Changes in the breeding bird population of a small clay river following flood alleviation works

Abstract

In 1979, a major, ecologically sensitive, flood alleviation scheme involving two-stage channel construction was implemented along 3 km of the River Roding in rural Essex. Disturbance to aquatic and waterside habitats and associated vegetation was minimized because the lowermost third of the river channel was retained to conduct dry-weather flow, while increased discharge capacity was achieved by excavating low-level flood berms. Vegetational change within the new river corridor was monitored and breeding bird territories were censused between mid-March and August 1980–82. In contrast to what happens after dredging, breeding river birds were able to re-establish quickly after berm excavation and within 3 years there were more than 17 . territories per km. Reeds proliferated along the unmanaged channel margins and on ungrazed flood berms, providing more nesting sites for Moorhen, Little Grebe, Mallard, Sedge Warbler, Reed Warbler and Reed Bunting. However, reeds impair hydraulic efficiency and remedial action to alleviate this would adversely affect river birds and nullify the benefit gained from berm excavation. Despite considerable conservation advantages over dredging, the use of hydraulically-efficient two-stage channels is limited by the amount of land they require. This increases the importance of determining the ecological and hydraulic implications of alternative flood alleviation techniques which have recently been implemented in lowland Britain.