Alteration of instream habitat

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Alteration of instream habitat

04. Morphological alterations

General description

Natural instream habitats offer refuge and shelter, food resources and spawning grounds to aquatic biota. Recognition of instream habitat alteration should be based on changes in surface flow type, hydraulic attributes (flow depth, velocity and bed roughness, shear velocity, Reynolds and Froude numbers), channel morphology, and bed substrate calibre. Instream habitat degradation may be an effect of a hydrogeomorphological process (natural or caused by other pressures), or of a direct human activity (e.g. channel dredging, gravel bed extraction). The latter activities are those pressures we are refering to here.

Effect/Impact on (including literature citations)

All hydromorphological pressures affect instream habitat, but in this section, we refer to those pressures that directly destroy the aquatic habitat, such as channel dredging and mining, and the reinforcement of channel bed and banks with introduced materials such as concrete or rip-rap. These activities generally reduce channel boundary roughness, leading to increased flow velocities and other consequences similar to those resulting from channelization. Assessing the effects of these specific pressures is difficult due to their association with other potential habitat-altering variables. For example, increases in turbidity and siltation can easily arise from agricultural land use (i.e. cattle grazing) in both channelized and reference streams.

  • Channel dredging

Channel dredging lowers and usually steepens the channel bed. Even if further incision is not induced, channel banks become higher and more exposed to erosion and, as a result, bank erosion is a likely consequence of dredging. The degree of impact of the dredging depends on the quantities of sediment delivered by the river to the dredged reach. The smaller the ratio of dredged to supplied sediment the smaller the likely HYMO effects. Lagasse and Winkley (1980) concluded that gravel dredging in the lower Mississippi River caused bed degradation, reduced flow resistance and thus reduced flood heights and groundwater table levels. Lou et al. (2007) identified increased grade slope, bank instability, and brackish-water intrusion as negative HYMO effects of dredging, while positive effects included decreased flooding. They also cited a study by Han et al. (2005, in Chinese) that identified changes in the river regime as a result of dredging, including lowered water levels, alteration of surface water and groundwater recharge, and re-balancing of salt and fresh water in tidal regions.

  • Bed Reinforcement

Not many references dealing with HYMO processes and variables have been found. Urban development transforms the hydrological system through construction of impervious surfaces and stormwater drainage systems, and river channels are completely reinforced precluding any morphological adjustment. Gurnell et al (2007) have analyzed of Urban River Survey data from 143 urban channel reaches in three European rivers (the River Tame, UK; the River Emscher, Germany; and the River Botic, Czech Republic) and have demonstrated the strong influence of river channel engineering on channel structure, physical habitat features and vegetation patterns.

Conceptual framework of alteration of in-stream habitat effects on HYMO processes and variables (LWD = Large Woody Debris).

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Other relevant information