Physical Parameters of Water Quality
Temperature is an important parameter in aquatic environments as it influences many aspects of stream physical, chemical, and biological health. Most aquatic organisms have limited optimal temperature ranges, which affect survival, spawning success, and metabolic rates. Annual temperature changes provide the stimulus for emergence of insects and spawning of fish. Because the high specific heat of water results in relatively slow rates of temperature change, aquatic species are buffered from the wide variations in temperature that terrestrial organisms are accustomed to. Therefore, large human-induced fluctuations in water temperature can have devastating impacts on stream biota. Increased water temperature also decreases the availability of dissolved oxygen, amplifies the stress caused to organisms by toxic compounds, and enhances algal and bacterial growth rates. Conductivity and pH are also influenced by the stream temperature, but most probes compensate for this effect. The greatest source of heat to the stream is solar radiation. Streams with little riparian canopy cover are therefore warmer than shaded stream reaches. Deep streams are generally colder than shallow streams, as evaporation at the surface level slows heat transfer through the water column. Stream temperature is also impacted by groundwater, tributaries, rainwater, and discharge pipes. These inputs may either raise or lower the temperature, depending on season and source. Temperature also varies between different habitats within a stream reach, with backwater pools often warmer than the main stream channel. Stream temperature can be measured with either a calibrated thermometer or with a dissolved oxygen, conductivity, or pH probe. Environmentally safe thermometers should be used to minimize the risks associated with potential mercury contamination. Data loggers which continuously record temperature measurements at set intervals may be used to show temperature fluctuations over time. For the LFRWMP, temperature measurements should be reported in °C to one decimal place. Temperature readings for streams in the Fox River Basin typically are between 10°C and 20°C during our sampling periods. Wisconsin water quality standards state that there shall be no temperature changes that adversely affect aquatic life, and that daily and seasonal temperature fluctuations shall be maintained. By law, the temperature for warm water fisheries shall not exceed 89°F (31.7°C). A maximum temperature limit for cold water fisheries is not specified.
Streamflow, or discharge, is the volume of water in the stream flowing past a given point within a specific period of time. For the school-based monitoring program, streamflow measurements are recorded in cubic feet per second (cfs) for ease of comparison to the data reported by USGS. Streamflow is calculated by multiplying the average width by the average depth by the average velocity by a bottom factor. To determine streamflow, a stream reach is selected that is at least 6” deep and fairly straight. The reach should contain few obstacles, and all the water should be flowing in a single channel (not braided). A run is typically a good selection for streamflow measurements because the water has sufficient speed and depth. Three transects across this stream reach provide the average width value. Along these three transects, equally spaced depth measurements are used to compute the average depth. Velocity is determined by the time it takes to float an orange or object of similar buoyancy the length of the stream reach. Oranges work well for measuring velocity because they float just below the water’s surface, where they cannot be influenced by wind. Finally, the stream bottom factor accounts for the friction of the water flowing past the stream substrate, with the assumption that average stream velocity is only 80% to 90% of the surface velocity. Rough bottom streams interspersed with submerged plants or rocks have slower water velocities as compared to smooth mud or bedrock. Thus, streams with cobble or gravel substrates or many aquatic plants have a bottom factor of 0.8, whereas streams with a smooth mud, silt, or bedrock substrate have a factor of 0.9.
The land use within a stream’s watershed greatly impacts stream discharge. Water enters the stream by direct precipitation into the channel, by surface runoff from the surrounding watershed, or from groundwater inputs. As a watershed becomes more urban, the increase in impervious surface cover causes less water to infiltrate to the groundwater and more surface runoff to enter the stream channel in a shorter period of time. Because of the reduced infiltration, the water table often drops and groundwater sources to the stream are reduced. This causes flashy conditions in which streams have higher peak discharges at the beginning of a precipitation event, and lower overall base flow conditions.
Streamflow has significant impacts on other water quality parameters. Fast-flowing, turbulent water at riffles increases dissolved oxygen levels through aeration. In contrast, low flow conditions typically result in higher water temperature and decreased oxygen levels. Periods of increased flow may result in greater turbidity, because the fast moving stream has enough energy to displace larger quantities and sizes of sediment particles. This erosive capacity is well illustrated by the exposed soil on the banks of a flashy urban stream. Finally, aquatic organisms are adapted to a variety of streamflow conditions based on feeding strategies, water temperatures, and dissolved oxygen concentrations. A diverse stream environment requires both slow and fast flowing habitats to support its biological community. Changes in streamflow dynamics, such as those brought on by an urbanizing watershed, can greatly impact the integrity of the stream ecosystem.
Turbidity / Transparency
The particulate matter carried by a stream determines its turbidity, or the relative muddiness or cloudiness of the water. Particulates in a stream consist of algae, sediment particles from erosion, coarse particulate organic matter (CPOM) such as leaves and twigs, and fine particulate organic matter (FPOM) that has been broken down by stream biota. Erosion is a natural geologic process. However, certain human activities such as farming, storm water discharge, and construction greatly increase the amount of erosion in a watershed. The increased sediment from these erosive activities blankets the stream bottom and destroys spawning areas and macroinvertebrate habitat. Sediment can also be resuspended into the water column by bottom feeders like carp or by walking through the stream. Suspended sediment blocks light needed by rooted aquatic plants, damages gills on fish and invertebrates, and decreases visibility for fish who must see their prey. Sediments can also carry adhered pollutants, such as heavy metals and phosphorus, into the stream.
There are several methods of measuring turbidity. The LFRWMP uses a transparency tube, which measures the depth at which a black and white crosshair pattern is visible at the bottom of a tube filled with stream water. Low transparency is highly correlated with high turbidity in streams. Another available method for measuring turbidity uses the Hach DR/850 colorimeter. This test reads the amount of light transmitted through the stream sample, and reports results in FAU, Formazin Attenuation Units. The most exact measurements of turbidity are made with a nephelometric turbidity meter. Turbidity meters report measurements in NTU, Nephelometric Turbidity Units, and have greater ability to determine lower levels of turbidity.
For the LFRWMP, measurements from the transparency tube should be reported in depth units of centimeters (cm). A transparency of about 25 to 35 cm is equivalent to about 25 NTU. A transparency of >60 cm is roughly equivalent to a turbidity of <10 NTU. A transparency of about 5 cm is roughly equivalent to a turbidity of about 200 – 300 NTU. A more detailed and robust relationship between turbidity and transparency for streams in Northeastern Wisconsin has not yet been developed. Turbidity and transparency can also be related to total suspended solids and streamflow results for specific streams or rivers. Although general relationships have been reported, the relationship must be established on a site-by-site basis.