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Chapter 7: Human Alterations to Riparian Areas
Construction of Dams
- In Arizona there are 431 registered dams with water storage capacity ranging from: 14 to 28,000,000 acre-feet.
Figure 7.4. Hoover dam (top left), Imperial dam (top right), and Lake Roosevelt (below).
- Benefits from dams include: production of hydroelectricity, storage of water for municipal, industrial and irrigation use, protection from flooding, and development of recreational areas
- Impacts upstream from dams.
- The river ecosystem shifts to a lake ecosystem.
- This leads to changes in plant, animal and fish communities.
- The larger water surface of the lakes leads to higher evaporation.
- Impacts downstream from dams.
- The overall stream discharge might decrease or remain the same. The main changes are the alterations of the natural flow regimes. Specifically:
- base flow discharge typically increases, while
- peak discharges (floods) almost always decrease,
- the magnitude of change depends largely on the principle purpose of the dam.
- Temperature and sediment transport are decreased. Sediment is typically trapped behind the dam.
- Decreased sediment will generally lead to increased incision, and stream bed and bank erosion (Figure 7.5). The extent of incision and erosion is dependent:
- on the degree of stream confinement (vertically and laterally) and
- the nature of the stream itself, if it is an alluvial system or not.
- These changes in the hydrology of the stream affect riparian areas and lead to decreases of the native plant, wildlife and fish biodiversity.
Figure 7.5. Decreasing the sediment transported (Qs) downstream of a dam changes the streams dynamic equilibrium based on Lane's equation (Qs D50 ~ Q S) (page 7, chapter 4). To get back to equilibrium conditions the stream bed and banks will in many cases erode (degrade) downstream from the dam.
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