Artificial riffles in river rehabilitation: setting the goals and measuring the successes

✍ Scribed by David Harper; Mohammad Ebrahimnezhad; Ferran Climent I Cot

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 178 KB
• Volume: 8
• Category: Article
• ISSN: 1052-7613
• DOI: 10.1002/(sici)1099-0755(199801/02)8:1<5::aid-aqc266>3.0.co;2-g

✦ Synopsis

1. Replacement of artificial riffles in canalized rivers is a widely-used tool in river rehabilitation but its effectiveness is only infrequently evaluated. In this paper artificial riffles, placed in a 2-km length of lowland stream in eastern England, were evaluated using geomorphological, functional habitat, and ecological techniques 3 years after installation.

2. Mean riffle spacing was approximately double that predicted from first principles, due to several large 'gaps' in the 2-km length where riffles had not been originally reconstructed. Two sequences, of 14 riffles, did approximate to the spacing predicted for natural riffles based upon the relationship with mean annual discharge and with bankfull width. Twenty out of 26 riffles retained their original physical character whilst six were deep, slow flowing and covered with sand or silt.

3. Shallow riffles in an approximately correct spacing retained their coarse particle dominance and caused the scouring between themselves of deeper pools than were found elsewhere in the stretch. Shallow riffles had high flow velocities which resulted in richness of functional habitats not found elsewhere in the stretch.

4. Invertebrate colonization showed a clear distinction between communities of shallow, fastflowing riffles and deeper slow-flowing runs and silted riffles. Indicator taxa of riffles were the genera Baetis, Simulium, Hydropsyche, Eukieferiella, and Rheotanytarsus. Indicator genera of silted riffles and runs were Caenis, Planorbis, Sphaerium, Microtendipes, and Stichtochironomus.

5. Riffle reinstatement in lowland rivers of low energy will produce desirable geomorphological and ecological changes if the riffles are spaced according to geomorphological 'first principles', and are shallow ( B 30 cm depth) under low-flow conditions: thus, simple science can set adequate goals at the design stage. Appraisal using the development of functional habitats at riffle sites provides a rapid technique for post-project appraisal, thus evaluating by ecological success the original hydrological goals.