Several different aquatic indicators can describe the ecological health of a stream and watershed.
Habitat Quantity and Quality
Stream organisms require water. Thus, more water means more habitat. Other examples of important habitat characteristics are stream temperature, stream bed composition (e.g., clay, sand, gravel, boulders), the frequency and depth of pools, and the availability of cover (e.g., undercut banks or large wood) in which fish and other creatures can hide from predators.
Fish Species Richness
The Willamette Basin contains about 31 known native fish species and 29 exotic or introduced species. Native fish species richness (number of native species) reflects the overall biodiversity of the aquatic ecosystem. Human disturbances that introduce toxic contaminants or non-native fish species (see Invasive species), increase temperatures, siltate stream beds, or produce other changes in stream habitats frequently result in the loss of one or more native fish species.
Macroinvertebrate Species Richness
In some ways, invertebrates are better indicators of stream condition than fish. Invertebrates are much less mobile than fish and thus more accurately reflect the condition of the stream reach in which they are found. There are also hundreds of species of invertebrates in the basin, in contrast to the comparatively small number of fish species. Different species tend to respond to different types of stresses and habitat features. Thus, the greater numbers of invertebrate species make invertebrate indicators sensitive to a wider array of human impacts. EPT richness is the number of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) taxa found in a stream reach. Mayflies, stoneflies, and caddisflies are typically used in biological monitoring because they are generally intolerant of silt, warm temperatures, and water quality degradation. As with native fish richness, stream degradation tends to result in the loss of species and a decline in EPT richness.
Alternative Futures Analyses
Changes in stream condition, projected for all three alternative futures, are distinctly less than the changes that occurred between Pre-EuroAmerican times and today. In fact, projected values for Plan Trend 2050 and Development 2050 are virtually indistinguishable from those for in 1990 for all indicators of stream condition in both the Lowland and Upland regions. Most of the land converted to urban and residential use under Plan Trend 2050 and Development 2050 was in agriculture 1990. Even under Development 2050, the amount and types of natural vegetation in riparian zones is fairly similar to levels in 1990. On average, the impacts of agriculture and urban/residential use on stream condition are similar in magnitude. Thus, converting agriculture to urban/residential uses will not, necessarily, cause additional stream degradation, beyond levels observed today. Specific management practices employed on either agriculture or urban/residential lands are likely to be more important in determining stream condition than the type of land use.
While changes between 1990 and Plan Trend 2050 and Development 2050 are also quite small for Upland streams overall, differences are apparent by land ownership. Under Plan Trend 2050, the cutthroat trout habitat suitability would improve slightly, relative to 1990, in streams draining federally managed forest lands, but continue to decline in streams draining privately managed and state managed forest lands. Smaller changes in habitat suitability are projected under Development 2050 than under Plan Trend 2050, primarily because of the smaller changes in the amount of riparian closed forest in Development 2050. Subbasin differences in habitat suitability reflect both land ownership patterns and the relative proportion of the subbasin in the Lowland and Upland regions.