Restoring vertical connectivity in rivers: geomorphic, hydrologic and biogeochemical responses to log sills in the Williams and Hunter Rivers, NSW, Australia

In alluvial rivers, groundwater and stream water are intimately connected via the saturated sediments lying below and beside the river channel, termed the 'hyporheic zone'. This zone is a spatially and temporally dynamic mosaic of biogeochemically distinct patches that are connected by multiple, hierarchical hydrological flowpaths that also vary in space and time. Active and diverse hyporheic zones promote resilience and resistance in rivers through thermal buffering, retention of water, solutes and organic matter, biogeochemical filtration, nutrient cycling, and biological production that occur within these ecotones between alluvial rivers and true groundwaters. However, alluvial river systems are among the most endangered ecosystems in the world, and in many the spatial and temporal configuration of hyporheic exchange has been impaired by human activities. Efforts to restore hyporheic zones are increasingly common. Typically, these projects have sought to reinstate geomorphic complexity through augmenting coarse sediment or installing wooden structures such as log sills. Most of these attempts have been on low-order reaches and focused at fine-scales (e.g. a single riffle). This thesis describes the first large-scale field experiment to assess the restoration outcomes and ecological success of large, engineered, multi-log structures such as those typically deployed by catchment managers. My study derived a conceptual model from the literature that hypothesized the mechanisms by which a log sill anchored within a riffle would increase hyporheic exchange and influence nutrient processing. I then tested these hypotheses using two log sills placed in each of two gravel-bed rivers, the Hunter River and the Williams River, New South Wales, Australia

Assessing change in riverine organic matter dynamics in the Hunter River, NSW, over the last 200 years: Implications for stream restoration

Successful river rehabilitation requires the restoration of self-sustaining ecosystem functions. One key function is organic matter cycling, including the sources, transfers and sinks of organic matter as it moves from the catchment, across floodplains, down streams, and exchanges with groundwater in the hyporheic zone. River food webs may depend heavily on organic matter generated in-stream by microbial and algal biofilms whereas flow pulses may import leaf litter from the floodplain. Bars and riffles retain this organic matter while generating diverse microhabitats whose particular biogeochemical conditions favour different suites of microbes. Poor land management has deprived the Hunter River of geomorphic complexity at the broad scale of bars and riffles. This paper reviews historical changes to channel shape and vegetation regime in the Hunter River and the repercussions of these on organic matter dynamics over the last 200 years. We conclude that introduction of wood will partly restore conditions closer to those pre-European settlement and alter hyporheic processes but that organic matter dynamics may never be fully restored