10^th Magdeburger Gewässerschutzseminar : Elbe, neue Horizonte des Flussgebietmanagements - Spindermühle, October 23th-21th, 2002.

Poster : Ecological impacts of channelization modes on the Elbe, Loire and Garonne rivers in comparison. Garcia XF, Pusch M.

Abstract

The west French rivers Loire and Garonne have been differently changed by hydraulic engineering in the past. Comparative studies on the biological colonization of these rivers have revealed the difference in impacts generated by the induced modification of habitat conditions on the fauna.

The lower part of the Loire river still possesses considerable fluvial dynamics due to the preservation of a 800 to 1000 m width free fluvial corridor between dykes, and a limited use of river water for water supply. This allows to the shifting bed to function and maintains the presence of different types of side channels as well as a strong connectivity between them.

Conversely, the main channel of the Garonne river is narrowly embanked since 1958 and the river is intensively used for irrigation. As consequences, the main channel is fixed and disconnected from the side channels. Habitat conditions in it become progressively homogeneous, especially by increasing the current velocities, as well as side channels disappear under land pressure. The regular pumping of water generates a high daily fluctuation of the water level impacting the bank habitats.

The measures of river engineering on the Elbe to promote navigation, especially embankment and historic construction of groyne structures perpendicular to the flow, partially exert similar ecological impacts as on the Garonne: increase of flow velocity in the middle of the channel, erosion of the river bottom and increase of the portion of the river bottom affected by sediment transport, loss of riverine standing water areas which are regularly flooded, disturbance of shoreline habitats by waves from ships passing by, alteration of the shore structure, especially loss of macrophytes and woody debris.

Using detailed examples from the comparison of Loire and Garonne experience, the modifications in structure and diversity of the chironomid community due to the Garonne embankment are shown. The parallels with the present and future impacts expected by the Elbe management on macro-invertebrate fauna are obvious. Impacts from embankment and construction of perpendicular groyne structures result in the disappearance of specialised species, a decrease in overall biodiversity, and in the reduction of the secondary productivity. The construction of alternative groyne structures parallel to the river flow, by recreating flooded standing waters at the margin of the main channel, will allow the renewal of habitat dynamics which means colonisation by frequent and rare species and finally increase of the global biodiversity.

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