What is a Chalk Stream?
The chalk streams mostly flow from the chalk hills of England's south and east. They are spring-fed, gin-clear, and mineral-rich. Lush with water plants harboring vast populations of insects, they sustain large heads of big fish. Fishing for trout on the chalk streams runs from late April to the end of September, with the cream of the season being mid-May to mid-June and September. Tactics are entirely restricted to the use of upstream dry flies and small nymphs. All the fishing is in private hands, but day rods can be acquired on many beats either from the U.K. websites that specialize in offering chalk stream fishing or via tackle shops or hotels in the main centers.
The Test, (Hampshire, England)
This is probably the most famous trout stream in the world, the place where dry flyfishing as a sport was refined and codified by a group of anglers led by Frederic Halford in the 1880s and 1890s. Anglers make pilgrimages from all over the world to fish it, and pressure on the river today is immense. Most of the fishing is for large, stocked browns and rainbows of two to six pounds and more. A typical day rod costs between #80 and #250. At last light, when the small spinners are up, even these fish can challenge the most skillful. Outside of mid-May to mid-June, when the huge mayfly Ephemera danica is up, most dry flyfishing will be with small upwinged and caddis imitations. Good base: Stockbridge.
http://gorp.away.com/gorp/activity/fishing/features/flyfish_topten2.htm#itchen