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CADDIS Volume 3: Examples & Applications

Worksheets: Little Scioto River, OH

Define the Case

In Ohio, biological impairment is defined by standard multimetric indices including the IBI, and ICI, and the modified index of well being (Ohio EPA 1989). These indices were used to identify the reaches of the Little Scioto River (Figure 1) that were considered to have impaired aquatic assemblages in comparison to the criteria for warm-water habitat (WWH) or modified warm-water habitat (MWH) designated uses. The Little Scioto River has both WWH and MWH sections. WWH is a designation shared by the majority of Ohio's rivers and streams, and is narratively defined as supporting a balanced, reproducing aquatic community. The minimum criteria required to be in attainment of WWH standards are defined as the 25th percentile values of reference condition scores for a given index, site type, and ecoregion. The MWH designation is a nonfishable aquatic life use, and is designed to protect streams that have been too physically impacted or modified to meet WWH standards.

A map
 highlighting the Little Scioto River
Figure 1. A map highlighting the Little Scioto River (outlined in blue in the center of the map) generated using Enviromapper for Water

Of seven sites sampled in 1992, the highest IBI score was at an upstream site with a 33 out of a possible score of 60 (Figure 2). The score translates into a fair ranking according to WWH standards. The remaining sites were described as severely impaired, with IBI scores between 25 and 12. Similarly, the ICI met WWH aquatic life use standards at the upstream site with a score of 38. The ICI score from RM 7.9 downstream was below MWH aquatic life use standards. Examination of the spatial distribution of the IBI, ICI and component metrics indicates that several distinct impairments occurred.

IBI and ICI scores on the Little Scioto River
Figure 2. IBI and ICI scores on the Little Scioto River. River miles are listed in upstream to downstream order from left to right.

The biological impairment and its basis

The Little Scioto River near Marion, Ohio, was listed as an impaired waterbody on the 1998 303(d) List of Impaired Waters. In 1992, Ohio EPA determined that the river was biologically impaired (Figure 1).

Specific effects

The IBI and ICI were disaggregated to identify more specific effects that were responsive to the environmental changes, had a sufficient range of variation, and appeared to indicate distinctive mechanisms (Figure 3).

Specific effects observed
                                      at the Little Scioto.
Figure 3. Specific effects observed at the Little Scioto.

Specific effects associated with the impairment observed at RM 7.9 (Impairment A) include an increase in relative fish weight and DELT anomalies. For the invertebrates, the percentages of mayflies decreased, and the percentage of tolerant invertebrates increased (Table 1). The specific effects observed at RM 7.9 were sufficiently distinctive from the other locations on the Little Scioto to suggest that either additional or completely different causes may be influencing the assemblage at other sites on the stream. Those sites were not addressed in the current example, but are discussed in Norton et al. (2002), and Cormier et al. (2002).

Table 1. Specific effects observed at Sites A and B in the Little Scioto River.
Response Impairment A: Change in assemblage at site A relative to Upstream Site Impairment B: Change in assemblage at site B relative to Site A
Relative fish weight Increased Decreased
DELT anomalies Increased Increased
% Mayflies Decreased Decreased
% Tolerant taxa Increased Increased

The investigation's purpose

The purpose of the investigation is to inform decisions about how best to return the Little Scioto River to a condition that meets State biocriteria. As such, we considered stressors from both point and non-point sources including toxic contaminants in water and sediment, nutrients, and habitat degradation.

The geographic area under investigation

The Little Scioto River is located near Marion Ohio (Figure 1). For the purposes of this example, the evaluation will be limited to identifying the probable cause of the biological impairment at RM 7.9 (Site A). The upstream site (RM 9.2) was used as a less-impaired reference site for comparison, and observations from six other downstream locations within the reach were brought in and analyzed as part of the case. For additional detail and an analysis of the biological impairments observed at other locations along the stream, see Norton et al. (2002) and Cormier et al. (2002).


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