The hot weather has caused water levels in brooks to drop to levels where a bone-dry bed is visible in many places.

Only two of the past 20 months have seen above-average rainfall, making this one of the driest periods in the past 100 years. The Environment Agency (EA) said conditions were unlikely to improve before the autumn.

Streams such as Dollis Brook, Deans Brook and Mutton Brook are parched of water in some sections, with levels either extremely low or non-existent.

Derek Warren, of the Finchley Society, said that wildlife seemed to be flourishing still, with regular visitors such as kingfishers, moorhens and foxes still surviving where the brooks do continue to flow. But the EA said that fish will struggle to survive and algae will breed. Some forms of algae can kill animals and pets that drink them.

Mr Warren, who monitors water levels in parts of the brook near Fursby Avenue, in Finchley, said that the source of much of the water in the brooks is likely to be from surface run-off fed by household water leaks and misconnected drains, rather than rainfall. He said: "The average water flow has been about four centimetres more than it is now, this year, but if it rains heavily it should add another six inches onto that. This is the longest it has been this low."

Dennis Pepper, chairman of Windsor Open Space, in Hendon, said: "It does seem surprisingly low. I've not seen it this low for several years. The water is concentrated in cloud pools, which indicates pollution. If this becomes stagnant because it's not flowing, then it becomes concentrated and starved of oxygen and you can see the sticklebacks coming up for air all the time it's a real worry."

An EA spokesman, said that, as much of the water courses in Barnet run through clay soils, once it did rain significantly, that it would be likely that the rivers would fill up again easily.