Physical Habitat Assessment

We assess physical habitat conditions in our streams because healthy aquatic communities cannot exist without suitable, stable habitats.

PWD aquatic biologists measuring stream channel dimensions and streamflow in Poquessing Creek.

Rapid Habitat Assessment

Descriptive habitat assessments are performed at the same time and same location as invertebrate sampling. Using Rapid Bioassessment Protocols developed by USEPA, aquatic biologists visually rate habitat conditions on a numeric scale. These methods are described briefly within this section. For more information, refer to the USEPA guidance documents, linked below under Additional Resources.

PWD aquatic biologists individually assign habitat scores for the following attributes:

  • Epifaunal Substrate/Available Cover
  • Pool Substrate Characterization
  • Pool Variability
  • Sediment Deposition
  • Embeddedness
  • Velocity/Depth Regime
  • Frequency of Riffles (or bends)
  • Channel Flow Status
  • Channel Alteration
  • Channel Sinuosity
  • Bank Stability*
  • Vegetative Protection*
  • Riparian Vegetative Zone Width*
*For these habitat properties, left and right streambanks are assessed separately.

Once all habitat attributes are rated, biologists' scores are averaged for each attribute and then summed to calculate a total score. The total score for each study site is compared to a reference site of approximately equal size. Results are usually expressed as a percentage of the reference site score.

Fish Habitat Survey and Assessment

While performing fish assessments, we collect fish from stream segments approximately 100 m in length. In order to provide more quantitative information about characteristics of the fish community, we measure the site to estimate total surface area and volume. In this way, fish abundance data can be expressed as number of individuals (or biomass) per unit area or unit volume.

Habitat requirements of fish and other wildlife are complex, but Habitat Suitability Index (HSI) models created by the U.S. Fish and Wildlife Service can be used to estimate habitat suitability. These models were primarily developed during the 1980s and most were never validated with real-world data. Electronic surveying tools are used to obtain hundreds of accurate survey points within the stream channel, along with more detailed information about the stream bottom characteristics. These detailed survey data are then analyzed to calculate characteristics of the stream segment that may be critical to fish habitat.

Modeling streamflow in Pennypack Creek with River2D software

Changes in Habitat Conditions over Time

Another consideration is how stable habitat conditions are over time. We perform detailed surveys of stream cross sections, restoration sites and streambank profiles. These sites are re-visited at appropriate intervals to see whether erosion or sedimentation has occurred. More information on this monitoring is available in the Fluvial Geomorphology page.

Additional Resources

USEPA Rapid Bioassessment Protocol (RBP) Physical Habitat Assessment

Habitat Suitability Index (HSI) models from U.S. Fish and Wildlife Service