How the Index is Calculated 

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How the Indices are Calculated

With all the data gathered, the StreamWatch Stream Health Index can finally be calculated. The index is designed to compare current conditions with ideal summer streamflow, water quality and habitat conditions for indicator fish at the sampling site in each stream. In setting out to evaluate habitat conditions for fish, we've assumed that protecting riverine fish and other aquatic wildlife is a worthwhile goal and we've had to make some assumptions about the parameters that affect fish habitat. There are certain limitations on the interpretation of the index:

• It does not reflect transient events affecting water quality or quantity,
• it does not rate the fish for consumption,
• and it does not test for the presence of toxic materials or fecal coliform bacteria in the water column.

Overview: The Stream Health index is arranged so that the environmental quality increases with increasing index (I = 1 for worst conditions; I = 100 for best conditions). Calculation of the index consists of three steps: (1) calculation of subindex scores for each of the parameters (2) aggregation of the subindex scores into index scores for flow, water quality, and habitat availability, and (3) aggregation of the three index scores into the overall "Stream Health" index . The table, below, shows the environmental parameters used and the criteria against which they are scored.

(Step 1) Calculating the individual scores: Water quality metrics are generally related to an index score along a rating curve. The SWSH index used a combination of the Massachusetts Water Quality Standards, regional averages, and biological tolerances of the indicator fish to develop curves for the water quality and flow parameters.

Below are sample curves developed for streamflow in Danforth Brook, and dissolved oxygen. For example, if the streamflow in Danforth Brook (a cold-water stream) were 1.7 cubic feet per second (cfs) and the dissolved oxygen concentration were 6.5 mg/L on a given date, then the subindex scores for streamflow and DO would be 50 and 64 respectively.

(Steps 2 & 3) Calculating the subindices and overall index: The trick in calculating an index is to combine the scores of the individual parameters in such as way that the overall index:

• gives a good picture of the overall score of all of the subindices,
• the most important subindices (for fish habitat) have the most influence over the overall index,
• and any low scoring subindex is not hidden by the averaging process.

For this index, the subindex scores within each group are combined using a harmonic mean to get the index score. (For example, the harmonic mean of the streamflow and the groundwater level scores gives the flow subindex.) Finally the three index scores are combined, again by calculating the harmonic mean, to get the overall Stream Health index score.

Using the harmonic mean (rather than an average or median) allows any low-scoring parameter to influence its subindex more heavily, and any low-scoring subindex to influence the overall index more heavily. For example, if water quality and flow conditions were all good except for dissolved oxygen, which was too low for fish to survive, the overall index should be low.

Other Indices: While this is the first time (that we are aware) that an index like this one has been used to report habitat conditions on a weekly basis, our index follows in the footsteps of many previous water quality indices. A quick review of the literature shows that much of the initial work on water quality indices was done in the 70's and early 80's, although interest seems to be reviving with indices now in use in Washington, Oregon, Ohio, Colorado, Kansas, Texas, and British Columbia (see the Links page under Water Quality Indices for links).

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