Mick Harper wrote:These are all excellently ingenious rationales but, since they apply more or less everywhere, don't come close to explaining the distribution of (classical) chalk streams.
The geologist is in.
The answer is blindingly simple. It's just lazy terminology. Even in England, these aren't all
chalk streams. But they are all
limestone streams. Chalk is just one type of limestone. e.g. Portland in Dorset, with "chalk" streams. But as soon as we look at the rock, we don't call that chalk, we call it limestone.
See :
https://en.wikipedia.org/wiki/List_of_t ... _limestoneElsewhere and all over the world there are limestone streams and rivers which are described in very similar terms, and equally praised for the clarity and purity of the water, and the quality of the fish and fly fishing.
e.g.
In the eastern U.S. (particularly Pennsylvania) there is a lot of limestone -- entire big valleys lined with the stuff. Limestone is a bit porous, so it's often riddled with springs, which seep up and form streams. These streams erode streambeds in the limestone. Doesn't sound like anything special, and as you can see from the above answer, you can work this out with a dictionary and common sense. However, limestone streams in the U.S. are kinda special. First of all, because they are mostly springfed, their temperatures are not as extreme as streams fed only by snow and rain. So, they're slightly warmer in winter and slightly cooler in summer (compared to ordinary streams fed by runoff). This is good for fish -- fewer extremes means better habitat. Also, the water picks up alkaloids from the limestone and is therefore less acidic than the water in other streams. This results in better aquatic vegetation and better aquatic insects, which leads to... you guessed it: more, bigger trout. Limestone streams often have substrates that change little and remain stable over time, too, making them great for wading -- limestone streams are great habitat for trout and anglers, alike, you might say.
Next please.