The mention of a "chalk stream" cunjures up pictures of trout lying, apparently motionless, amongst the beds of swirling green waterweeds at the bottom of crystal clear streams. Any investigation of the particular qualities of chalkland rivers must start at their source within the chalk itself.
The majority of the water falling as rain on chalk downland does not run off along the surface but rapidly moves through the soil into the underlying chalk itself. Not only is the chalk porous, but it is also full of fissures and the water accumulates in the rocky reservoir in what is known as an "aquifer".
The percolation of the groundwater into the chalk reservior and its subsequent emergence as natural springs is quite a lengthy process and chalk streams have several unique characteristics.
Chalk streams tend to have comparatively stable flow regimes. The winter rains replenish the aquifer producing peak flows in the rivers in late winter and early spring. The flow gradually decreases throughout the summer and autumn months.
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Because of the length of time which the groundwater spends in the chalk, the water in the springs tends to emerge at a steady temperature of 11°C throughout the year. Chalk streams thus tend to be warm during winter and cold in summer when compared to other rivers which lack a major groundwater component, rel;ying more on [[surface runoff]] and thus more liable to the temperature changes of the seasons.
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Because there is little opportunity for [[surface runoff]] to rinse soil and other materials into the stream and the bulk of the water has been filtered through the chalk, the waters of chalk streams are very clear.
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The water emerging from chalk aquifers is calcerous itself, containing a considerable quantity of calcium carbonate ("hard water") and is rich in many plant nutrients.
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Because the source of the water in chalk streams is the aquifer within the chalk, the flow is more constant than in rivers which are more reliant on [[surface runoff]]. They are, however, reliant upon the level of water which is present in the aquifer and the source spring has no fixed location. As the aquifer fills with winter rain, the source of the river rises to form a temporary "winterbourne" section. As the level of the water in the aquifer drops, the source of the river drops to the level of the permanent springs known as the "perennial head".
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The winterbourne section, under water in the winter and dry in the summer, poses considerable problems for the wildlife which inhabits it emerging from chalk aquifers is calcerous itself, containing a considerable quantity of calcium carbonate ("hard water") and is rich in many plant nutrients.
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