Establishing the real Bourne Identity:  A twin track approach to assessing habitat diversity and identifying appropriate river restoration needs

David Bradley  (APEM Ltd - d.bradley@apemltd.co.uk)

Stuart Clough  (APEM Ltd)

Sally German  (ARUP)

Kate Robinson  (ARUP)

Fiona Bowles  (Wessex Water Services Ltd)

Proceedings paper from the 10th Annual River Restoration Centre Network Conference, 1st - 2nd April 2009

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Abstract

Wessex Water are currently undertaking investigations to assess the environmental impact of groundwater abstraction in the River Avon Special Area of Conservation (SAC) catchment, and are reviewing options for mitigating any adverse effects, if they are found. Whilst river restoration does not address abstraction stress at source, rehabilitating the physical condition of the habitat can potentially provide mitigation through minimising the ecological effects of abstraction pressure. The River Bourne is not considered to be in Favourable Condition due to extensive channel modifications by management practices for flood defence, agriculture and fisheries, in addition to the effects of abstraction, and, therefore, habitat rehabilitation was considered to be an essential step towards achieving Favourable Condition.

Geomorphological fluvial audit surveys are often used to inform appropriate river rehabilitation at the reach-scale. In this paper, we compare the outputs of a geomorphological assessment and ecological habitat assessments of selected reaches of the River Bourne, using published methods, to assess to what extent the standard geomorphological assessment is ecologically meaningful for the purposes of informing river rehabilitation on SAC rivers.  Additionally, using real biological data for fish and macroinvertebrates, that were collected within the geomorphological survey reaches in a dry year (2006) and a wet year (2007), we test the hypothesis that ecological community stability between a dry and a wet year was positively correlated to an index of standard geomorphological diversity.

The results of the geomorphological survey and the ecological habitat survey were weakly correlated and several habitat features that are functionally important for fish (including Atlantic salmon, bullheads and lampreys which are features of interest of the River Avon SAC) were not accounted for by the geomorphological assessment. Ecological community change from 2006 to 2007 was not significantly correlated to the geomorphological standard diversity scores. However, the ratio of the abundance of some macroinvertebrate groups (mayflies, stoneflies and caddisflies) that are functionally important in rivers, are particularly sensitive to environmental perturbation and are apparently declining at many locations, to the abundance of ubiquitous groups (Chironomidae and Oligochaeta), was positively correlated to the geomorphological standard diversity scores.

Notwithstanding this important finding, this study suggests that geomorphological surveys for river restoration do not convey enough information on
their own about the ecological implications of undertaking habitat modifications, particularly for fish which are often highly mobile, have specific habitat requirements for different life-stages and are of conservation interest. A critical consideration in this respect is the longitudinal order in which geomorphological features occur between survey reaches and the importance of this order to different life-stages of fish; this information is not conveyed through standard fluvial audit methods.

Fluvial audit surveys for informing river restoration on SAC rivers should be made more ecologically relevant by incorporating ecological habitat classifications. In this way, efforts can be focused towards restoring the appropriate habitats, in the right longitudinal order, to support the characteristic biota of the river and to protect the listed features of interest.

Keywords

low flows; abstraction; chalk streams; geomorphology; ecology