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

Abstract

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.