The water temperature of a river is very important for water quality. It influences: the amount of oxygen that can be dissolved in water, the rate of photosynthesis by aquatic plants, metabolic rates of organisms, and the sensitivity of organisms to toxic waste and diseases. Cool water holds more oxygen than warm water, because gases are more readily dissolved in cool water. Humans change the temperature of rivers through thermal pollution.

Thermal pollution may be caused by industries discharging water used to cool machinery. People affect water temperature by cutting down trees used to shade the water. Soil erosion caused by human activities can also raise water temperatures because cloudy water (turbid) absorbs more of the sun’s rays.

Water (H20) contains both hydrogen (H+) and hydroxyl (OH-) ions. The pH test measures the H+ ion concentration of liquids and other substances. Each measured substance is given a pH value on a scale that ranges from 0 – 14. Pure deionized water contains equal numbers of H+ and OH- ions, is neutral and has a pH value of 7. If a water sample has more H+ than OH- ions, it’s considered acidic and has a pH less than 7. If the sample has more OH- ions than H+ ions, it’s considered basic with a pH greater than 7. Every one unit of change on the pH scale there’s a ten-fold change in how acidic or basic the sample is. Most organisms have adapted to life in water of a specific pH and may die if it changes.

Most aquatic plants and animals need oxygen to survive. They are adapted to extract oxygen from the water they live in. Much of the dissolved oxygen in water comes from the atmosphere. Waves on lakes and tumbling water on rivers at to mix atmospheric oxygen with water. Aquatic plants also add oxygen to water through photosynthesis. Water temperature and the volume of water affect oxygen levels. Depletion of dissolved oxygen can negatively affect the aquatic organisms that live in water.

This water quality measure includes: dissolved solids (or solids that pass through a water filter) and suspended solids that are trapped by a filter. They include anything from silt to plankton to waste. Dissolved or inorganic materials include calcium, nitrogen, phosphorus and other ions found in a water body. A constant level of these materials is essential for the maintenance of aquatic life because the density to total solids determines flow of water in and out of an organism’s cells. Soil erosion, plant material and salts in urban runoff, fertilizers, waste from treatment plants, etc. are all sources of total solids. High concentrations of total solids can lower water quality and cause water balance problems for individual organisms.

Phosphorus is usually present in natural waters as phosphate. Organic phosphate is a part of living things, their by-products and remains. Inorganic phosphates include ions bonded to soil particles and phosphates present in laundry detergents. Phosphorus is an essential element for life. It’s a plant nutrient needed for growth and a fundamental element in the metabolic reactions of plants and animals. Phosphorus functions as a “growth- limiting” factor because it is usually present in very low concentrations.

Nitrogen is an element needed by all living plants and animals to build protein. In aquatic ecosystems t’s present in many forms. It’s more abundant than phosphorus in nature. Nitrogen is most commonly found in its molecular form (N2), which makes up 79% of the air we breathe. This form is useless to aquatic life. Algae are able to use N2 and convert it into forms of nitrogen that plants can take up through their roots and use for growth: ammonia (NH3) and nitrate (NO-3). Animals obtain the nitrogen they need to form proteins in two ways: they either eat aquatic plants and convert plant proteins to animal proteins, or they eat other aquatic organisms that feed on plants. As aquatic plants and animals die, bacteria break down large protein molecules into ammonia. Ammonia is then oxidized (combined with oxygen) by specialized bacteria to from nitrites (NO-2) and nitrates (NO-3). Excretions of aquatic organisms (high in ammonia) and decomposition of dead plants releases nitrogen that was previously tied up which causes eutrophication. Sewage is the main source of nitrates added by humans to rivers.

Fecal coliform bacteria are found in the feces of warm-blooded animals. These bacteria naturally occur in the digestive tract and aid in the digestion of food. In infected individuals, pathogenic organisms are found along with fecal coliform bacteria. These bacteria can enter rivers through direct discharge from mammals and birds, from agricultural and stormwater run-off and from sewage treatment plant discharges. Diseases and illnesses such as typhoid fever, dysentery and ear infections can be contracted from water with high fecal coliform counts.

When organic matter decomposes, it is fed upon by aerobic bacteria. In this process, organic matter is broken down and oxidized (combined with oxygen). BOD is a measure of the quantity of oxygen used by these microorganisms in the aerobic oxidation of organic matter. When aquatic plants die, they are fed upon by aerobic bacteria. The input of nutrients into a river, such as nitrates and phosphates, stimulates plant growth. More plant growth leads to more plant decay. Nutrients then can be a prime contributor to high BOD. In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need. As organic pollution increases biodiversity decreases.

Turbidity is a measure of the relative clarity of water; the greater the turbidity, the murkier the water. Turbidity increases as a result of suspended solids in water that reduces the transmission of light. Suspended solids vary from soil to plankton, industrial waste and sewage. High turbidity may be caused soil erosion after a rainstorm, carp stirring up the bottom or algal growth. At higher turbidity levels, water loses its ability to support biodiversity. Water becomes warmer as suspended particles absorb heat from sunlight, causing oxygen levels to fall. Photosynthesis decreases because less light penetrates the water, causing further drops in oxygen levels. Suspended solids can clog fish gills, reduce growth rates, decrease resistance to disease and prevent eggs and larval development. Turbidity can be measured by a Secchi disk or transparency tube.

A biotic index is a scale for showing the quality of an environment by indicating the types of organisms present in it. It is often used to assess the quality of water in rivers. The evaluation of water quality can be linked to the numbers of pollution tolerant aquatic organisms at a site compared with intolerant species. A large number of tolerant organisms and few or no intolerant species would indicate pollution. In general, communities with high diversity (more different types of organisms) tend to be more stable than those with less diversity. Pollution reduces the number of species in an area by eliminating organisms that are sensitive to poor water quality.