What's in the Oregon Water Quality Index?
Parameters, calculation and methods
What is the Oregon Water Quality Index?
The Oregon Water Quality Index (OWQI) is one way that the Oregon Department of Environmental Quality assesses the health of Oregon's water resources. The index is used to communicate water quality information in an easy-to-understand, non-technical manner.
The OWQI is calculated each year with data from a network of 160 sites across the state. The network is focused on rivers and streams and does not include lakes, estuaries, wetlands, marine water or groundwater resources.
The index includes eight common water quality variables. Check out the next section to find out why these eight were included.
Temperature
What is temperature?
Water temperature is a measure of how hot or cold the water is.
Why is temperature important?
If water is too hot or too cold, then fish, insects, bacteria and other aquatic organisms may be negatively affected. This could be because of the stress put on the organism, or it could be from a lack of oxygen in the water. Water temperatures are a critical factor in maintaining and restoring healthy salmonid populations throughout Oregon.
What affects temperature?
Water temperatures are naturally influenced by the sun, shade, air temperature, and groundwater. Human activities like releasing heated water from power plants or cold water from dams, removing trees along streams and removing water for irrigation, can all influence water temperature.
pH
What is pH?
pH is an indicator of the acid/base balance of water. The pH scale ranges from 1 to 14 with values less than 7 being acidic, values greater than 7 being basic or alkaline, and values of 7 being neutral.
Why is pH important?
pH is an important water quality parameter that has many effects on aquatic life, including direct impacts on the health and survival of aquatic organisms and indirect effects on the overall water chemistry. The pH of freshwater streams is usually between 6.0 and 8.0.
What affects pH?
In most natural water, pH is determined by the relationship between photosynthesis, cellular respiration, daylight and temperature. Photosynthesis by aquatic plants makes the water more alkaline and increases pH. Cellular respiration by microbes and bacteria makes the water more acidic and lowers pH.
Dissolved Oxygen (DO)
What is DO?
Dissolved oxygen (DO) is the amount of oxygen gas dissolved in the water. It is measured in milligrams per liter and also expressed as a percent of saturation. Saturation is the total amount of oxygen gas the water can contain.
Why is DO important?
DO is critically important to living things in the water. Without enough oxygen, fish, insects and other aquatic life quickly die. The minimum oxygen requirements can vary with different species and different life stages, but sensitive species like salmon and trout need fairly high oxygen levels at or near saturation at all life stages to survive and thrive.
What affects DO?
DO can vary widely over the course of a day as photosynthesis and cellular respiration change with shifting daylight and temperature. Other factors that determine how much oxygen the water can hold at saturation are water temperature and atmospheric pressure. Warmer water holds less oxygen than colder water. The amount of oxygen at saturation will be lower at higher elevations because the atmospheric pressure is lower.
Biological Oxygen Demand (BOD)
What is BOD?
Biochemical oxygen demand is the amount of dissolved oxygen needed by aerobic organisms in a water body to break down organic material present in the water in a certain amount of time and at a certain temperature. Most natural, unpolluted waters contain at least some BOD.
Why is BOD important?
Aerobic aquatic microorganisms use organic matter in water for food. As BOD increases, the microorganism activity increases. If BOD is high enough, the microorganisms will deplete the DO in the water to levels below what fish and other aquatic life need to survive.
What affects BOD?
Anything that increases the input of organic material into the water increases the BOD. This includes soil erosion, dead plant material, sewage treatment plant and industrial discharges, animal waste, or anything else that can be decomposed by microorganisms.
Total Solids (TS)
What is TS?
Total solids is a measure of the amount of sediment found in the water column. It includes both dissolved and suspended solids.
Why is TS important?
All natural water contains at least some solids. High total solids can impair the ability of fish to find prey and avoid predators, can smother fish eggs and benthic organisms, and eliminate or impair benthic fish and macroinvertebrate habitat.
What affects TS?
Anything that increases the input of particulate or dissolved matter into the river increases total solids. Erosion from the disturbed land surfaces and poor riparian conditions are often major inputs of total solids.
Nitrogen (N) and Phosphorus (P)
What are N and P?
Nitrogen and phosphorus are naturally occurring nutrients that are essential for plant growth. Both are commonly added to crops, gardens and lawns to promote additional growth. Natural waters contain at least some of each of these nutrients.
Why are N and P important?
Like on land, nitrogen and phosphorus in water help promote growth of algae and other aquatic plants. High amounts of either can stimulate additional growth. Under the right conditions, nutrient levels can trigger a blue-green algae bloom, which can release toxins harmful to humans and wildlife.
What influences N and P?
Nitrogen and phosphorus can make their way into rivers, lakes and groundwater in runoff from crops, lawns, soil erosion, sewage plant discharges or the natural decomposition of plant material.
Bacteria
What is Escherichia coli (E. coli)?
Escherichia coli is a bacterium that lives in the intestines of warm blooded animals like mammals and birds.
Why is E. coli important?
There are many bacteria, viruses and protozoa that cause disease in people that can be present in water. People can swallow these organisms and get sick when swimming or drinking. Most types of E. coli are harmless, but its presence is a clear indication of fecal contamination.
What influences E. coli?
E. coli can come from a variety of sources including failing septic systems, stormwater runoff, domesticated animals and wildlife and discharges of untreated or poorly treated sewage.
Great, but how do these water quality variables become the Oregon Water Quality Index?
Keep scrolling to find out!
Sampling to Results
DEQ staff samples all 160 sites in the ambient water quality network every two months. Temperature, DO and pH samples are recorded on site. The rest of the variables require water to be collected, held on ice and returned to the lab. In all, DEQ staff travels about 27,000 miles and collects approximately 900 gallons of water a year for this project. The collected samples are analyzed by chemists at the DEQ Lab in Hillsboro.
Every two months ODEQ staff visit all 160 ambient network sites.
Results into Subindex Scores
For each variable, mathematical equations are then applied to the results to determine a score for each sample collected. The equations are designed to protect Oregon's salmonid populations (temperature, DO), to account for regional variations (pH), to include all forms of variable (nitrogen, TS), and to indicate potentially hazardous conditions (bacteria).
Subindexes into OWQI
The eight subindex scores calculated from the sampling results are then used to determine the OWQI score for the sampling event. For example, if a site is sampled in January, March and May, then a OWQI score would be calculated for each of those events. To calculate the final OWQI score for a particular site, DEQ staff use a statistical test called the Seasonal Kendall test to compare values between two seasons: summer (June, July, August, September), and fall-winter-spring (October through May) for a ten-year period. The lowest average of the two seasonal values is used as the final OWQI.
OWQI Status Range
OWQI Scores to Status and Trends
OWQI scores range from 10 to 100. These scores relate directly to the status assigned to each site (see figure). Trends are calculated based on the change in OWQI score over the 10-year period. ODEQ reports significant improving or declining trends. All others are designated as having no trend for the 10-year period.
What does the OWQI say about water quality in your area?
Interactive map for the 2023 OWQI.
Limitations and alternatives to the OWQI
Creating an index like the OWQI simplifies very complex systems and creates a potential loss or distortion of information. The strength of the OWQI is in its simplicity. However, for more technical reports DEQ also assesses the health of Oregon's water resources in the Oregon Status and Trends Report as well as the EPA Integrated Report. The buttons below take you to the web pages for these reports where you can find more information.
