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floodplain soils, which reduces infiltration and increases runoff and erosion.
Following established trails, grazing animals break down banks and enhance sediment yields to channels. As woody riparian vegetation density declines, channel
margins are less shaded, which influences water temperature and photosynthesis
within the channel. Grazing animals add excess nutrients to rivers through their
wastes. Finally, bank trampling and removal of woody vegetation result in the
formation of relatively wide, shallow channels with mobile beds of fine-grained
sediment and a lack of pools, all of which limits habitat abundance and stability for
stream organisms. Numerous studies tie riparian grazing to reduced water quality
and declines in macroinvertebrate and fish populations (e.g., Kauffman and Krueger
1984; Belsky et al. 1999).
3.3 Cumulative Effects
Although the previous section reviews the effects on river ecosystems of individual
forms of land and resource use, an important consideration is that each type of
human alteration rarely occurs in isolation. In most altered river basins, multiple
human activities have occurred simultaneously or in succession through time. These
activities tend to be particularly well documented in regions that experienced intensive resource use within the past two to three centuries, such as North America,
Australia, and New Zealand, but the combined effects of human-induced changes
now characterize most rivers around the world (Nilsson et al. 2005a).
The predominant cumulative effect of indirect and direct alteration of river ecosystems is to simplify and homogenize water and sediment inputs, physical form,
and biota (Moyle and Mount 2007; Peipoch et al. 2015). As noted in the first chapter, the secondary effects of this simplification and homogenization show up in loss
of freshwater species, declines in water quality and quantity, flood hazards, exacerbated channel erosion, eutrophication, and coastal erosion. Four major drainage
basins can be used to illustrate these cumulative effects: the Mississippi River of
North America; the Murray-Darling River of Australia; the Danube River of Europe;
and the Huanghe (Yellow River) of Asia.
3.3.1 The Mississippi River Drainage Basin of North America
The Mississippi River drains 3.5 million square kilometers of North America, predominantly within the United States (Fig. 3.13). The majority of the river’s water
flow comes from the central and eastern half of the drainage via the Ohio, Illinois,
and Upper Mississippi Rivers. The majority of the sediment discharges comes from
the Missouri River in the western half of the drainage (Meade and Moody 2010).
The larger rivers within the drainage basin have been used for navigation for centuries. Modification of the rivers to enhance navigation started with commercial
3 Human Alterations of Rivers
floodplain soils, which reduces infiltration and increases runoff and erosion.
Following established trails, grazing animals break down banks and enhance sediment yields to channels. As woody riparian vegetation density declines, channel
margins are less shaded, which influences water temperature and photosynthesis
within the channel. Grazing animals add excess nutrients to rivers through their
wastes. Finally, bank trampling and removal of woody vegetation result in the
formation of relatively wide, shallow channels with mobile beds of fine-grained
sediment and a lack of pools, all of which limits habitat abundance and stability for
stream organisms. Numerous studies tie riparian grazing to reduced water quality
and declines in macroinvertebrate and fish populations (e.g., Kauffman and Krueger
1984; Belsky et al. 1999).
3.3 Cumulative Effects
Although the previous section reviews the effects on river ecosystems of individual
forms of land and resource use, an important consideration is that each type of
human alteration rarely occurs in isolation. In most altered river basins, multiple
human activities have occurred simultaneously or in succession through time. These
activities tend to be particularly well documented in regions that experienced intensive resource use within the past two to three centuries, such as North America,
Australia, and New Zealand, but the combined effects of human-induced changes
now characterize most rivers around the world (Nilsson et al. 2005a).
The predominant cumulative effect of indirect and direct alteration of river ecosystems is to simplify and homogenize water and sediment inputs, physical form,
and biota (Moyle and Mount 2007; Peipoch et al. 2015). As noted in the first chapter, the secondary effects of this simplification and homogenization show up in loss
of freshwater species, declines in water quality and quantity, flood hazards, exacerbated channel erosion, eutrophication, and coastal erosion. Four major drainage
basins can be used to illustrate these cumulative effects: the Mississippi River of
North America; the Murray-Darling River of Australia; the Danube River of Europe;
and the Huanghe (Yellow River) of Asia.
3.3.1 The Mississippi River Drainage Basin of North America
The Mississippi River drains 3.5 million square kilometers of North America, predominantly within the United States (Fig. 3.13). The majority of the river’s water
flow comes from the central and eastern half of the drainage via the Ohio, Illinois,
and Upper Mississippi Rivers. The majority of the sediment discharges comes from
the Missouri River in the western half of the drainage (Meade and Moody 2010).
The larger rivers within the drainage basin have been used for navigation for centuries. Modification of the rivers to enhance navigation started with commercial
3 Human Alterations of Rivers