Many factors listed in the last section (e.g., flow and substrata size, light and primary production, and discharge and turbidity) have a direct relationship to one another. Some of these interactions are obvious; however, many others are more complex. In addition to the unpredictability of the direct results from manipulating a given ecosystem component, interactions can vary temporally and spatially as conditions in the ecosystem change. For example, primary production in a stream may be limited by nutrients in winter when riparian shading is low, but may shift to light limitation during spring and summer. Additionally, changes in a particular component often have both positive and negative effects on other ecosystem components. The establishment of riparian vegetation reduces light and nutrients entering a stream, which can reduce primary production, but increase leaf and woody debris inputs which may increase hetero-trophic microbes and macroinvertebrate shredder and collector production.
Ecological stream management is based on component interactions, and its success depends on the accuracy of predicting these interactions. A good example of the complex nature of stream management is currently taking place in the southwestern United States. The Colorado River is one of the most managed river systems in the world. Construction of the first dam (Hoover Dam) was completed in 1936, and now more than 20 dams have been constructed on the river and its tributaries. Dams were originally built to supply hydroelectric power, and irrigation and drinking water for the surrounding communities. Hydrological modifications and water withdrawals from the dams have severely altered the native river ecosystem, changing discharge intensity and frequency, water temperature, sediment movement, and native fish community structure (Figure 2). It has also unintentionally produced a productive coldwater trout fishery and endangered native fishes that depend upon warm waters and seasonal flooding. As a result, tremendous amounts of time and money have been spent managing this system to restore native fish
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Figure 2 Example of multiple ecosystem alterations due to a single anthropogenic implementation. River discharge and water temperature for the Colorado River, USA, below the Glen Canyon Dam. Construction of the Glen Canyon Dam in 1964 reduced natural flow variability and also lowered average water temperatures due to a hypolemnetic release. Data courtesy of the United States Geological Survey.
populations, maintain a non-native trout fishery, sustain current and future power generation and water needs, attempt to reproduce natural flooding, provide recreational areas, and satisfy local Native American requirements.
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