The character and age structure of valley fills in upper Wolumla Creek catchment, south coast, New South Wales, Australia

Abstract

Extensive valley fills at the base of the escarpment in upper Wolumla Creek, on the south coast of New South Wales, Australia, have formed from a combination of 'cut and fill' processes. The valley fills comprise series of alternating, horizontally bedded sand and mud units, reflecting reworking of detritus from deeply weathered granites of the Bega Batholith. Sand units are deposited as sand sheets or splays on floodplain surfaces or in floodouts that form atop intact valley fill surfaces downstream of discontinuous gullies. Alternatively, sands are deposited from bedload and form bars or part of the valley floor within channel fills. Organic-rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts. Within 5 km ofthe escarpment, valley deposits grade downstream from sand sheet and splay deposition in floodouts, to mud deposition in swamp and seepage zones. Radiocarbon dates indicate that virtually the entire valley fill of upper Wolumla Creek was excavated prior to 6000 years BP. Remnant terraces are evident at valley margins. The valley subsequently filled between 6000 years BP and 1000 years BP producing valley fills around 12 m deep, but no greater than 300 m wide. Reincision into the valley fill, on a scale smaller than the present incision phase, is indicated at around 1000 years BP, following which the channel refilled. Portion plans dated from 1865 refer to the study area as 'Wolumla Big Flat', and show large areas of swampy terrain, suggesting that the valley fill had re-established by this time. Within a few decades of European settlement the valley fill incised once more. Upper Wolumla Creek now has a channel over 10 m deep and 100 m wide in places, draining a catchment area of less than 20 km 2 .

Wetlands in drylands: diverse perspectives for dynamic landscapes

“…Leigh et al ( 2010 ) suggest that this flow variability is essential in maintaining dryland river and wetland habitats and biodiversity. Similarly, some valley-bottom WiDs (also referred to as valley fill swamps, valley mire fens or ciénegas in literature) have been characterised by phases of incision, whether induced by crossing of an intrinsic geomorphic threshold, or by crossing of an extrinsic geomorphic threshold triggered by anthropogenic or climatic stressors (e.g., Fryirs and Brierley 1998 ; Tooth et al 2014 ; Pulley et al 2018 ; Grenfell et al 2019 , 2020 ). These types of systems oscillate spatio-temporally between palustrine valley-fill wetlands and incised gully networks over centennial to millennial timescales.…”

“…In addition to the potential impact of climate on sediment accumulation rates, Wiener et al ( 2022 ) postulate that seasonal cycles of desiccation of sediments in WiDs will likely strengthen phosphorus retention due to oxidation. Further, process dynamics in WiDs are equally as vulnerable as humid region wetlands to human-induced changes in riverine sediment flux, that may cause either siltation (e.g., Wolanski et al 2001 , Gell et al 2009 ) or erosion (Fryirs and Brierley 1998 ) of wetlands downstream.…”

What type of gully is that? The need for a classification of gullies

“…Discontinuous gullies (or ‘independent’ gullies – Geyik, 1986) however form on the backslope and lower slopes of the valley side, which was a major, classifying factor introduced by Heede (1970). They do not join up with other gullies or stream networks but, instead, distribute their flow as fans over gentle slopes, or in valley‐fill aggradation in ponds or swamps, for example, the headwater ‘swampy meadows’/‘chain‐of‐ponds’/‘cut‐and‐fill’ landscapes (Eyles, 1977; Fryirs & Brierley, 1998; Prosser et al, 1994) described in south‐eastern Australia. Their discontinuity is explained by changes in slope gradient affecting the hydraulic capacity of the gully flows to deposit the eroded sediment from upslope.…”

Valley‐bottom wetlands as temporary buffers for source‐to‐sink dispersal of sediment and associated phosphorus in dryland landscapes

“…A detailed understanding of trends in wetland sediment trapping dynamics at different temporal scales not only provides insight into catchment sediment budgets, but also into morphodynamic changes, resilience and evolutionary trajectories of these geomorphic landforms. Previous investigations that explored rates of vertical accretion at centennial and longer timescales have attempted to address the influence of geomorphological processes in the formation of valley‐bottom wetlands (e.g., Fryirs et al, 2014; Fryirs & Brierley, 1998; Pulley et al, 2018). Reported rates of long‐term sediment accretion in valley‐bottom wetlands, in a variety of climatic settings, were low and highly variable (0.01 to 0.40 cm year −1 ) (for review, see Wiener et al, 2022).…”

“…Valley‐bottom wetlands maintain a fragile balance between depositional and erosional processes, such that a change in the ratio of water to sediment supply may result in a significant shift in the dominant geomorphic processes (Grenfell et al, 2019). Studies on geomorphic processes have demonstrated that this wetland type alternates between multi‐decadal intervals of storage, followed by periods of net erosion (Fryirs & Brierley, 1998; Pulley et al, 2018). Thus, as many wetlands have the potential to intermittently recycle deposited fine sediment and associated phosphorous, they may contribute to downstream water quality deterioration (Fryirs et al, 2007).…”