I've been for a bike ride this evening alongside the Hamps where it dries up (below Waterhouses, some of you may know it).

Anyway, on a part where its been effectively dry for at least two months there was a nice hatch of olives going on!

Does this sound feasible? I'm 99.9% sure they were a medium sized olive and not a terrestial fly.

Fresh water shrimp can emerge from a dry river bed, as their eggs can survive in the dry. Its more likely that your 'duns' have come from above or they may be spinner that have flown upstream. It is amazing how quickly a winterbourne stream can be populated.

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http://www.haddonestate.co.uk/rivers/rivers.php
http://www.thepeacockatrowsley.com/fishing.html

Mature riverflies fly upstream to lay the eggs. This is to counter the natural downstream drift of eggs and nymphs. Coming back from fishing the Monnow, in Hereford and on sunny evenings I have seen clouds of grannom sedge adults more than a mile away from the river, following the roads in the mistaken belief that they are rivers.

Instinct is a wonderful thing but how do they know which way "upstream" is?

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"The sun was so delightfully warm, and the stone, which had been sitting in it for a long time, was so warm, too, that Pooh had almost decided to go on being Pooh in the middle of the stream for the rest of the morning."

www.monnow.org