Factors Influencing Stream Health

A watershed is a large area of land from which water contributes to only one stream or river. A watershed is bounded by the ridges or hilltops that divide it from other watersheds. When rain falls onto hillsides or when snow melts, the water runs downhill and gathers in streams. Each small tributary stream eventually joins the main river channel at a confluence. The amount of rainwater that falls and the geology of the watershed control the size and flow rate of the river, and the drainage pattern of the watershed.  

Many watersheds have been extensively changed by people, so they no longer function as they originally did, and the plants and animals there have faced new challenges to their survival. What kinds of changes and challenges do they face? Sometimes people channelize a stream by smoothing it out and lining it with concrete. This makes the entire stream bed the same, and eliminates many of the little holes and refuges of the animals there, and also the special places where food can be found. Channelization also makes the stream flow much faster, and since there is no texture to the bottom, a lot more dirt and trash is carried through.  People may also use the stream to carry away their wastewater. This may add bacteria to the water, and also increase the amount of nutrients in the form of Nitrogen and Phosphorous, two chemicals that all living things need. In small amounts, nutrients are good. If there are too many of them, then only one kind of plant or animal that is the fastest at using these nutrients may grow. Then, that plant or animal overgrows everything else, and when it dies, its decomposition uses up vital oxygen. Other kinds of pollution also effect the plants and animals in the watershed. Many of them are poisonous. Introduction of invasive species into the stream system also affects native animals and plants. Invasive species may eat natives, or they may compete with them, and use up all the food and space so the natives starve to death or have no refuge from predators.

Dissolved Oxygen

All living things need oxygen to live, even those animals and plants that live in water require oxygen. Several factors affect the amount of oxygen dissolved in the water in streams. Colder water usually has more oxygen, because gasses dissolve better in cold temperatures. Lots of water motion and splashing also helps mix oxygen into the water. Fertilizers and other nutrients used to promote plant growth on farms and in gardens may find their way into water. At first, these nutrients encourage the growth of plants and algae in water. However, as they grow, they use a lot of oxygen, and when they die and settle underwater, microorganisms decompose them. In the process of decomposition, these microorganisms consume oxygen that is dissolved in the water. Oxygen levels in the water may drop to such dangerously low levels that oxygen-dependent animals in the water, such as fish, die. This process of depleting oxygen to deadly levels is called eutrophication.


All living things require nitrogen, and this element is recycled through the environment in the nitrogen cycle. Humans disrupt the nitrogen cycle which can result in not enough nitrogen, when harvesting of crops, and cutting of forests all have caused a steady decline of nitrogen in the soil. (Some of the losses on agricultural lands are replaced only by applying energy-expensive nitrogenous fertilizers. Human disruption can also result in too much nitrogen. The escape of nitrogen from overfertilized croplands, cut forestland, and animal wastes and sewage has added too much nitrogen to aquatic ecosystems, resulting in reduced water quality and the too much algal growth. We now know that this can lead to not enough oxygen.


Phosphorous is another important nutrient that all living things need. But like nitrogen, too much is a bad thing. Wastewater from laundry detergents containing phosphates is known to be a water pollutant because like nitrogen, phosphates are a primary nutrient of algae; when it grows in excess, algae can choke a lake or river and draw off needed oxygen from aquatic life. Beginning in the 1970s, some regions of the United States and Canada banned or put strict limits on the amount of phosphates that detergents could contain. Along with nitrate, phosphate is also a component of many fertilizers, and when added in quantity to the water leads to over nutrification, increased algal growth, depleted oxygen, and death of other organisms.


This is a very serious water pollution problem.  A 1994 study by the Centers for Disease Control and Prevention (CDC) estimated that about 900,000 people get sick annually in the United States because of organisms in their drinking water, and around 900 people die. Many disease-causing organisms that are present in small numbers in most natural waters are considered pollutants when found in drinking water. Our test is for coliform bacteria, which when found in the lining of our gut is considered naturally harmless, even helpful to our digestive processes. However, if consumed, it can lead to serious intestinal illness, and even death.  The presence of coliform bacteria indicates contamination of the stream with human waste. Here in Honolulu, that is probably unlikely because we have an urban sewage treatment system.


pH is a measure of the acidity of a solution. On the pH scale of 0 to 14, 0 is highly acidic, and 14 is highly alkaline. A pH of 7 is considered neutral, and most organisms can only tolerate a pH close to 7. If the pH is too high, or too low, it can be extremely harmful. Imagine the effect of dipping your hand into acid. What can cause increased or decreased pH in streams? Sulfur dioxide released into the air by power plants mixes with moisture in the atmosphere to produce sulfuric acid, better known as acid rain. Acid rain entering the watershed can decrease the pH of a stream, making it more acidic. In general, an increase in pH is usually attributable to lime, a calcium compound, which is used in the preparation of cement and mortar and as a neutralizer of acid soils in agriculture. It is also used in the manufacture of paper, glass, and whitewash, in leather tanning and sugar refining. Run-off wastewater from these industries can lead to increased pH, of a stream, making it more alkaline. Areas with a lot of limestone may also have more alkaline waters naturally. 

Turbidity is a measure of the amount of suspended particles in the water. It is the amount of dirt “hanging” in the water. It is important because the amount of dirt suspended in the water affects the depth that sunlight can reach. If the water is very dirty (has a high turbidity) the sunlight will not be able to penetrate very far. All plants need sunlight to grow, and in dirty water, there may not be enough light to support plant growth to feed grazing stream animals like o’opu. The washing away of soil caused by the removal of soil-trapping trees near waterways, or carried by rainwater and floodwater from croplands, strip mines, and roads, can also damage a stream by introducing too much nutrient matter. This leads to eutrophication. A sudden increase in sediment can also choke plants, and cover streambed gravel in which many fish and other stream animals lay their eggs, killing those eggs. You may notice as you work to collect your samples that by disturbing the soil on the banks, or in the water, you increase turbidity. If you personally have this much effect, imagine the impact that a large-scale construction activity might have on the turbidity of a stream!


Thermal Pollution, a harmful increase in water temperature, is caused by either dumping hot water from factories and power plants or removing trees and vegetation that shade streams, permitting sunlight to raise the temperature of these waters. A temperature increase as small as 1 or 2 Celsius degrees (about 2 to 4 Fahrenheit degrees) can kill native fish, shellfish, and plants. The second type of thermal pollution is much more widespread. Streams and small lakes are naturally kept cool by trees and other tall plants that block sunlight. People often remove this shading vegetation in order to harvest the wood in the trees, to make room for crops, or to construct buildings, roads, and other structures. Left unshaded, the water warms by as much as 10 Celsius degrees (18 Fahrenheit degrees). In a similar manner, grazing sheep and cattle can strip streamsides of low vegetation, including young trees. Even the removal of vegetation far away from a stream or lake can contribute to thermal pollution by speeding up the erosion of soil into the water, making it muddy. Muddy water absorbs more energy from the sun than clear water does, resulting in further heating. Finally, water running off of artificial surfaces, such as streets, parking lots, and roofs, is warmer than water running off vegetated land and, thus, contributes to thermal pollution. All plant and animal species that live in water are adapted to temperatures within a certain range. When water in an area warms more than they can tolerate, species that cannot move, such as rooted plants and shellfish, will die. Species that can move, such as fish, will leave the area in search of cooler conditions, and they will die if they cannot find them. Typically, other species, often less desirable, will move into the area to fill the vacancy.  Algae and other plants grow more rapidly in warm water than in cold, but they also die more rapidly; the bacteria that decompose their dead tissue use up oxygen (remember that eutrophication?), further reducing the amount available for animals. The dead and decaying algae make the water look, taste, and smell unpleasant.


Salinity is a measurement in grams of salt per kilogram of solution (g/kg), which can also be expressed as parts per thousand (ppt or ‰). Seawater contains a mixture of salts, the most abundant being sodium chloride (NaCl), or table salt. The oceans contain an average of 35 grams of salts per kilogram of seawater (35 ppt). Salinity is an important measurement in biology because salt is dissolved in the bodily fluids of all living things. The level of dissolved salts in a fluid controls many of the processes of life. Most animals are adapted to a narrow range of salinity and cannot live in water that is outside of that range. Think about what happens if you are thirsty and you take a big drink of saltwater! It is the same for animals that live in freshwater and are suddenly exposed to lots of salt. It can kill them. Of course, some very special animals are able to tolerate a wide range of salinities. O’opu can, but so can some of the alien species here, like Tilapia and mosquito fish.

Flow rate

The flow rate of a stream is the result of the volume of water flowing down the stream and the cross sectional area (width x average depth) of the stream bed. Think of it this way: You are holding a garden hose, and turn on the water faucet. Water flows out of the hose fairly slowly. If you place your thumb over the opening of the hose, the water comes out faster and harder. What did you change? The amount of water coming out of the hose did not change. But, the size of the hole through which the water could flow got smaller. The same thing happens in a stream. If you have a wide stream the water flows “slowly”. If you do nothing but narrow the stream, the water must flow by faster in order for the same volume of water to get by. Flow rate is important because many animals are specialized to an environment with a certain kind of flow rate. If it changes, they may not be successful. The amount of water motion is related to the dissolved oxygen, and also to the temperature. A slow moving stream will heat up more quickly if it is unshaded.

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