Afaik Frank Sawyer, the famous head keeper of the Army's stretch of the 'upper' Hampshire Avon above Salisbury in Wiltshire and the inventor of the Pheasant Tail Nymph, was the first person to do it. This was in the mid to late 1950's as I remember reading somewhere.Thanks for that,
It was tremendously successful but the effect doesn't last and it is expensive to keep doing it every couple of years so they gave it up.
Note that the Avon, even above Salisbury, isn't a chalkstream. It just looks like one. But only over a relatively small part of its length, the part where it flows over Salisbury plain.
Because the banks/surroundings are full of plants and trees that like chalky soils. And the same goes for the river weeds. They don't get their nutrients from the water. That, which is pretty sparse in nutrients anyway, due to its non-chalk origin, flows past too quickly to deposit much in the way of nutrients. The waterweeds get it from the soil their roots are in, which is chalk, gravel, flint, and 'earth'. So fertilize the soil with even more chalk directly tipped in and their roots grow better.
THAT is where Sawyer was so clever. His critics entirely miss the underlying processes that he noted. He was a far deeper thinker than most others in the field.
It rises from 'greensand' not chalk aquifers. And for most of its length (both above and below the chalk Salisbury plain) it does not flow over chalk. Its few tributaries, notably excepting the 'nine mile river', are chalkstreams but those don't join the main river until Salisbury. The trout fishing is above that.
The upper Avon wasn't fished much, not for trout or anything else. It was Sawyers work, and slightly later, Kite's 'publicisation' in his books and TV series that made it well known and fished. Plus the later efforts of the excellent Salisbury club. But it still rises and falls a lot with the weather and is often coloured.
It was apparently in a terrible condition when the Army purchased their long section. Hardly any weed and few fish with almost zero natural regeneration.
Sawyer wrote a book called 'Keeper of the Stream' and you MAY find it in there. I don't have a copy so I can't check.