A chalk stream is a globally scarce type of spring fed river.   Although some chalk rivers can be found in other parts of eastern England and also in Normandy, the majority of the world’s chalk rivers can be found within a 50 mile radius of Stockbridge in Hampshire.  The geology of the area derives from thick layers of sea shell deposited during the cretaceous period, some 70 million years ago, when what is now southern England was under the sea.   The key component of this crushed and solidified sea shell is calcium carbonate, or chalk.   As the (slightly acid) winter rains are filtered by the chalk, they become alkaline and crystal clear.   When there is sufficient pressure in the underground aquifer, the springs burst and this cold, clear, alkaline water starts its journey to the sea. 
In essence, then, a chalk stream is a heightened version of what Americans call a “limestone spring creek”

Good light transmission and constant flows encourage lush weedgrowth, providing both cover and feeding stations for trout, as well as habitat for invertebrates such as freshwater shrimps and the nymphs of upwinged flies, which thrive in the alkaline water, effectively recycling the dissolved sea shell in the water.

This fortuitous combination of clear water and abundant food provide some of the world’s most exciting fly fishing, where large fish are individually spotted, stalked and cast to with upstream dry fly or sometimes artificial nymph, depending on the rules on a particular stretch of river.   This highly visual aspect of chalkstream fishing, along with the stunning surroundings in which it takes place, makes it one of the most desirable forms of our sport.
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