Factors That Affect Downstream Changes in Water Quality
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Climate Change
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Long-term changes in weather patterns affect the way different species are distributed across the planet, which leads to a change in the hydrology of an area. Climate change is typified by steadily increasing temperatures, which have an irreversible effect on biomass. One example of climate-induced hydrology change can be seen in the Colorado forests, where climate change has affected downstream water volumes over the course of around 70 years, say researchers at Colorado State University. The watersheds formed during this time have more sand and silt and are of poorer quality.
Dams
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Similar in effect to climate change, but on a localized scale, dam water is warmer than natural cold-water systems, particularly during the hotter summer months. Researchers at Michigan State University found that water temperatures below dams can rise by 5 degrees or more and that this reduces the populations of cold-water fish such as trout and several macro-invertebrates.
Land Use
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Climate change is not the only factor that determines the planet's hydrology. Water quality is affected by human activity on the land, including logging, deforestation, mining, livestock grazing and hunting. Continual changes in the type of activity occurring on a certain tract of land can negatively impact the quality of nearby streams and rivers because it is more difficult to measure and monitor a continually changing problem. The discipline of predictive water management is being developed to cope with the complex problem of land use.
Pollution
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Perhaps the most significant factor affecting downstream water quality is pollution. The source of the pollution varies from point-sources (a specific addition point) to non-point-sources (outbursts of pollution over a wide area). Point-source pollution can include industrial waste dumped into water further upstream, leaking pipes sending contaminated effluent into the wrong part of a river system or untreated, domestic water accidentally being added to waterways. Non-point-source pollution can include the spreading of contaminants during heavy rainfall. One of the reasons why pollution is so damaging to the natural balance of rivers is its high temperature. Effluent is usually warmer than the water it is added to.
Drought
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Rare compared with forest fires, which happen intermittently throughout the life of a forest, drought can affect water flow and downstream water quality. Droughts cause water quality to change indirectly. The sustained lack of rainfall again reduces the viable biomass (the kind that actively converts carbon dioxide into oxygen during photosynthesis), and this in turn changes the water quality further downstream. Domestic water and water used for agriculture can be affected by the change in water quality. Some politicians have called for deforestation to increase the flow of water, but scientifically this does not make sense in the longterm, as explained by The National Academy of Sciences.
Japanese Knotweed
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A nonnative, invasive species, Japanese knotweed (Fallopia japonica) changes the quality of river water because its shallow roots collect particles of sediment. As more sediment becomes suspended, the turbidity of the surface water increases, which affects the amount of light getting through from the sun. Plants beneath the surface die, and the animals that rely on those plants for food, such as fish, cannot survive. The ecosystem of a freshwater stream can be left imbalanced by one rapid growth of persistent Japanese knotweed -- and it is notoriously hard to remove.
Fire
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Forest fires and human-caused fires represent a dramatic, often sudden reduction in photosynthesizing biomass. As the canopy cover -- the surface layer of the forest -- is reduced by burning, the quality of downstream water is also reduced. Fires can be made more prevalent and frequent by the growth of large populations of invading plants, so the two factors combine to cause an even lower quality of downstream water with limited oxygen content.
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