THE THREAT OF STREAM CHANNELLING AND STRAIGHTENING...

A recently cleared drain in a North Island orchard.

Channelled streams are likely to be prone to excessive shear forces encountered during storm events, as a result of this lack of channel meandering and roughness. This could remove almost all the aquatic invertebrate species within them within minutes of such a flow, and is why so many of the species found in thse waterways are also commonly found in temporary point habitats, with their high ability to redisperse due to flying e.g. waterboatmen (Sigara species) and water beetles (e.g. Dytiscid species). Where possible the concrete stream base in riffle/ run areas should be broken to facilitate infiltration of water back into the watertable during periods of rain, with a following release back into the stream bed then occurring during periods of low flow when the watertable is consequently higher than the stream bed. Artificial meanders, pools, and wetlands may need to be placed in areas of already channeled streambed exposed to high shear forces and water velocities during flood events though their smoothness and straightness. In addition to preoviding refugia from flood volume and shear-force stresses on aquatic life, these formations will also provide instream and riparian habitat diversity for greater consequent aquatic invertebrate and fish biodiversity and abundances.

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