How clean is your stream? Ask the insects

Fly fishing in a stream

Clean water is critical not just so that we can safely drink from it, but also to support any and all life within it. What’s more, a detrimental change in water quality can have a profound effect not only on that particular body of water, but also on any adjoining streams, tributaries, rivers or even the ocean. Poor water quality can have a huge impact on all of these other bodies of water, which is why it is so essential that careful measures be put in place for assessing water quality and reacting to any changes that are detected.

When it comes to assessing water quality, there are of course a number of chemical tests that can be performed. Temperature, salinity, pH, turbidity, as well as the levels of oxygen, nitrogen, phosphates and other elements are all important factors in assessing water quality, and are commonly used in routine testing. However, if you’re out fishing, hiking or camping and are wondering how clean and pure the water around you is, you don’t need to have a test kit handy to find the answer. Insects and other aquatic macroinvertebrates such as snails, leeches, worms, mussels and more spend at least part of their life cycle in the water, and can tell us a great deal about what sort of quality a given stream, river or lake has to offer.

Why use macroinvertebrates?

Macroinvertebrates are easy to study because they don’t tend to move to different areas throughout their lifespan unless they are forced to, they are easy to collect and identify, and some are highly sensitive to pollution, meaning that you may be able to use them to assess when even a minor change to water quality has occurred. However, other macroinvertebrates are highly tolerant of pollution, so it is important to know which are which when it comes to using these organisms to assess water quality.

Fussy macroinvertebrates

Stone Fly Nymph
Photo Credit: Dave Huth

As mentioned, certain macroinvertebrates are extremely sensitive to even minor changes in water quality, an attribute that makes them ideal for assessing the quality of your local stream or river. While the absence of these insects and other organisms from a body of water doesn’t necessarily indicate that the area is polluted, their presence, especially in large numbers, certainly suggests that it is in good condition. Similarly, if these organisms used to be abundant and are now rarely seen, this may be an indication that a pollution event has occurred. The fussy macroinvertebrates that fall into the category of being highly sensitive to pollution include the following:

– Mayfly nymphs

– Gilled snails

– Adult riffle beetles

– Caddisfly larvae

– Stonefly nymphs

– Water pennies

Generally speaking, these organisms all require high levels of dissolved oxygen, which is an indication in itself that water quality is high.

Moderately tolerant macroinvertebrates

damselfly nymphs
Photo Credit: USGS Bee Inventory and Monitoring Lab

This encompasses a large group of aquatic organisms that are able to tolerate some degradation in water quality. Seeing these organisms in large numbers can be an indication that the water quality is moderate to good. Generally speaking, though, it is difficult to ascertain the quality of water based on the presence of these invertebrates. Organisms falling into this category include, but are not limited to, the following:

– Alderfly larvae

– Dragonfly and damselfly nymph

– Mussels

– Sowbugs

– Clams

– Crayfish

– Scuds

– Fishfly larvae

Highly tolerant macroinvertebrates

Aquatic Insect Larva
Photo Credit: Dave Huth

There are many macroinvertebrates that actually prefer poor quality water. If you see these organisms in your local stream or river, it is probably an indication that the water is polluted. Some organisms which fall into this group include:

– Black fly and midge fly larvae

– Aquatic worms

– Leeches

– Lunged snails

If you notice a change in the organisms that are present in the river or stream you fish in, it may be a sign that the water quality has changed. You may like to inform the relevant authorities in your area so that chemical testing can be performed.


I highly recommend learning more about entomology and macroinvertebrates if you are interested in fly fishing. It will give you a better understanding of what fish eat and how to mimic your fly patterns to the right type of insect species. I have these books personally and enjoy both of them:

Pocket Guide to Western Hatches Hardcover Book by Dave Hughes

Bugwater by Arlen Thomason (My personal favorite.. great photos.)

I spend time between the love of my life (my wife), my digital agency, and trying to squeak in time for fly fishing when I can hide from responsibilities. If you want to strike up a conversation with me try subjects on: fly fishing, fly tying, cooking, photography, reading, camping or anything related to social media or marketing.