An Evaluation of the Spatial Configuration and Temporal Dynamics of Hydraulic Patches in Three UK Lowland Rivers.

Accurate characterisation of the hydraulic environment is a key step in describing
hydromorphology at an ecologically relevant scale which has relevance to several aspects of
river management, including monitoring river health, designing environmental flows and
evaluating river rehabilitation measures. However, current hydraulic habitat quantification
methods oversimplify the spatial heterogeneity of the hydraulic environment and do not
explain or interpret the spatial arrangement of different habitat units sufficiently or define the
dynamics of these shifting patterns. This research applied a novel numerical classification
method and a landscape ecology framework to quantify the composition and configuration
hydraulic patches in three UK lowland river reaches at five different flows. Five spatially
coherent hydraulic patches, defined by the joint distribution of depth-velocity, were optimally
delineated from hydraulic point data at each reach using the Gustafson-Kessel fuzzy
clustering algorithm. Transitional zones between hydraulic patches occupied between 18-
30% and represent an application of the ecotone concept to the instream environment.
Hydraulic patch diversity increased with discharge, peaking at high flow (Q38-Q22),
suggesting that the provision of high flows is important for maximising hydraulic
heterogeneity. The dominance of shallow, slow patches at low flow was gradually replaced
by faster, deeper hydraulic patches at high flow illustrating the effect of discharge on the
availability of different hydraulic patch types. The spatial arrangement of patches, quantified
using a range of spatial metrics from the field of landscape ecology at two spatial scales
(class and reachscape), was relatively invariant to changes in discharge suggesting that the
configuration of the hydraulic patch mosaic is determined by channel morphology and
remains stable between channel forming discharges. The majority of hydraulic patch types
occurred in relatively fixed locations in the channel, moving relatively small distances as
discharge increased, associated with the gradual expansion or contraction of patch area. The
results suggest that sub-bankfull flow variations will primarily affect the composition rather
than the configuration of hydraulic patches, however large fluctuations are likely to result in
high rates of patch turnover (change in location), with potential implications for instream
biota. The hydraulic patch/transition zone model of the hydraulic environment provides a
new approach for exploring the link between physical and biological heterogeneity in the
instream environment, including the role of instream ecotones. Whilst the application of
numerical classification is currently limited by the large hydraulic data requirement, future
advances in remote-sensing technology and hydrodynamic modelling are likely to widen its
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applicability at a range of spatial scales. The results highlight the need for further research on
the ecological significance of hydraulic patches and transition zones and ecological
sensitivity to changes in hydraulic patch configuration. Wider application of the landscape
ecology approach to hydraulic habitat assessment in different reach types is recommended to
improve understanding of the links between geomorphic and hydraulic diversity.