What do you measure for WQ? is your index based on chemical measure or waterorganisms?

You could to a gradation of distance to pollution source, then there is no need for categorising your sites. A  bit more context would help to improve the answers...

Aquatic biologist experts will often rate organisms based on pollution tolerance or intolerance.  Such as in USA, the presence of diverse population with stoneflies, mayflies and caddis flies is usually a good water quality indicator.  But there are individual species within these genera that are pollution tolerant, so it is inappropriate to overgeneralize.  So if I remember a rating, an aquatic invertebrate rated 1 = very good water quality, a rating of 5 is poor.  I think after you sample the benthics, you take number in each category, times the rating factor, sum all this up for all categories, and divide by number in sample.  So if I had 5 individual aquatic insects in category 1, 10 in category 2, 4 in category 3, 1in category 4, and 0 in category 5, my total is 41 divided by 20 or rating of 2.05, so a good water quality.  The key to using these systems is having knowledgeble aquatic biologists that have linked and rated aquatic insects to water quality.

If I did not respond on another question, I have a report on my researchgate site with a moth on the cover -- spruce budworm.  We used aquatic Macroinvertebrates as an indicator of accidental pesticide over spray and monitoring of spills.  The report is pretty old, but if you have not seen, you may find interesting.  

The added questions from Dr. Herr were appropriate.  A variety of water quality components might be addressed with aquatic insects, but their tolerance to pollution might vary with the pollutant or suite of pollutants.  I think poor water quslity waters will have one or more of these conditions, very low dissolved oxygen, high nutrient loading, high fecal coliform, excessive fine sediment loading, pesticide issues, etc.  If you find that your aquatic insects have been rated, I would see if I could find and read about the rating, and apply it, keeping good records.  If you find the rating has errors for your specific water quality application, you will want to go back and adjust your data, methods used, reasoning, and reports.  

In Germany a variety of metrics are used to assess the ecological quality of a river section. The selection of the metrics depends on the type of river (mountain creeks require a different metric set than lowland streams), so knowing what type of a river you are analysing will be helpful.

Each river type is associated with a number of core metrics from several categories (e.g. one diversity index, one functionality index and so on). One of these metrics is for example the number of EPTCBO taxa (ephemeroptera, plecoptera, trichoptera, coleoptera, bivalia and odonata), another one would be the percentage of EPT taxa (ephemeroptera, plecoptera, trichoptera).

Each core metric is assessed individually and the result is then transformed into a score between 1 (very good) and 0 (very bad).

The combination of the scores of all core metrics is then the final result for the analysed river stretch.

You can read more about this approach here:

www.fliessgewaesserbewertung.de/en/

There is a section about the river typology:

http://www.fliessgewaesserbewertung.de/en/gewaesserbewertung/typologie/

And about the software used for the assessment:

http://www.fliessgewaesserbewertung.de/en/gewaesserbewertung/berechnung/

Of course you cannot use that method/software as the taxa in your catchment will be vastly different from the one used in Germany. But the general approach is worth checking out, especially the documentation of the metrics. You can find some of the documents in English here:

http://www.fliessgewaesserbewertung.de/en/download/berechnung/

But I am not sure if all the information about the different metrics is available in English.

My advice would be reading more about the diferent available metrics and then chosing maybe just one or two (to keep it simple) that are most suitable for your research area. Depending on the metrics you choose you might also need additional information about the value of the taxa found in your catchment as indicators (How narrow is their ecological niche? Are they indicating a rather good or bad water/habitat quality or are they ubiquitous?)

I hope that helps you getting started.

All the best,

Michael

We use a physical habitat assessment on our sites and it results in a single metric for each site.  It is independent of the macroinvertebrates and you can therefore use the macroinvertebrate metrics as response variables.  The EPT index responds wuite well to this metric.  Links to the US EPA protocols we use are here: http://epscor.w3.uvm.edu/2/node/899#habitat