Only 15% of the UK's lowland rivers and streams remain in their natural state.

The remainder have been hit by a combination of agricultural land drainage and urban development.

The damage done to rivers is also hurting the wildlife living on its banks. Even rivers that look natural are often little more than agricultural drains offering little to animals.

Only 15% of streams are in their original state

The Environment Agency also found Giant Hogweed, Himalayan Balsam and Japanese Knotweed are taking over the land around British rivers.

One in four streams are overrun with the plants, which first came to the UK as 19th century ornamental decorations.

The River Habitat Survey found that the Mersey has the largest number of all three weeds.

While Giant Hogweed can cause blistering and irritation in those who touch it, Himalayan Balsam is the tallest plant growing in the UK can force out others.

The Environment Agency gathered data at more than over 5,600 sites between 1994 and 1996.

Head of Conservation Paul Raven said: "This survey has provided us with valuable information on the extent and distribution pattern of these extremely hardy and invasive plants.

"The spread is a real problem and one which is hard to tackle, particularly along river banks where we are restricted to using only herbicides approved for use near watercourse.

"We provide advice to the public and landowners who need to contact the Agency before using such chemicals near water.