Structural characteristics Species diversity Gradients and zonations in species

Figure 1 Cause and effect chain of factors influencing a river system. Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

regime, the sediment and organic matter, the thermal and light characteristics, the chemical and nutrient characteristics, and the biota.

The flow regime defines the rates and pathways by which runoff enters and circulates within river channels and connecting groundwaters, and the residence time of water in the ecosystem. The sediment and organic matter inputs create the physical habitat structure, refugia, and nutrient storage and supply. The thermal and light characteristics regulate metabolism, activity level, and ecosystem productivity. The chemical and nutrient characteristics regulate pH, productivity, and water quality. The biotic community influences ecosystem process rates and structure.

In naturally functioning systems, these drivers vary in significance depending on the seasons and their associated climates and day lengths. Ecosystems and species have evolved to accommodate annual cycles of these drivers, and they have also developed strategies for surviving -indeed requiring - periodic hydrologic extremes caused by floods and droughts that may not occur every year. All drivers must be considered jointly when evaluating freshwater ecosystem integrity; managing one at a time will not produce a functional aquatic ecosystem.

Plant and animal communities associated with rivers and their floodplains are dependent upon change: changing flows, moving sediments, and shifting channels. They depend on inputs of organic matter from vegetation in the riparian zone. They depend on the exchange of nutrients, minerals, organic matter, and organisms between the river and its floodplain. This is provided by variable flows and sediment transport. All of these factors influence the structural character of the stream or river and its aquatic and terrestrial ecosystems -their species distribution, diversity, and abundance.

Humans also influence the structural character of a stream or river and its associated ecosystem. Assessments of the effect of human activities on river systems require indicators relating cause to effect. A cause-effect chain is illustrated in Figure 1. Insight into connections between processes and structures and their temporal and spatial scales leads to a more integrated interdisciplinary approach to river system monitoring and management.

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