Human activities can introduce thermal pollution into streams in several ways. Industries and power plants may use water to cool machinery and then discharge the warmed water into a stream. In the summer, storm water warmed by urban surfaces, such as roads, roofs, and parking lots, can flow into nearby streams. Water temperature rises when trees and tall vegetation providing shade are cut down. Soil erosion caused by construction, removal of streamside vegetation, poor farming practices, overgrazing, and recreation increases the amount of suspended solids in the water. The suspended particles absorb the sun's rays and also increase water temperature.

Chemical processes involved in the metabolism, growth, reproduction and behavior of aquatic organisms are sensitive to water temperature. Thermal stress and even shock can occur when the temperature changes more than 1 or 2C in less than 24 hours. In addition, the sensitivity of an aquatic organism to toxic wastes, parasites, and disease often increases with rising temperatures.

Water temperature affects the amount of dissolved oxygen and other gases that water can hold at specific atmospheric pressure. A rise in temperature decreases the ability of water to hold oxygen molecules.

Dissolved Oxygen: There are two main sources of dissolved oxygen in stream water: the atmosphere and photosynthesis. Waves and tumbling water mix air into the water where oxygen readily dissolves until saturation occurs. Oxygen is also introduced by aquatic plants and algae as a byproduct of photosynthesis.

The amount of dissolved oxygen is limited by physical conditions, such as water temperature and atmospheric pressure.

Lower temperature ---> higher potential dissolved oxygen level

Higher temperature ---> lower potential dissolved oxygen level

Activity of living organisms increases in warmer water, requiring more oxygen to support their metabolism and magnifying the temperature effect on dissolved oxygen.

Oxygen is essential for fish, invertebrate, plant, and aerobic bacteria respiration. Dissolved oxygen levels below 3 parts per million (ppm) are stressful to most aquatic organisms. Levels below 2 or 1 ppm will not support fish. Fish growth and activity usually require 5-6 ppm of dissolved oxygen.

Low dissolved oxygen indicates a demand on the oxygen of the system. Build up of organic material from human activities is one source of oxygen depletion. Microorganisms in the stream consume oxygen as they decompose inadequately treated sewage, urban and agricultural runoff, and discharge from food-processing plants, meat-packaging plants, and dairies that has entered the stream.

Natural organic materials, such as leaves, also accumulate in the stream and create an oxygen demand as they decompose.

Some pollutants, such as acid mine drainage, produce direct chemical demands on oxygen in the water. Dissolved oxygen is consumed in the oxidation-reduction reactions of introduced chemical compounds, such as nitrate (NO₃^1-) and ammonia (NH₄¹⁺), sulfate (SO₄^2-) and sulfite (SO₃^2-), and ferrous (Fe²⁺) and ferric (Fe³⁺) ions.

One measure of dissolved oxygen in water is parts per million (ppm), which is the number of oxygen molecules (O²) per million total molecules in a sample. Calculating the percent saturation is another way to analyze dissolved oxygen levels. Percent saturation is the measured dissolved oxygen level divided by the greatest amount of oxygen that the water can hold under various temperature and atmospheric pressure conditions multiplied by 100.

pH: The pH of a liquid is the negative logarithm of the concentration of hydrogen ions in the solution.

pH = - log [H⁺]

Because the pH scale is logarithmic, every single unit change in pH actually represents a ten fold change in acidity. For instance, at pH 7 there are 1 x 10^-7 hydrogen ions, and at pH 6 there are 1 x 10^-6 hydrogen ions present.

The pH of natural water depends on several factors: the carbonate system, types of rock, types of soil, and nature of discharged pollutants. The concentration of carbonates (CO₄^2-, HCO₃^1-) and carbon dioxide (CO₂(aq)) is the main influence on the pH of clean water. High concentrations produce alkaline waters (high pH), while low concentrations usually produce acidic waters (low pH).

Acidic and alkaline compounds can be weathered into the stream from the different types of rock present. When limestone (CaCO₃) is present, carbonates can be released, affecting the alkalinity of the water. The types of soil in the drainage area also affect the pH. Drainage water from forests and marshes is often slightly acidic due to the presence of acids produced by decaying vegetation.

Nitrogen oxides (NO, NO₂) and sulfur dioxide (SO₂) from automobile and power plant emissions are converted into nitric acid (HNO₃) and sulfuric acid (H₂SO₄) in the atmosphere. The acids can affect the pH of streams by combining with moisture in the air and falling to the earth as acid rain or snow.

Surface waters can sometimes act as weak buffer solutions depending on the concentration of carbonates and hydrogen carbonates. Buffer solutions are usually a mixture of a weak acid and a strong base. The pH of a buffer solution changes only slightly when small amounts of acid or base are added.

The pH values of natural surface waters usually range from 5.5 to 8.5. Extremely high (9.6) and low (4.5) values are unsuitable for most aquatic organisms. Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5.

Changes in pH can also affect aquatic life indirectly by altering other aspects of water chemistry. Low pH levels accelerate the release of heavy metals from sediments on the stream bottom. The heavy metals can accumulate on the gills of fish, reducing their chance of survival.

Turbidity: This is the measure of the relative cloudiness of water. Turbidity is caused by suspended solid matter scattering light as it passes through water. Suspended solids include clay, silt, plankton, industrial waste, and sewage. Soil erosion introduces soil and mineral particles to surface water. Stream bed sediments can be stirred up by organisms feeding off the bottom. Particles remain suspended by water currents for some time. Urban runoff introduces a wide variety of particles to stream water. Algal growth from added nutrients and sunlight can also increase turbidity.

As the amount of suspended solids increases, photosynthesis decreases, fish gills become clogged, and eggs are smothered. Material settling into spaces between rocks makes these microhabitats unsuitable for the macroinvertebrates living there. Surface water temperature also rises as suspended particles near the surface absorb heat from the sunlight, which in turn affects dissolved oxygen levels.

Another concern of suspended sediments is that attached nutrients, metals, and pesticides can be carried throughout the water system.

Hardness: Water hardness is a historical term expressing the total concentration of cations, specifically calcium (Ca²⁺), magnesium (Mg²⁺), iron (Fe²⁺) and manganese (Mn²⁺) in water. Hardness, however, refers primarily to the amount of calcium and magnesium ions present. Calcium and magnesium enter the stream mainly through the weathering of rocks.

A stream's hardness reflects the geology of the catchment area and provides a measure of the influence of human activity in a watershed. For instance, acid mine drainage often results in the addition of iron (Fe²⁺) into a stream.

Calcium is an important component of plant cell walls and the shells and bones of many aquatic organisms, while magnesium is an essential nutrient for plants and a component of the chlorophyll cycle. Waters with calcium levels of 10 ppm or less are usually oligotrophic, supporting only sparse plant and animal life. Eutrophic waters typically have calcium levels above 25 ppm.

When the total hardness of water exceeds the total alkalinity, the excess is called "noncarbonate hardness" and indicates the presence of chloride and sulfate ions.

Alkalinity: The buffering capacity of water is measured as the "alkalinity." Alkalinity does not refer to pH, but instead refers to the ability of the water to resist change in pH. The presence of buffering materials, principally the bases HCO₃^1-, CO₃^2-, and OH^1-, help neutralize acids as they are added to or created within the water column. A total alkalinity level of 100-200 ppm will stabilize the pH level in a stream. Levels of 20-200 ppm are typical of fresh water. Levels below 10 ppm indicate that the system is poorly buffered. Poorly buffered waters are susceptible to changes in pH from natural and anthropogenic (human-caused) sources.

As increasing amounts of acid are produced, the buffering capacity is consumed. Natural buffering materials in water slow the decline of pH to around 6. A rapid pH drop follows this gradual decline as the bicarbonate buffering capacity is used up. At pH 5.5, only very weak buffering ability remains and the pH drops further with additional acid.

Conductivity: The ability of an aqueous solution to carry an electric current is called conductivity. The current is conducted in the solution by the movement of ions. Conductivity increases with increasing amounts and mobility of ions. In natural water, the dissociation of inorganic compounds is the main source of ions in the solution. Therefore, measuring conductivity reveals the concentration of dissolved salts in water. Conductivity is also affected by heavy metal ions released into water by acid mine drainage.

Total Solids: This is the sum of dissolved and suspended solids.

The quantity of dissolved material is mainly determined by the solubility of rocks and soils that the water contacts. Water that flows through limestone and gypsum dissolves calcium, carbonates, and sulfates, resulting in high total dissolved solid levels. The amount of material dissolved in a water sample affects its ability to conduct electricity. Total dissolved solids can be estimated by measuring conductivity, because as total solids increase, conductivity also increases.

Runoff from urban areas can carry salt from streets, fertilizers from lawns, along with other types of materials to contribute dissolved solids. Wastewater treatment plants can add phosphorus, nitrogen, and organic matter. Leaves and other plant materials dumped into streams are another source of dissolved solids. Soil particles are introduced by soil erosion and runoff. Decayed plant and animal matter is naturally converted to particulate matter within the water.

High concentrations of total solids can lower water quality and cause water balance problems for individual organisms. Low concentrations may limit the growth of different aquatic life. High concentrations of dissolved solids can lead to laxative effects and unpleasant mineral taste in drinking water.

Fecal Coliform: These are bacteria that are naturally abundant in the lower intestines of humans and other warm-blooded animals. They are not pathogenic, but their presence serves as a reliable indication of sewage or fecal contamination in water. Fecal coliform can enter water through various sources, including mammal and bird discharge, agricultural and storm runoff, and human sewage discharge.

Biological oxygen demand: (BOD) is the measure of the amount of oxygen consumed by microorganisms in aerobic oxidation of organic material. Unpolluted natural waters will have a BOD of 5 mg/L or less.

The organic matter available for decomposition has both natural and human origins. Nutrients are the main culprit for high BOD in river water. Calm stretches of water ways also collect organic wastes that settle out from upstream. Swamps, bogs, and vegetation along the water provide organic matter for decomposition.

Human activities can result in point source and nonpoint source contribution. Discharge from wastewater treatment plants, pulp and paper mills, meat-packing plants, and food-processing plants are examples of point sources of organic matter. Urban runoff is a nonpoint source provider. Rain and melting snow carry sewage form improper sewer connections into storm drains. Pet wastes wash off sidewalks. Nutrients from lawn fertilizers, leaves, grass clippings, and paper from residential areas also find their way to the water. Nitrates, phosphates, and other nutrients are carried from fields by agricultural runoff. Chemical oxidation of sulfides, ferrous ions, and ammonia also consume oxygen in water.

Nitrates: Nitrogen is an element needed by all plants and animals to build protein. It most commonly exists in its molecular form (N²) where it is unusable for most aquatic plant growth. Blue-green algae converts N² to ammonia (NH₄⁺¹) and nitrate (NO₃^-1) that can be taken in and utilized by aquatic plants. Ammonia is also released as bacteria break down aquatic plant and animal remains. Specialized bacteria can then oxidize the ammonia to form nitrites (NO) and nitrates (NO₃^-1).

Excretions of aquatic organisms are very rich in ammonia. In large groups, duck and geese can contribute heavy loads.

Nitrogen in the forms of ammonia and nitrates functions as a plant nutrient and initiates eutrophication.

Sewage is the main source of human-influenced nitrate addition to streams. Nitrates are introduced by inadequately treated wastewater from sewage treatment plants, effluent from illegal sanitary sewer connections, and poorly functioning septic systems. Fertilizers from fields and runoff from cattle feedlots, dairies, and barnyards are other nitrate sources.

Total Phosphate: Phosphorus present in natural waters is usually found in the form of phosphates (PO₄^-3). Phosphates accumulate from living plants and animals, their byproducts, and their remains. Phosphate ions bonded to soil particles and in laundry detergents also end up in streams. Other sources of phosphorus include sewage, animal waste, soil erosion, fertilizers, and drained swamps and marshlands.

Phosphate acts as a "growth-limiting" factor of aquatic plants and algae. Excess phosphate creates blooms of extensive algal growth. Forest fires and fallout from volcanic eruptions are responsible for natural eutrophication in streams, while humans are the instigators of cultural eutrophication.

Phosphorus initially stimulates aquatic plant growth, which unlocks even more phosphorus from bottom sediments. The first key symptom of eutrophication is the pea-soup green color of the water caused by an algal bloom. Algal blooms then become more frequent and further deplete the water of dissolved oxygen as the algae decays.

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