SI-2D (2/23/05 version)
Environmental Targets And Priorities, Stream Integrity, Stream/Watershed Components: HUC 11 Watersheds


On a routine annual basis, Ohio EPA conducts fully integrated river and stream biological surveys (biosurveys) in 20-25 U.S. Geological Survey 11-digit HUC watersheds and 2-3 large rivers with an aggregate total of 400-450 sampling sites. Biological, chemical, and physical monitoring and assessment techniques are employed in biosurveys to determine:

  • A current and thorough assessment of water quality conditions in watersheds that are scheduled for TMDLs in the near future (1-3 years);
  • The extent to which use designations assigned in the Ohio Water Quality Standards (WQS) are either attained or not attained;
The designated aquatic life uses as codified in the Ohio Water Quality Standards include:
EWH (Exceptional Warmwater Habitat)
WWH (Warmwater Habitat)
CWH (Coldwater Habitat)
MWH-C (Modified Warmwater Habitat-Channelized)
MWH-MD (Modified Warmwater Habitat-Mine Drainage)
MWH-I (Modified Warmwater Habitat-Impounded)
LRW/LRW-S (Limited Resource Water)
LWH/WWH-L (Limited Warmwater Habitat)
SSH (Seasonal Salmonid Habitat).
  • If use designations assigned to a given water body are appropriate and attainable;
  • If any changes in key ambient biological, chemical, or physical performance or response indicators have taken place over time, particularly before and after the implementation of point and nonpoint source controls;

The key to having a successful watershed approach is in using the different types of indicators within the roles that are the most appropriate for each. The inappropriate use of stressor and exposure indicators as substitutes for response indicators is at the root of the national problem of widely divergent 305(b) statistics reported between the States. This issue is discussed in the 1994 Ohio Water Resource Inventory (Ohio EPA 1995).

The principal biological evaluation tools used by Ohio EPA to determine aquatic life use status are the Index of Biotic integrity (IBI) (PDF 1164kb), the Modified Index of Well-Being (MIWB) (PDF 1164kb) and the Invertebrate Community Index (ICI) (PDF 801kb). These three indices are based on species richness, trophic composition, diversity, presence of pollution-tolerant individuals or species, abundance of biomass, and the presence of diseased or abnormal organisms. The IBI and the MIWB apply to fish; the ICI applies to macroinvertebrates. Ohio EPA uses the results of sampling reference sites to set minimum criteria index scores for use designations in water quality standards. Provisions addressing biological criteria are in paragraph (A)(6) of rule 3745-1-07 (PDF 68K) of the OAC.

Simultaneous with biological evaluation, habitat is assessed using the Qualitative Habitat Evaluation Index (QHEI). The QHEI scores help to explain:

  • Biological performance using the indices (ICI, IBI, and MIWB) above;
  • Stream morphology (hyperlink to SIcomponentsmorph.html);

both of which explain:

  • The aquatic life use the stream is capable of supporting;

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Evaluating the Status of HUC 11 Watersheds

The specific design of Ohio EPA biosurveys are dependent on stream size. There are 331 HUC 11 watersheds in Ohio. The average drainage area for a HUC 11 watershed is 130 square miles.

Beginning with the 2002 Integrated Water Quality Report, Ohio EPA chose to use HUC 11 watersheds as the principle water quality assessment unit in Ohio. HUC 11 watershed characteristics are fundamentally different both biologically and morphologically from those found in large rivers. HUC 11 watersheds have higher stream gradients and consequently higher velocity flows under average conditions. The volume of water is lower and there are more and shallower riffles. Consequently, the fish and macro-invertebrates communities are different and more diverse than those found in large rivers, because the habitat is more diverse throughout the watershed.

Therefore the HUC 11 water quality sampling approach includes a watershed grading technique that incorporates a spatial analysis of sub-basins, in addition to the traditional segment approach. This technique fosters direct comparisons of aquatic life use attainment status from one HUC 11 watershed to another. More details are available in Section 4 of the 2002 Integrated Water Quality Report.