To study water quality, scientists study both chemistry (what is dissolved in water) and biology (what is alive in the water). Chemical monitoring provides a "snap shot" of the condition of the stream at the time the sample is collected. In contrast, the living organisms show what is happening in the stream over a period of time. Certain types of plants and animals are more tolerant than others to changes in habitat and water quality. The most commonly studied organisms in streams are fish, algae, and macroinvertebrates. Of these groups, the macroinvertebrate community is the most commonly used because they:

• are fairly easy to sample and identify
• are sensitive to pollution and changes in their habitats
• have a relatively long life cycle and so are indicators of water quality over a period of time
• are common in most streams or rivers

Although the Leaf Pack Experiment is not designed to monitor water quality, some basic assessment measurements may still yield interesting information. Two commonly used indices are:

• Percent of EPT
• Biotic Index

The EPT and Biotic Index are used to monitor the sensitivity of taxa to pollution. Some insects are more tolerant of pollution than others. Although these pollution tolerant groups, which include certain true flies and worms, can be found in the cleanest streams, their numbers should not dominate the community. On the other hand, the presence of mayflies, stoneflies, and caddisflies—sometimes referred to as the "canaries of the stream" because they are so sensitive to pollution—indicates good water quality.

Percent EPT : The total number of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera* (caddisflies). Many species within these three groups are sensitive to changes in water quality. In general, the more EPT taxa, the better the water quality.

*Note : Certain netspinning Trichoptera (Hydropsychidae) are more tolerant to pollution than other caddisflies and are therefore not counted as part of the total Percent EPT.

Biotic Index: A comparison of the abundance of taxa and their tolerance to environmental stress. This widely used index can indicate organic and nutrient pollution. Organisms are assigned tolerance values which range from 0 to 10, depending on the organism's sensitivity to changes in water quality and habitat (tolerance values increase as water quality decreases). Although originally designed for use at the species level, this index is now more commonly used at the family level. A modified version at the order/family level is provided. While the modified version is less accurate than its unmodified counterpart, it is useful for quick assessments.