Sedimentologically significant tributaries: catchment‐scale controls on sediment (dis)connectivity in the Lockyer Valley, SEQ, Australia

Abstract

The nature of catchment-scale sediment (dis)connectivity is the primary influence on sediment delivery to trunk streams and controls the particle size distribution of channel bed sediments. Here, we examine the distribution of major sediment buffers (floodplains, terraces, alluvial fans, trapped tributary fills), barriers (weirs), and effective catchment area (i.e. sediment contributing area) to characterize the potential for coarse sediment (dis)connectivity in 20 tributaries of Lockyer Creek, in the Lockyer Valley, SEQ. We then analyse the distribution of trunk stream sedimentary links to determine how certain tributaries or disconnecting features (buffers and barriers) influence downstream patterns of bed sediment fining along Lockyer Creek. We find that buffering increases downstream in the Lockyer Valley, and that tributary position and shape influence the space available for sediment buffering. Correspondingly, the spatial extent of sediment buffers impacts the distribution of effective catchment area, which influences the sedimentological significance of individual tributaries. Tributary sediment connectivity, the extent of overbank flows (floodwater zones), and weir locations all exert an additional influence on the distribution of sediment links along the trunk stream. These controls are related to the physiographic and climatic setting of the Lockyer Valley, and anthropogenic influences in this system. We conclude that controls on sediment connectivity and bed load sediment characteristics are highly variable between catchments, and that sediment (dis)connectivity merits equal consideration with tributary basin/channel size when determining controls on tributary-trunk stream relationships and channel sediment regime.

‘Out with the Old?’ Why coarse spatial datasets are still useful for catchment‐scale investigations of sediment (dis)connectivity

“…We modelled a slope threshold at 2° (cf. Fryirs et al , ; Lisenby and Fryirs, in press) on the catchment DEMs, where cells with a slope of <2° were deleted (Figure ). For each DEM, we then determined the location of every drainage point in each tributary basin and summed the drainage areas (effective catchment areas) for all points that fell within 25 m (cf.…”

“…However, when characterizing the catchment‐scale nature of physical processes, using a simple model with coarse‐scaled inputs can generate quick, meaningful, and reasonably accurate insights that can be used to help focus further research and management efforts. Full results of the Lockyer Valley case study are reported in the accompanying paper (Lisenby and Fryirs, in press).…”

Connectivity as an emergent property of geomorphic systems

“…In this context, geomorphologists have also begun to assimilate notions of connectivity from other disciplines, especially ecology (Merriam, ; Amoros and Roux, ; Ward and Stanford, ; Ward, ) and hydrology (Pringle, , ) (cf. Bracken and Croke, ; Poeppl et al ., ), seeking to better describe water and sediment dynamics in catchment systems (Croke et al ., ; Brierley et al ., ; Fryirs et al ., , ; Turnbull et al ., ; Wainwright et al ., ; Fryirs, ; Gomez‐Velez and Harvey, ; Bracken et al ., ; Lisenby and Fryirs, , ). Depending on the respective disciplinary basis, three types of connectivity have commonly been differentiated in geomorphic contexts, although all of the types are interdependent: (1) sediment connectivity, which is the potential for sediment to move through geomorphic systems (Hooke, ) as governed by the physical coupling of landforms; (2) landscape connectivity, which is the physical coupling of landforms; and (3) hydrological connectivity, which describes the passage of the transporting medium from one part of the landscape to another.…”

“…Given differences in landscape connectivity relationships in differing environmental and landscape settings, there is profound variability in the ways and rates with which responses to disturbances that disrupt the sediment regime are mediated through a catchment (Fryirs et al ., , ; Lane et al ., ; Surian et al ., , ; Kuo and Brierley, , ; Lisenby and Fryirs, , ). Fryirs et al .…”

“…Configuration and state are interrelated. Places in a river network with local sediment disconnectivity, for example, can accumulate sediment through time and become sites with higher potential for geomorphic change, or they can be areas that absorb change and limit manifestation of disturbance at off‐site locations (Czuba and Foufoula‐Georgiou, ; Lisenby and Fryirs, , ).…”