Pollution can be defined as "any impairment of the suitability of water for any of its beneficial uses, actual or potential, by man caused changes in the quality of the water" (Warren, 1971).
Usually we think of pollution as the addition of something, like sewage, to the water. This fulfills the definition above. But it is also possible to damage a river without adding anything to it. We can damage the river by changing it, but without adding anything to it. This also fits the definition above. What are some of the ways this could happen?
In addition to the material below we also have a series of interactive models, some of which can help you better understand certain aspects of pollution and land use.
This term usually refers to the practice of straightening and smoothing a section of the river bed. Bulldozers and other heavy equipment are used to straighten the river into a uniform channel, in the hope that the water will flow through the channel more quickly. Usually these projects are conceived in a "mitigation panic" after an local episode of flooding.
Unfortunately, these stream channelization projects are now known to have many negative side effects. While the channelization may remove the water more quickly from a channelized area, it simply dumps that same water at the end of the channel creating a flooding problem for the downstream residents.
In addition to the increased threat of flooding downstream, there is also an increase in erosion downstream. In a natural stream bed the energy of the falling water is dissipated slowly by turbulence and friction with the streambed. In a channelized stream the water emerges with much of this energy still present in the form of kinetic energy. When this water hits the downstream section it expends that energy as an increase in erosion.
The channelization is also a disaster for the living things in the stream. Not only does the actual channelization process kill everything in the stream, the stream habitat that remains after the project is unsuitable for most forms of aquatic life. There are few eddies where fish can rest and hide; there is little suitable habitat for stream insects.
Channelization changes a beautiful and complex natural stream into a virtual culvert.
There are a number of types of projects of more limited scope than channelization that often involve only the river bank and not the streambed. These include:
Because of their more limited scope these projects usually do not cause changes of the same magnitude as does stream channelization. Nonetheless, they can cause changes. For example, erosion is more likely opposite and downstream from the structure, and they can interfere with the maintenance of the flood plain. Each of these projects should be carefully evaluated for its positive and negative consequences before being embarked upon.
As the name implies, this refers to the practice of removing sands and/or gravel from the stream bed for use elsewhere as a construction material. As you would guess, it has a severe impact on the stream.
Every river has a certain capacity to transport gravel. The amount of water flowing and the slope of the channel are proportional to the size and amount of sediment a river carries. Therefore, there is a balance between the production of sediment (the river's erosional processes) and the ability of the river to keep it moving. When gravel is taken out in large quantities, a river adjusts upstream by eroding more gravels from its streambanks or streambed to replace what was removed. Gravel mining, therefore, sometimes causes the undermining of bridge supports, roads and culverts. This undermining can cause even more bank erosion.
The immediate effect is the complete destruction of the habitat and stream life in the area where gravel mining occurs. In addition it produces large amounts of sediments which are moved downstream by the water for a ways before they fall out of the water. This sediment interferes with the trout and other creatures that live in the area. Also, the deposition may be severe enough to cause the stream's bed to reroute itself.
These were covered in the AASF material Human Impacts on a Watershed. Remember that dams can cause sedimentation, warming of the water, and irregular changes in water level above the dam. Below the dam they can cause erosion and irregular changes of water level, temperature and dissolved oxygen.
In many cases the natural vegetation along the river has been removed. This may be to increase tillable land in fields next to the river, or it may be done simply to form a lawn or enhance a view of the river.
These practices have a negative effect because they can increase the speed of runoff and the amount of sediments entering the river. When this practice includes allowing livestock access to the riverbank, the amount of erosion can increase significantly.
Even when only the overhanging trees are cut, the effects can be serious. Allowing the sun to shine directly on the river increases the temperature of the water, in many cases enough so to make it unsuitable for sports fish. In addition with the increased amount of light the fish become more vulnerable to predators.
The AASF material already covers many of the important sources of pollution caused when things are added to rivers and streams. There are a few additional ones you might discuss with other students:
All of the sand and salt we put on the roads during the winter has to go somewhere. The salt dissolves and the stream usually carries it away quickly to a lake or it enters the groundwater. Salt can kill roadside and riparian vegetation and when it enters the groundwater can make well water unfit or even dangerous to drink. There are only a few instances in our watersheds where road salt is known to have had severe effects, but it is a problem that needs to be monitored.
Not all of the sand reaches the rivers, but some does wash along the drainage ditches and makes its way into the stream. In towns the sand gets flushed into storm drains that often dump into streams. How much sand reaches the river and whether it is a significant amount should be determined on an individual basis. When the sand does reach the river, it has the same effects as sediment from any other source.
Although the distinction between point source pollution and non-point source pollution was covered earlier, it needs to be emphasized that the amount each contributes to a river depends on the individual situation and that each river is unique.
Some generators of non-point source pollution might be:
In both of these cases sediments are likely to be a major concern, but nutrient minerals such as phosphorous and other types of pollutants may also be important and are often adsorbed onto sediments.
Each year species which are native to other parts of the world are accidentally (or sometimes purposefully) introduced into our area. Some of these species die, but sometimes the new habitat allows the species to thrive and perhaps to become a pest.
Some examples of these exotic nuisances you may be familiar with that affect our aquatic habitats are Eurasian Water Milfoil, Water Chestnut, Purple Loosestrife, and Zebra Mussels. Each exotic species has its own history of where it came from and just why it becomes so numerous as to be a pest. Scientists have found that the factor that allows many of these organisms to produce the large populations that qualify them as pests is that their predators, which in their homelands kept their populations small, did not migrate along with the pests. Free from their predators, the pest populations explode.
In order to protect our habitats and their native species we need to educate people to prevent the introduction of pests and to control their spread. We also need to continue our search for ways to control species that have already become a nuisance.