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Life in Freshwater


In the headwaters and middle reaches of a river (erosion and sediment transfer zones) water will invariably be contained within a channel. This channel is determined by the amount of discharge and of course this may vary over several years. As a consequence the channel is rarely running at full capacity. The photo below was taken in spring when catchment is high due to higher rainfall. Also the terrain is boggy on either side. This consists of bog mosses (Sphagnum species) that act like sponges and will slowly release water over time.

Channel in the headwaters

In the transfer zone the fluctuation in discharge may be more dramatic. Flow maybe in decline and a flatter topography. The water should have a clear channel but when a river in the headwaters is in spate (fast discharge, see below) the river will go above the top of the channel and flood the surrounding land. This creates a wetland area around the river. In the meandering river below this is made up of rushes and sedges.

Meandering river in the transfer zone

River in Spate
A river in spate beginning to overspill the channel. The brown coloration is due to peat (organic matter) brought down by the fast flowing water.

The meandering river shows the bank (edge of the channel) to vary considerably along its length. At times it is cut away and on the opposite bank there is deposition of material.

Riffles and Pools

Along short lengths of low order streams (see the first photograph on this page) you can see alternate sections of where the channel changes. A smooth area of water (pooling) maybe followed by a turbulent, rough one called a riffle. The diagram below shows a longitudinal section along a channel. The riffle occurs where the depth decreases due to the accumulation of large pebbles and stones or even an outcrop of rock. As the depth increases a smooth run may occur until the water may seem to stop flowing. Here the water is at its deepest and at the bottom will be finer sediments.

A riffle / pool sequence
A pool-riffle sequence

As well as this sequence occurring under the water, expect to see a change across the channel. In the photograph below you can see smooth water at the back and disturbed water in the foreground on the left. Here pebbles have been deposited and the shallow nature of the bend creates the riffle. The channel is at its deepest on the right-hand side, where the current is at its fastest, removing material.

A fast current is undercutting the bank

This fast current is undercutting the bank causing it to collapse into the water. However, the speed of flow is such that it will remove the sediment and keep the channel clear.

The variation in the conditions within the channel will therefore generate different micro-habitats within the greater river habitat. Each of these micro-habitats exhibits their own niche so expect to see a variation in the distribution of species.


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