Water pH is an important indicator of river and ecosystem health. Most species of game fishes prefer pH in specific, narrow ranges, although there are exceptions to this.

Temperature can be a good indicator to the health of a stream. Streams with healthy plant communities, including trees, along their banks will tend to have lower temperatures than streams that are open to sunlight. In general, streams maintained by ground-water discharge will have lower temperatures than those maintained primarily by surface runoff. Typically, the cooler the stream the more likely it is to support a sport fishery. Most species of game fishes prefer cooler waters, although there are exceptions to this.

Conductivity is an indication of the amount of dissolved ions (i.e., salt, metals, toxins) present in the water. It is measured with a conductivity meter, which measures how easily electricity can flow through the sample water. If the average conductivity measured at a site is 800 microSiemens (µS) or less, it is considered natural for stream water. At sites with high levels of development and impervious surfaces (roads, driveways, roofs), rainwater washes chemicals, such as fertilizers and pet wastes, from the developed landscape into the creek. Development can lead to a conductivity above 800 µS, which is considered excessive and may indicate the presence of toxic substances. (We must note that there are many toxins that are harmful and are not measured by conductivity.)

Aquatic Insects and Stream Habitat: River health can also be measured by the kinds of insects and in-stream habitat that are present.

The hydrology of a river system, which is mainly determined by soils, geology, and land use, is a critical physical factor to aquatic life. In streams where groundwater is the principal water source, stable flow patterns occur, characterized by low seasonal and daily fluctuations in discharge. Stable flows promote stable habitat for aquatic life in the form of diverse bottom substrates, secure in-stream cover, and moderate water velocities and temperatures. Unstable flows scour river bottoms and may cause excessive sedimentation which in turn covers up critical in-stream habitat.

Aquatic insects such as mayflies, caddisflies, and stoneflies depend on stable flows, high water quality, and sufficient in-stream habitat for survival. These are insects in the orders Ephemeroptera (the mayflies), Plecoptera (the stoneflies), and Trichoptera (the caddisflies), generally called the EPT orders. These insects are particularly sensitive to factors that reduce oxygen or flow, or increase temperature or otherwise raise metabolic rates in a stream.