Geomorphic and hydraulic unit richness and complexity in a coastal plain river

Abstract

Geomorphic and hydraulic units in river channels are closely linked to geodiversity and habitats, and thus to biodiversity. In a ~ 200 km reach of the lower Sabine River, in the northern Gulf of Mexico Coastal Plain, 72 different hydraulic units (HU) were identified in six geomorphic zones or river styles. Richness–area relationships indicate a linear or logarithmic increase of HUs, as opposed to the less steep power functions generally found in biogeographic species–area curves or in soil richness–area analyses. Different results are obtained when starting from the upstream or downstream end of the study area, indicating the importance of directionality in such analyses. These results show that HUs (and related habitats and biotopes) are both richer and more variable than a repeated sequence of units. The number of HUs inundated increases linearly with flow stage categories, indicating the importance of high within‐bank flows in maintaining and activating HUs. Aggregated HUs (AHUs) associated with similar geomorphic units are highly connected, both with respect to patterns of spatial adjacency and potential connectivity at similar flow levels. Spectral graph theory metrics applied to a graph representation of spatial adjacency shows a highly complex network with a high potential for rapid propagation of changes—and even more so for a graph based on flow connectivity. The flow connectivity graph shows far higher synchronization as indicated by algebraic connectivity. Thus suggests more rapid and coherent changes for processes driven by river flow, as opposed to phenomena driven by other factors between flow events. These findings have important implications for understanding relationships between geodiversity and habitat diversity, managing habitat and biodiversity, and linking the latter to instream flows. Copyright © 2017 John Wiley & Sons, Ltd.

Geomorphology of the fluvial–estuarine transition zone, lower Neuse River, North Carolina

“…Examination of maps and images, and field experience in several Gulf of Mexico coastal plain rivers (e.g. Phillips, 2013, 2014, 2017; Phillips & Slattery, 2006, 2007) suggests that the phenomena found in the Neuse are widespread in similar coastal plain settings.…”

Assemblages of geomorphic units: A building block approach to analysis and interpretation of river character, behaviour, condition and recovery

“…Analysis of assemblages of geomorphic units provides a powerful tool to interpret river type and behaviour, condition and recovery (Brierley & Fryirs, 2005). In conceptualizing each of these components, such analyses can be unpacked to undertake reach‐scale interpretations of landforms (individual geomorphic unit) that underpin assessments of patterns and sequences along longitudinal profiles, across catchments or across regions (Brierley & Fryirs, 2005; Church, 2002; Newson & Newson, 2000; Phillips, 2017).…”

“…In some instances, river types have a simple character and behaviour with a small number and a low richness of geomorphic units in their assemblage (Fryirs & Brierley, 2009; Phillips, 2001, 2017). For example, a chain of ponds has a simple assemblage comprising ponds and valley fill (Figure 3).…”

“…Without such information it is difficult to model and interpret process–form and behavioural traits that define the expected range of variability of different river types, the causes of different condition states or the triggers of river recovery. However, getting the geomorphic analysis and interpretation of assemblages of geomorphic units ‘correct’ is critical if such work is to be effectively and consistently used in eco‐hydrology (habitat) and hydro‐morphology (hydraulic) assessment, monitoring and management of a dynamic physical template (cf., Newson & Newson, 2000; Phillips, 2017; Poff et al, 1997; Wheaton, Brasington, Darby, Merz, et al, 2010).…”

“…Each geomorphic unit has fundamental morphometric properties (Minár & Evans, 2008) that include a characteristic shape (geometry), sedimentary composition, and position on the valley bottom. A mix of these attributes can be used to map and identify geomorphic units, and classify them (Belletti et al, 2017; Evans, 2012; Phillips, 2017; Wheaton et al, 2015). In hierarchical approaches to geomorphic analysis of rivers, geomorphic units sit between hydraulic units and reaches (Brierley & Fryirs, 2005; Fryirs, Ralph, et al, 2018; Gurnell et al, 1994; Gurnell, Corenblit, et al, 2016).…”