Healthy Streams: Components and Measures

Healthy Streams: Components and Measures
by Jack E. Williams and David B. Hohler

The linear, branching nature of stream drainages implies connectivity throughout watersheds. As such, stream habitat conditions and the communities they support are largely determined by processes occurring in the watershed and the riparian areas adjacent to streams. Healthy streams result from the interrelated function of riparian and channel condition as determined by watershed processes. The condition of upland habitats substantially influences the condition of riparian habitats, which in turn, influence the condition of stream habitats.
Healthy streams are characterized by a high diversity of habitat types that support animal and plant communities that are similar to those historically found in the region. Healthy streams are resistant to natural and artificial disturbances, but once disturbed, recover quickly.

Stream health is a composite of the condition of the physical, habitat and biological components of riparian areas as well as the condition of physical, habitat, and biological components of stream channels (see attached matrix). Stream health, like human health, is too complex to evaluate with a single measure. The Index of Biotic Integrity (IBI) is a composite measure of various attributes of fish and/or macroinvertebrate communities and can be used as a surrogate of stream health. Proper Functioning Condition (PFC) measures the ability of vegetation, water, soil and natural structures -- the physical elements -- within riparian areas to properly perform ecological functions. PFC measures the condition of these physical elements and when supplemented by measures of the biotic conditions of riparian areas (i.e., animal and plant communities), provides a comprehensive view of riparian health.

In general, riparian and stream habitats supporting native communities of a high biotic integrity occur within riparian areas and streams that are in proper functioning condition. The reverse, however, is not necessarily true. That is, just because habitats are in good condition does not necessarily mean that intact biotic communities are present. Populations may be slow to respond to improving habitat conditions. Also, populations and/or species may have been extirpated from the habitat and substantial time may be required for recolonization. In some cases, recolonization is impossible or cannot occur without human-assisted translocations.

It is also possible to have relatively intact biotic communities during early phases of declining habitat condition. In those cases where streams are rapidly declining from proper functioning condition to nonfunctional, biotic communities may be intact relative to habitat conditions because of a lag time in responding to the degradation.

Nonnative species are a problem in many stream and riparian habitats and contribute to reduced biotic integrity. These species often compete with or prey upon native species or serve as sources of disease and parasites. Establishment of nonnative species is an indicator of reduced biotic integrity. Normally, streams with flow, turbidity, and temperature regimes that are similar to historic conditions will favor native species over nonnative forms.

Matrix of Stream Health Components and Measures

Riparian Areas Stream Channels

Physical components: adequate amounts of
vegetation, large woody debris and/or other natural structure are present to dissipate stream energy, filter sediment, stabilize streambanks, and perform other natural ecological functions
measure: Proper Functioning Condition
components: adequate water quality to support native fish and invertebrate communities; complex channel structure to support diversity of habitat types

measures: state water quality standards, water temperature

Habitat components: multi-story vegetation, snags

measure: measures of vegetation diversity
components: deep pools, riffles, pocket pools, large woody debris, undercut banks
measure: number of deep pools per mile, pool:riffle ratios, amount of large woody debris in stream, amount of undercut bank

Biological components: native amphibian, reptile, migratory bird, mammal, and invertebrate communities equal or similar to that which historically occurred in representative habitats region
measures: Indicator Species, diversity of neotropical migratory birds, status of rare species
components: native fish, amphibian, and macroinvertebrate communities equal or similar to that which of historically occurred in representative habitats of region

measures: Index of Biotic Integrity, EPT Index, status of rare species.

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	Riparian Areas	Stream Channels
Physical	components: adequate amounts of vegetation, large woody debris and/or other natural structure are present to dissipate stream energy, filter sediment, stabilize streambanks, and perform other natural ecological functions measure: Proper Functioning Condition	components: adequate water quality to support native fish and invertebrate communities; complex channel structure to support diversity of habitat types measures: state water quality standards, water temperature
Habitat	components: multi-story vegetation, snags measure: measures of vegetation diversity	components: deep pools, riffles, pocket pools, large woody debris, undercut banks measure: number of deep pools per mile, pool:riffle ratios, amount of large woody debris in stream, amount of undercut bank
Biological	components: native amphibian, reptile, migratory bird, mammal, and invertebrate communities equal or similar to that which historically occurred in representative habitats region measures: Indicator Species, diversity of neotropical migratory birds, status of rare species	components: native fish, amphibian, and macroinvertebrate communities equal or similar to that which of historically occurred in representative habitats of region measures: Index of Biotic Integrity, EPT Index, status of rare species.