Physical assessment involves observing and measuring general characteristics of a stream system and is considered the foundation for any monitoring study (Behar, 1996). Measuring the physical aspects of a stream such as width, depth, and flow rate is important because these parameters can significantly affect chemical water quality. Large, fast flowing streams can receive pollution discharges and not be significantly affected due to high dilution factors. In contrast, smaller streams with slower flowing waters have less capacity to dilute pollution discharges and are more subject to pollution impacts (USEPA, 1997).
Flow rate can affect the water clarity, or turbidity, of the stream by either keeping sediments suspended in the water column or allowing solids to settle to the stream bottom. Stream velocity also determines the type of aquatic organisms that can inhabit a stream system. Observing characteristics such as water clarity, odor, streambed composition, and aquatic vegetation is important since variations in these parameters can be indicative of water quality problems. For example, dense mats or blooms of algae can be indicative of an enriched system due to excessive nutrient inputs to the stream, a problem known as cultural eutrophication.
Physical observations are recorded on the Central New York Save Our Streams Program Field Data Survey Form (Appendix B). The first part of the survey asks for the location of the monitoring site as well as the air temperature and weather conditions during the monitoring event. The Director determines the air temperature (°C) and notes the weather conditions. Using a Garmin GPS 12 Receiver, the Director also determines the location of the sampling site. The location is recorded in latitude/longitude or UTM coordinates so the site can easily be located on a 7.5-minute United States Geological Survey (USGS) topographic map. The teacher is responsible for filling out the heading portion of the form as well as the general characteristics of a stream section. The group observes water appearance, odor, streambed composition, stream stability, algae color-texture-amount, and stream bank cover. For each characteristic category, all options that apply should be checked off.
An important physical parameter that is measured is stream flow, or discharge. The flow rate is first estimated through measurements and calculations, while the actual flow rate is measured with a flow meter. The stream flow will be calculated by multiplying the transect area and average velocity. The average stream depth and width of the stream are needed to determine the transect area of the stream in order to calculate the flow rate. The students through what we call the “float technique” will collect the average velocity. Flow rates are recorded in units of cubic feet per second (ft3/s or cfs). Stream flow (discharge) is expressed in the following equation:
Stream flow = area x velocity = cubic feet per second (cfs)
A transect is prepared across the stream site where the data will be collected. The transect is selected as an average visual representation of the stream and by accessibility. The transect provides the cross-sectional area of the stream. The monitoring group records the width (measured in feet) and depth (measured in feet) of the stream at this selected site, using a measuring tape. The width is marked across the stream with a rope. Five evenly distributed locations across the stream transect are marked using clothespins. The depth is measured at the five marked locations with a straight edge measuring stick. The average of the depth measurements is recorded on the field data survey form.The velocity of the stream at the monitoring site is measured using the "float technique." The float technique involves volunteers timing how long it takes for a float (tennis ball or whiffle ball works well) to travel a specified distance from upstream to downstream. The length of the stream run, typically 20 to 30 feet, is determined by natural obstacles and finding a safe area for the ball to be released and retrieved. The length of the stream run where the group release and retrieve the ball is measured off with a measuring tape and marked. The end of the stream run where the ball is released is marked with the rope used to measure the transect width, while downstream the retrieval area is marked with an item placed at the streams edge or string tied to brush or trees. One person releases the float in the main current at the beginning (upstream) of the run while another person waits downstream to call time and retrieve the float once it has reached the end of the measured distance. A third individual uses a stopwatch to determine the amount of time it takes the float to travel the specified distance. The five float trails are released at the five evenly distributed and previously points where the stream depth was also measured. Velocity is then calculated by dividing the distance the float traveled by the amount of time it took the float to go that distance. Average velocity is recorded in a unit of feet per second (ft/s). The average velocity is calculated and recorded on the field data survey form.
**English units are used for physical measurements since this system is what is most commonly used in the field of hydrology. English units are also utilized in the American Public Health Association's publication, Standard Methods for the Examination of Water and Wastewater.
Even though the program director will measure the stream flow with a flow meter, stream flow is still calculated through measurements by the students, because it provides conceptual understanding. They would also be able to find the find stream flow on their own without the presents of a flow meter.
When using the flow meter, the director finds the actual average velocity of the stream. By placing the “measuring stick” attached to the calculator in the five evenly spaced locations on the transect, we find the velocity in each location. The stick should be held still and at mid-depth in the stream. The average of these five velocities will be the actual average velocity multiplied by area to find actual stream flow.