Because much ecological research in rivers applies theories developed elsewhere to a diverse array of habitats renowned for their spatial and temporal complexity, riverine ecology lacks a clear conceptual cohesiveness (Fisher 1997). Hence, the quest to identify, explain, and predict dominant ecological patterns and processes has led to the proposition of many conceptual models that also vary across spatial and temporal scales. These models range from the structure of river networks through to reach-scale models of flow regimes, patch dynamics, sediment organization, and stream hydraulics. Not surprisingly, the explicitness of these conceptual models to specific river types (e.g., headwaters, alluvial rivers, floodplain rivers) contributes significantly to the processes and linkages emphasized by the models. Despite the obvious lack of cohesion in conceptual models of river function, three themes are common to all such models and these are fundamental to riverine ecology: (1) identifying interactions between structure and function; (2) understanding the processes driving the arrangement of structural components in space and time; and (3) identifying how specific habitats and processes are connected in space and time. Critical reviews of conceptual models of river function are given elsewhere (see Thorp et al. 2006). Our aim here is to discuss these three themes as they relate to understanding river function.

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.

Since European settlement, vast amounts of wood have been removed from Australian rivers. In recognition of its ecological value, including as habitat for aquatic invertebrates, wood is being reintroduced but with little understanding of optimum placement (pools v. riffles) or structural complexity to enhance invertebrate diversity. We hypothesised that complex woody debris would support higher numbers and more macroinvertebrate taxa, especially in riffles. Wood substrates of two complexities but similar surface areas were introduced into pools and riffles at three sites along the Hunter River, Australia. After 30 days, more taxa and individuals occurred on the complex substrates in pools and riffles at all sites. Substrates in riffles usually supported more taxa and individuals but responses were site-specific. Community composition varied among sites, substrates and habitats. Complex substrates, especially in riffles, trapped drifting organic matter that increased abundance and taxa richness but did not alter overall trends among substrates or habitats within sites. However, densities of rheophilic (flow-loving) taxa were reduced by entrained organic matter. Our results indicated that complex woody debris introduced into riffles could enhance diversity and abundance of macroinvertebrates in the Hunter River. However, these conclusions from a short-term, small-scale experiment need validation from longer-term, large-scale river rehabilitation projects.

The lack of scientific knowledge on the flow thresholds and environmental water requirements for specific water sources within the North Coast is a significant limitation in the allocation and implementation of environmental flows. This uncertainty, the limited volume of ECA water and changing NRM platforms has resulted in the limited use of environmental flow allocations and monitoring to date. The aim of this review is to evaluate the existing scientific knowledge of the environmental flow requirements of the regulated water sources; in the Hunter, Chichester and Paterson Rivers, and Glennies Creek in the Hunter-Central Rivers CMA region and parts of the Richmond River in the Northern Rivers CMA region. Outcomes will inform the future development of environmental flow monitoring and research, and decision-making about the management of environmental flows in the North Coast of NSW.