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Although the harbour was formed by river valleys being drowned at the end of the last Ice Age, there are no major rivers flowing into the harbour today. However there are several freshwater streams. These flow from springs in the chalk downland several miles north of the harbour and are very clean. They flow directly into the harbour. The section nearest the harbour tends to be brackish as salt water from the incoming tide mixes with fresh water in the stream.
Fishbourne Stream rises from the chalk downs to the north of Chichester and flows to meet the sea at the top of Fishbourne channel. It is very clean and is home to both freshwater creatures and some sea creatures that enter the stream at high tide.
The stream flows fairly fast in the middle over a stony streambed. At the sides are plants that can grow in water such as watercress and yellow flag iris.
The most common freshwater creatures tend to be freshwater shrimps that feed on dead animal and plant matter.
Other creatures which live among the plants for shelter and food include water beetles, leeches and flat worms.
The stream has a number of species of fish, both freshwater and marine. These include sticklebacks and the young of several marine species such as flounders and eels. Fish use the stream as a nursery and come here to spawn. The adults enter the stream at high tide and lay their eggs in the stream where the young can grow up in a more sheltered environment than the tidal harbour. Young flounders disguise themselves among the stones on the bottom of the stream. Young eels, called elvers, will eventually swim all the way to the Sargasso Sea in the middle of the Atlantic.
There are a group of creatures that rely on both the stream and the meadow in order to complete their life cycle. The young of caddis flies, midges and dragonflies are all aquatic creatures.
Caddis fly larvae live inside shelters they make by sticking material they find to their cases, to hide from fish which like to eat them. Small clusters of stones stuck to a larger stone may not be what they seem! The adult caddis flies emerge from the stream as insects and live a very short adult life of about a week. Dragonfly larvae spend several years in the stream as voracious predators, eating anything they can catch with jaws they can shoot out! They emerge as adults to spend a few weeks flying over the meadows catching insects before breeding and then dying.
Fishbourne stream is also home to the rare water vole It is often mistaken for a rat. Ratty in ‘Wind in the Willows’ was a water vole not a rat! It eats water-side plants and lives in burrows in the banks of the stream, often with entrances above and below the water.