The hydro‑geomorphic structure, function and evolution of chains‑of‑ponds: implications for recognition of these discontinuous watercourses in river management
thesisposted on 2022-03-29, 01:55 authored by Rory T. Williams
The hydro‑geomorphic structure and function of river and wetland systems provide a physical template for ecosystem functions, allowing for their recognition, conservation and rehabilitation. This thesis examines the diverse geomorphic structure and hydrological processes of a spectrum of chain‑of‑ponds morphologies, identifying the intrinsic and extrinsic controls on their formation, evolution and impairment or loss. Chains‑of‑ponds consist of steep‑sided ponds separated by densely vegetated alluvial valley‑fill sediments that contain shallow ephemeral channels or preferential flow paths. They are part of the spectrum of discontinuous watercourses that are important landscape features due to their unique geomorphological, hydrological and ecological function. Analysis of seven headwater chain‑of‑ponds reaches reveal morphological diversity, in both planform and stratigraphy, and in varying stages of evolution. Sensitivity assessments of these reaches, combined with two‑dimensional hydraulic modelling, precipitated the quantification of measurable pond and planform characteristics associated with geomorphic condition. This examination developed three quantitative metrics that provide a measure of event sensitivity, morphological sensitivity and change sensitivity that can be used to guide the management of smaller‑scale headwater systems. Stratigraphic analysis and luminescence dating of larger‑scale chains‑of‑ponds on higher‑order streams show different formation processes that result from threshold changes in fluvial energy over the late Quaternary, with antecedent controls on the size and position of much deeper (up to 7 m) and more stable ponds. This adds important insight to the range of non‑linear responses through both intrinsic controls and also more complex responses from the persistence of palaeo landforms. This analyses improve the understanding of intrinsic controls on fluvial evolution and will help assess recovery potential through forecasting future trajectories, guiding approaches to maintaining or restoring ecosystem function of discontinuous headwater systems in southeast Australia and similar systems internationally. Stable isotopes (δ18O and δ2H), 222Rn, and pond and floodplain water levels revealed that the hydrology and ecosystem function of large‑scale chains‑of‑ponds, and discontinuous fluvial systems more generally, could be highly sensitive to local‑scale water extraction, shallow groundwater aquifer interference and changes to groundwater recharge due to climatic changes. The surficial appearance of discontinuity masks the continuous longitudinal flow of the alluvial aquifer during both high‑flow and no‑flow stages, highlighting the importance of subsurface and hyporheic processes in defining the hydrological character of a river. This thesis strengthens the understanding of the hydro‑geomorphological processes of discontinuous systems to facilitate ecosystem managers to develop workable strategies for protecting and managing systems that, due to their discontinuous form, do not as yet receive the same legal status or protection as other rivers.