Rivers and Their Watersheds

Flows in streams and rivers come from and are influenced by watersheds and their land cover. Riverine networks and their connecting groundwaters are the sinks into which landscapes drain. The amount and quality of water in streams and rivers are substantially influenced by terrestrial processes, and these in turn are influenced by human uses or modifications of land and water. Freshwater ecosystems are impacted by the regime of water quantities and qualities over time. Flow regime, sediment and organic matter inputs, thermal and light characteristics, chemical and nutrient characteristics, and biotic assemblages are fundamental defining attributes of freshwater ecosystems. These attributes impart relatively unique characteristics of productivity and biodiversity to each ecosystem. The natural range of variation in each of these attributes is critical to maintaining the integrity and dynamic potential of aquatic ecosystems. Managers of water and watersheds should allow for these dynamic changes.

As water flows on its way to the sea, it moves through freshwater systems in three spatial dimensions: longitudinal (upstream-downstream), lateral (channel-floodplain), and vertical (surface water-groundwater). These dimensions represent functional linkages among ecosystem compartments over time. Water bodies are ultimately the recipients of materials generated from the landscape; hence they are greatly influenced by terrestrial processes, including human modifications of land and water resources.

Environmental drivers regulate much of the structure and function of any river ecosystem, although their relative importance varies depending on the particular aquatic ecosystem (Figure 1). The interaction of these drivers in space and time defines the dynamic nature of freshwater ecosystems. These drivers include the flow

Human system

* Land-use emissions

* River regulation

• Wastewater discharges

River system

Abiotic system

Hydrology Geomorphology Water quality

Biotic system

Functional characteristics Flux of matter Retention

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