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Watersheds, rivers, and floodplains in desert environments have been dramatically altered to meet human needs. Modifications include river engineering projects, floodplain development, dams and diversions, and watershed development, amongst others. In the southwestern United States (the Southwest), the exploitation of rivers for human needs has enabled a remarkable level of economic and population growth over the past century. The implications of these development pressures, however, have been profound for aquatic ecosystems. Modifications to hydrologic and geomorphic processes have contributed to population decline, endangerment, extirpation, and in several cases extinction of native species. This chapter provides a synopsis of the recent history of watershed, river, and floodplain development in the Southwest. Using examples from across the region, some of the major activities that have contributed to the interruption of natural hydrologic and geomorphic processes are highlighted. Connections are made between human-induced stressors and subsequent geomorphic and ecological responses. Further, efforts to ameliorate these impacts through physical stream restoration projects and the adoption of environmental flows are discussed. Additional research and adaptive management strategies are needed to improve understanding of the connections between underlying processes and ecosystem conditions to achieve restoration success.

Environmental flow assessments take place in many different management contexts, at various spatial scales, within different biophysical systems, and in contrasting socioeconomic and political settings. These settings and circumstances have a strong bearing on the scope for provision of an environmental flow regime and the methods most suited to achieving the desired ecological outcomes. This chapter describes techniques for assessing environmental flow requirements for rivers. They range from the simplistic use of available hydrologic records to establish minimum and flushing flows to sophisticated modeling procedures that link changes in river discharge with geomorphological and ecological responses at species, community, and ecosystem scales. Hydrological methods include the Montana (Tennant) method, flow duration curve analysis, Range of Variability Approach (RVA), and flow translucency approach.

River ecosystems are four-dimensional systems, with longitudinal, lateral, and vertical components, relationships, and processes; temporal vectors add the fourth dimension. The overriding importance of dynamic flow patterns in streams and rivers is captured in the Natural Flow Regime Paradigm. More general hydro-ecological principles conceptualise and quantify the ecological roles of flow, and contribute to the understanding of the flow requirements of rivers and their biota. Explicit hydro-ecological principles and conceptual models describe in-stream biodiversity (aquatic plants, invertebrates, and fish), riparian vegetation, and nitrogen dynamics. Although common to most stream and river ecosystems, particulars vary with climate, biogeographic region, topography, catchment vegetation, and flow regime type. Hydro-ecological principles and concepts described in this chapter provide the ecological foundations for environmental flow management.

Escalating human demands for fresh water are jeopardising the intrinsic biodiversity values, ecological health, and vital ecosystem services of the rivers, wetlands, and estuaries upon which millions of humans depend for water, food, and secure housing, as well as for quality of life, health, and prosperity. Climate change exacerbates these pressures by affecting the global water cycle, freshwater availability, and the ecological health of aquatic ecosystems. Restoration of biodiversity, ecosystem function, and resiliency is a global imperative for water managers, scientists, and civil society. This chapter introduces the main argument of the book, which is that to sustain freshwater and estuarine ecosystems the natural volumes and variability patterns of standing and flowing water must be maintained through provision of “environmental flows.”

Estuaries can be classed as embayments and drowned river valleys, wave-dominated estuaries, wave-dominated deltas, coastal lakes and lagoons and strandplain-associated creeks, tide-dominated estuaries, tide-dominated deltas, and tidal creeks. They form the lower end of the river continuum and their ecological character is deeply dependent upon the incoming freshwater flow regime. This chapter reviews the impacts of land-use changes, urbanization, and water engineering in river corridors, large water-supply dams, flow diversion, water extraction from rivers, and groundwater pumping on freshwater inflow regimes and water quality. Environmental flow methods for estuaries can be divided into three types based on achieving: inflows (e.g., a percentage relative to natural inflows), a specified estuarine condition, or the needs of particular estuarine resources.

Common carp (Cyprinus carpio) is a major and widespread pest species in southeast Australia. The recent emphasis on increasing regulated river flows to improve environmental values requires prevention of carp spread into carp-free areas while still permitting the passage of major water flows. Biological control techniques are still many years away from effective implementation. In the meantime, the use of engineered structures represents an important mechanism for the exclusion of carp. Two types of structure are discussed herein. With the first, the principle of species separation is based on the carp’s jumping ability, in contrast to the non-jumping behavior of indigenous species, and is thus discriminatory. The second type of structure is designed to capture all biomass and, accordingly, is non-discriminatory. Case studies indicate that a discriminatory structure works well within fishways, but is not particularly effective in a riverine situation where all species tended to avoid the structure. In contrast, a non-discriminatory structure tested by way of a physical model was very effective, capturing better than 97% of introduced samples of carp eggs and carp fingerlings. Of the less than 3% that did pass through the structure, one-tenth — or less than 0.3% of the total introduced sample — was considered viable.

In this chapter we review techniques that managers may use to respond to climate change. First, modelling the impacts of climate change on freshwater ecosystems is discussed. While hydro-climatic projections can be used their imprecision requires the selection of robust adaptation options that provide benefits under a range of possible climate outcomes. Second, contested concepts for managing freshwater ecosystems and resources are summarised, and we conclude that they may be used to develop and implement cross-sectoral policies that sustain freshwater ecosystems. Third, options for climate change adaptation for freshwater ecosystems recommends application of six principles, emphasising: accommodation of change; application of ecological and socio-economic targets across multiple scales; maintaining connectivity, conservation of refugia, and representative habitats; initial implementation of no- and low-regret adaptation interventions; agreeing on thresholds for ecological change that trigger new management interventions; and scientific monitoring and evaluation. We conclude by considering how to manage the negative impacts and seize positive synergies in climate change responses: conservation advocates must engage with agriculture, energy, and water resources sectors if freshwater ecosystems are to be incorporated in their decisions.