WRM 2000/Dam and river ecosystem basics

Water quality

Water quality is important to the river's ability to support aquatic life, provide clean drinking water, and enable desired recreational opportunities. Characteristics that determine water quality include temperature, dissolved oxygen, minerals, metals, nutrients, organic chemicals (including pesticides), pathogens, and sediment. The water quality in a river system, free-flowing or impounded, greatly depends on the types of activities that people undertake within the watershed, and the methods by which those activities are carried out. Some of these activities introduce pollution to rivers at a single point (called a point source), such as the end-of-pipe discharge from a wastewater- treatment plant. Nonpoint sources of pollution do not enter the river system at discrete locations; examples include runoff from agricultural operations, residential lawns, construction sites, roads, and parking lots.

High concentrations of sediment in a river cause water-quality problems. Fine sediments (clays and silts) increase the turbidity of the water. Turbidity is a measure of the clarity of the water: the more turbid water is, the less clear it is. Turbidity may be a problem for fish and other aquatic life, such as sight-feeding fish and mussels. If fish spawning beds are covered with sediment, populations may be reduced. Chemical contaminants from human activities can also be bound to sediments, including heavy metals, polychlorinated biphenyls (PCBs), and other organic chemicals.

Impounded systems

Free-flowing rivers are constantly flushing themselves, but impoundments can trap water–quality problems. Any chemical or biological water-quality problems, such as too many nutrients, pesticides, pathogens, and contaminated sediments, build up in the impoundment. As mentioned previously, rivers deposit much of their sediment when they enter an impoundment. Any contamination deposited with the sediments is trapped as well. For example, during the planning process for the removal of the Oak Street Dam in Baraboo, coal tar was found in the sediments in part of the impoundment. The coal tar was stabilized and removed before the dam was removed to prevent contamination of the river downstream. Sediment cores can be taken if there is any contamination suspected.

Nutrients tend to build up within an impounded system more quickly than in free-flowing systems. High nutrient levels in impoundments can lead to eutrophication of the impoundment. This means that the water contains enough nutrients to support nuisance growths of aquatic plants and algae blooms. When these algae blooms die, the decay process uses up oxygen dissolved in the water, reducing the amount of oxygen available to fish and other aquatic life. The turbidity caused by decaying algae and suspended sediment can cause problems for other plants. Plants rely on sunlight reaching them to carry out photosynthesis. Turbidity causes problems for fish, not allowing them to be able to see their prey and predators. Turbidity also promotes typically undesirable species, such as carp, which can tolerate lower dissolved oxygen conditions and exacerbate turbidity problems themselves by stirring up the bottom sediments.

The upper layer of the still water in an impoundment also tends to have higher temperatures than in a free-flowing system. This increase in temperature decreases the amount of dissolved oxygen present in the water. Also, the still water of an impoundment mixes less with the air above than free-flowing rivers, reducing the amount of oxygen added by contact with the air. This increase in temperature and decrease in dissolved oxygen affects the quality of the water in the impoundment and also affects the quality of the river downstream. Similarly, water–quality problems caused by any other dissolved chemical in the water will affect the impoundment and the river downstream of the impoundment.

Free-flowing systems

The water quality in free-flowing river systems depends on the quality of the water entering and flowing through the system. As water–quality problems occur, they tend to be flushed through the system by the flow of the river. The quality of the water in the river reflects the quality of the water from overland runoff and groundwater entering the river. As with impounded river systems, point and nonpoint (runoff) pollution sources impact water quality. Dissolved oxygen is an important water–quality parameter, especially for fish. Free-flowing rivers tend to have high amounts of dissolved oxygen compared to impoundments. The turbulent flow of rivers through riffles increases the amount of dissolved oxygen by mixing air into the water.

Increases in the sediment load of a river can lead to increases in the amounts of nutrients, such as nitrogen and phosphorous because these nutrients are commonly bound to sediments. Higher levels of nutrients may allow greater biological productivity, but eventually these organisms die, and their decay uses up some of the available dissolved oxygen. If all the oxygen is used up, the decay continues through other biological mechanisms that give off unpleasant chemicals, such as methane (rotten egg smell) and hydrogen sulfide (sewer gas smell). Some of these gases can cause organic sediments to rise to the surface and form unsightly scum, which also reduces light penetration. High nutrient levels can lead to poor water quality.

Water-quality restoration

The restoration of water quality in either free-flowing or impounded systems requires assessing the effects of land-use activities in the watershed and acting to eliminate or mitigate any water-quality problems they may cause. For example, excessive amounts of sedimentation to a river or impoundment can be reduced by the implementation of agricultural best management practices, the treatment of storm-sewer runoff, and the maintenance of vegetative buffer zones along riverbanks and impoundment shores. Reducing the amounts of residential and agricultural fertilizers applied to the land surface reduces the loading of nutrients to rivers and impoundments. Sewering rather than the use of septic tanks and improving the technology used in sewage-treatment plants may reduce nutrient and pathogen loadings.

The removal of a dam may lead to improvements in the water quality of the former impounded area and to the river downstream. Removing the dam will flush out the warm, low dissolved oxygen, high nutrient water from the impoundment. The water quality in the restored area should return to that of the river upstream from the impoundment. The initial drawdown of the impoundment (prior to the physical removal of the dam), however, may cause temporary water–quality problems downstream until the flushing is complete.

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URL: http://www.ies.wisc.edu/research/wrm00/educqual.htm