out by advancing glaciers. When glaciers melted they left a deep channel carved into the earth with a shallow barrier or sill near the ocean. Fjords usually have a high input of freshwater and because of the sill, the deep waters of the fjord are prevented from mixing with the deep waters of the sea. This results in anoxic (low oxygen) water on the bottom of the fjord, with the river water mixing with the salty ocean water near the surface.
Coastal Plain Estuary: NIWA - New Zealand
Coastal plain estuaries were formed at the end of the last ice age between 10,000-18,000 years ago. As glaciers melted and sea levels rose, some coastal streams and river valleys became somewhat covered with water. Coastal plain estuaries are also called drowned river valleys.
Cowichan River Delta: Open Government License - Canada
The Cowichan Valley was formed during the last ice age, as glaciers carved out the valley and left a large amount of gravel "till" which has become the floor of the valley. The Cowichan and Koksilah rivers flow over this till and empty into the ocean through a complex network of ever-changing channels, sandbars and marshes here at Cowichan Bay.
The Cowichan Estuary at Cowichan Bay is an estuary fed by freshwater from both the Cowichan and Kosilah Rivers. These rivers, along with the streams that feed them, are known as the Cowichan Bay watershed. The 277 hectare estuary intertidal area is one of the largest estuaries in British Columbia.
At this location, where the river's freshwater meets the ocean's salty seawater, the liquids combine and turn into 'brackish' water which is 'slightly salty water'. Salinity is the saltiness of a body of water and is an important ecological factor because it influences the type of organisms, plants and animals that can grow or be fed by the water. Because of the tidal movement of sea water into and out of the estuary's intertidal zone, the salinity of estuarine water is constantly changing. During tidal ebb the tide pulls estuary water back to the sea and during tidal flow the tide pushes sea water into the fresh water. During the ebb is when shore birds come to feed on the stranded fish and invertebrates on the estuary mud flats. Additionally, the volume of freshwater entering the estuary changes throughout the year due to seasonal conditions such as ice melting at higher elevations or drought conditions.
In order to log this cache, send us an email (through our geocaching profile) that has the answers to the following questions:
1. During a drought in the Cowichan Bay watershed what would happen to the salinity of the water in the intertidal zone?
2. What type of estuary is the Cowichan Bay estuary? How did you come to this conclusion?
3. According to the nearby interpretive sign, when should you look for Short-eared Owls?
The Cowichan Estuary was a favourite settling place for local First Nations because of the abundance of shellfish, salmon, herring roe and seaweed.
Nearby Cowichan Bay grew up as the main access point for people and goods to enter the Cowichan and Chemainus valley communities from Victoria. In the late 1800's the Malahat was not a friendly transportation route.
Because of the negative impacts of human use of the watershed and estuary, The Cowichan Estuary Environmental Management Plan was ratified in 1987. You can read more about it here: www.cowichanestuary.com