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BASIN Introduction to Water Quality

Water Quality

Image of South Boulder Creek near Tolland by Harry H. Lake around 1900.
Copyright Denver Public Library. For more information visit the BASIN Gallery.

Boulder Creek begins in the Rocky Mountains west of Boulder. The creek, as it flows through the mountains and canyons toward Boulder, picks up pollution particulates from the air, travels past old mines, and through some agricultural and residential areas. When it reaches Boulder, the creek flows through the middle of town. Here it is subject to the strains of the city: automobile oil, paint, fertilizers, treated wastewater effluent, food wastes, trash, etc. The creek eventually meanders into eastern Boulder County where it traverses farms and new rural subdivisions. On its journey from the high mountains to its convergence with the St. Vrain River, Boulder Creek is influenced by every type of urban and rural impact.

The health of a stream can be monitored with physical, chemical and biological parameters. The primary physical and chemical parameters include turbidity and color, temperature, flow, dissolved oxygen, pH, and chemical pollutants such as heavy metals, nitrate, and phosphate.

Color and Clarity

We can learn a lot about a stream by its color and clarity. Rainwater, groundwater and snowmelt run clear unless a stream’s banks are eroding. Glacial meltwater, on the other hand, appears milky gray from glacial flour, tiny suspended particles of rock crushed by the grinding force of moving glaciers. Brown water might be a sign of a mudslide in the mountains, a heavy storm event, or local erosion. Greenish water may represent blooms of algae nourished by excess fertilizers from yards, golf courses, or farms. Suds might point to detergents or to septic tank failure.

An iridescent sheen on the water may have been released from rotting leaves; however it could mean that used motor oil or oil-based pesticides leaked or were dumped into the stream, or perhaps it was picked up in runoff from roads, driveways, and parking lots. To test the clarity of a creek, turbidity measures are taken. Turbidity is defined as the light scattering capability of a particle. In natural waters, it may be caused by algae or by organic acids, which stain the water a dark brown. But it usually indicates suspended sediments, fine soil particles, held in suspension by the turbulence of the current which scatter light passing through the water. Excess sediment reduces the clarity of water and it can harm the gills of fish and aquatic insects. It also makes it more difficult for aquatic species to find food or escape predators.


Water temperature is another easy test that tells a lot about a waterway. Temperature determines where aquatic organisms can live. Plants and animals that live in the creek are completely surrounded by water, and so their climate depends on water temperature rather than air temperature. In fact, since fish and aquatic insects are cold-blooded, they become the temperature of the water. Their metabolism, growth rate, and all internal chemical reactions are regulated by the water temperature.

The amount of oxygen found in a waterway is directly connected to the temperature. Cooler water holds more dissolved oxygen than warmer water. Different aquatic species have found their habitat in streams according to food availability, oxygen needs and temperature. Trout like cold, oxygen-rich waters, whereas bass and sunfish prefer to live in warmer water.

Current and Flow

Current and flow are also easy characteristics to monitor. Stream Gauging: A Study of Flow activity (coming in Fall 1999) provides directions for taking flow on your waterway. Current is determined in part by the gradient, or slope, of the stream. Mountainous headwaters have swift currents because they drop quickly over a short reach. Current also varies with the flow, which is the amount of water carried by the stream channel at any given time. Water levels fluctuate throughout the year. During the spring, the creeks in the Front Range rise quickly and have swift currents because of melting snow in the mountains.

By midsummer, when most of the snow has melted and the hot, dry weather sets in, creeks slow down and sometime even dry up. Creeks may rise for a short time during the summer if a heavy thunderstorm releases a lot of rain over the headwaters of a creek. During fall and winter, watershed creeks maintain a fairly slow, constant flow, as rain and snow contribute more consistent water to the creeks. During normal flow, currents are controlled by instream structures, both natural and human-made. Large boulders, fallen trees, and beaver dams are examples of natural current checks. On a larger scale, hydroelectric, irrigation and flood-control dams also check the current. Knowing the flow and current of a stream helps us know how quickly something, such as a pollutant, might travel down a creek. We can also determine the types of animals that live in the creek depending on the site.

Dissolved Oxygen (D.O.)

Some important stream measurements are not as visible as the first ones mentioned. There are certain things that need to be measured with sampling equipment, such as the levels of dissolved oxygen in a stream.

Instream creatures depend on oxygen that is dissolved in the stream water. Still or slow-moving water gets some oxygen from the air above it. In a pond, for example, only the upper layers receive much dissolved oxygen, while the bottom layers are often depleted by the respiration of animals in the mud and by bacteria decomposition.

Rushing water, as it churns over rocks and plunges over hundreds of tiny falls, is aerated by the bubbles of air that get caught by the water. Plants add oxygen to the water during daylight as a byproduct of photosynthesis, but they also deplete it as they respire a night. Water never contains large amounts of oxygen: our atmosphere contains 23% oxygen, whereas even oxygen-saturated water has less than 1%. Small losses or gains in D.O. levels can be critical to instream species.

The amount of D.O. in a stream is dependent on three things: water temperature, the amount of oxygen taken out of the water by respiring and decaying organisms, and the amount put back by physical aeration and photosynthesis. The cooler the water is, the more dissolved oxygen it can hold. Large active fish like trout, which need a lot of oxygen, are restricted to cool streams.


pH is another invisible, yet very important parameter to measure. pH is the amount of hydrogen ions in the water (p stands for negative logarthim of; H for hydrogen ions). The lower the number, the higher the acidity. The pH scale goes from 1-14. pH 7 is neutral (distilled water); pH 2 is acidic (vinegar); pH 13 is basic (ammonia). pH affects the solubility of many nutritive and toxic chemicals in the water and thus their availability to stream creatures. AlkalinityÑthe opposite of acidityÑis a measure or the stream’s capacity to buffer, or neutralize, the effects of acidity.

Chemical Pollutants

A final test to measure a stream’s health is to monitor for chemical and heavy metal pollutants. Some pollutants, not soluble in water, can be easily detected. Petroleum products spread over the surface of water creating a rainbow sheen. Other chemicals may have volatile gasses that can be detected by smell. And some, like detergents, give a slippery feeling to the water or create suds on the surface. Unfortunately, most chemicals dissolve in water and cannot be detected except by expensive laboratory instruments. The only way we see them is by noting their effects on stream lifeÑorganisms sensitive to chemicals may die out while more tolerant ones thrive.

Major chemical spills will cause dramatic plant and animal deaths throughout the impacted stream. However, most waterways are slowly degraded over a period of months or years. Certain chemicals and metals are tested regularly because of their effects on the aquatic ecosystem and their potential danger to humans. Lead is tested in our drinking water because its compounds are poisonous and accumulate in the bone structure. Lead is seldom found in surface waters because it is precipitated by a variety of substances. Nitrate and phosphate are two chemicals tested for because they are nutrients. High levels of nitrates and phosphates can lead to the excessive growth of aquatic plants. High levels in natural water supplies may indicate some nonpoint source pollution problems such as fertilizer runoff from lawn or fields.

One other constituent often tested for is the fecal coliform bacteria. It is important to find out if our water sources are being polluted by septic tank or agricultural runoff. If fecal coliform is detected in a waterway, it means that certain disease causing pathogens are present. These pathogens can cause great harm to both aquatic species and humans.

Stream Insects

Benthic (creekbottom) mircoinvertebrates, or stream insects are an important feature of the stream ecosystem. Stream insects are monitored to check the overall health of the stream. This bioassessment is conducted for a number of reasons:

1) macroinvertebrate communities have limited migration patterns, thus they are good indicators of localized conditions;

2) they integrate the effects of different pollutants and other short-term environmental variations, thus providing a holistic measure of the impact of various contaminants; and

3) as a primary food source, they are important to the overall health of the stream ecosystem.

As you can see, the creek ecosystem is dependent on many things working together to maintain a healthy balance of clarity, temperature, current and flow, dissolved oxygen, pH and chemicals. To make sure our waterways are protected, these parameters should be monitored regularly.

Source: Yates, Steve. Adopting a Stream: A Northwest Handbook. Published by The Adopt-A- Stream Foundation. 1988.

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Last Page Update - Tuesday December 27, 2005