4.1 The physical properties of a stream
The physical properties of a stream, such as the channel shape, the substrate size, and the available habitats, play an important role in determining the type and diversity of organisms that the stream can support. These physical attributes provide useful information on ecosystem function and stream health.
In the field, scientists measure a variety of aspects of the physical environment in a stream. One important physical attribute is habitat diversity. A typical stream contains three major types of habitats: riffles, runs, and pools. A riffle is an area of fast moving, churning water. A run is an area of smoothly moving water. A pool is an area of stagnant water. Each habitat type supports different types of aquatic life, so a stream with a greater diversity of habitat types will generally support a greater diversity of organisms.
One simple measure of habitat diversity is the number of habitat transitions. A researcher in the field walks along a 50 meter or 100 meter section of a stream and records the location and type (riffle, run, pool) of each new habitat encountered. Simply counting the number of habitat transitions within the transect provides an informative measure of habitat diversity in a given stream. Another measure of stream habitat diversity involves calculation of the total length of riffle and run habitat divided by the total length of pool habitat. This ratio is generally greater for streams with higher water quality, because riffles and runs tend to be higher in oxygen and pools tend to be lower in oxygen. The amount of variation in stream width or depth can also provide a measure of habitat diversity in a stream. Researchers simply measure the width or depth of a stream at regular intervals and then asses the amount of variation in the set of data using a frequency distribution (histogram) or statistical measure of variation (e.g., coefficient of variation).
Another useful physical measure is the shape of the cross section of a stream channel. A deeper, more incised channel is typically associated with urban streams and is often the result of direct human impacts (e.g., dredging the stream channel) or indirect human impacts (e.g., a decreased percentage of impervious surfaces resulting in high energy stream flows and erosion during rainstorms). Incised channels have faster flow rates and greater turbidity (due to more sediment in the water). Scientists can measure the channel depth (from bank height to deepest point) and width (from bank to bank) and then calculate the depth to width ratio, called the incision ratio. A greater incision ratio indicates a more incised channel.
A third aspect of a stream’s physical environment is the substrate size. Stream substrates may include sand, gravel, rock, or even bedrock. Since different organisms have different habitat requirements, a stream with a variety of substrate sizes will generally support a greater diversity of organisms. Some urban streams are mostly sand, while other urban streams are mostly rock. A more natural stream generally has a greater variety of substrate sizes. Scientists cannot measure all the substrate particles in a given stream, so instead they measure a randomly selected sample of substrate particles. A common sampling method is called a “pebble count.” In this method a researcher walks in a zigzag pattern along a streambed. After each step the researcher measures the “pebble” (substrate particle) at the tip of his/her foot. A substrate particle has three axes (dimensions), and the researcher measures the length of the intermediate axis. This measurement is generally made for a large sample of substrate particles. Then calculations are made to determine the amount of variation in substrate size. A frequency distribution (histogram) can be created to examine the amount of variation in substrate size. There are also statistical measures of the amount of variation (e.g., range or coefficient of variation) that can be calculated.
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