Secrets of the Credit River


Upper Credit River, ON (photo by Merridy Cox)

I began my limnology career teasing out the secrets of stream life as a grad student at Concordia University, Quebec. My master’s research focused on several rural and urban streams in the Eastern Townships, not far from where I grew up.

Later, as a limnologist for various environmental consulting companies in British Columbia, I used stream macro-benthos communities as indicators of environmental impact from industrial discharges, agriculture and municipal development. Macro-benthos are bottom-dwelling life you can see with the naked eye. They’re mostly made up of aquatic worms and juvenile stages of insect species (benthic invertebrates). Many of these insects spend most of their lives voraciously feeding in the water stage (several months to several years), before they emerge as adults to live briefly (some from days to weeks) to mate and create new life. Adults don’t feed; in fact, they don’t have usable mouthparts. The Dolania americana adult female mayfly lives a brief five minutes. Similarly, the Gripopterygidae stonefly larva, which clings to the underside of rocks and debris in fast flowing water, takes from one to three years to mature but once emerged may last from 1-4 weeks.

mayfly larva-creditStream ecologists identify benthic invertebrates by their form, but they also recognize them by how they feed:

Shredders use scissor-like mouths to cut and shred apart coarse particulate matter. These include amphipods, mayflies, stoneflies, midges, and some caddisflies.

Collector–gatherers (e.g., worms, nematodes, crustaceans, and gastropods) use their broom-like mouths to sweep in fine and ultra-fine organic matter.

Grazers or scrapers, such as beetles, mayflies, and stoneflies, feed on attached algae and biofilms. Their mouths chisel against periphyton (attached algae) on rocks, woody debris and aquatic plants.

Filtering collectors, such as blackfly larvae, collect suspended fine and ultra-fine organic matter, which can include phytoplankton (floating algae) in their finely spun nets. The caddisfly larva Arctopsychegrandis, for instance, builds a rough house made from twigs, leaf fragments, and small pebbles and spins silk nets across its threshold to capture organic matter suspended in the stream.

Predatory benthic invertebrates, such as damselflies and dragonflies, have piercing mouthparts that act like a straw, allowing them to suck the nutrients from their prey without having to chew or shred it.caddis fly larva-kv

Since 1909, when Kolkwitz and Marsson demonstrated that benthic invertebrates showed specific tolerances to organic enrichment and other sources of pollution, scientists have used these communities to study impacts to stream health from chemical pollution, flow disruption and habitat destruction. The EPT Richness Index was developed, based on the EPT groups being generally pollution-intolerant. EPT stands for Ephemoroptera (mayflies), Plecoptera (stoneflies) and Tricoptera (caddisflies) and the index corresponds to their percentage in the stream. EPT benthos will disappear in areas of poor water quality, organic enrichment, low oxygen, and high metal levels.

I recently tested this in several ad hoc field trips I made with my naturalist friend Merridy Cox along the Credit River in Ontario. We started our explorations with the lower Credit River watershed, within the urban setting of Mississauga, Ontario, where we sampled the river and a few small tributaries in Riverwood Park, a few kilometres from where the river empties into Lake Ontario.

Originally named “trusting creek” (Missinnihe) by the Mississauga First Nation people, the salmon-bearing Credit River drains some 860 km2 of Ontario and flows 90 km from its source at Orangeville, over the Niagara Escarpment, through several suburbs, and into Lake Ontario at Port Credit.

stonefly larvaGreat efforts have been made to restore and maintain the health of the Credit River and its watershed, mostly through the work of the Credit Valley Conservation Authority, together with the provincial and various municipal governments.

While the water quality of the lower river is considered generally fair to poor, the river is partially saved by its gradient and turbulent flow. The length of the Credit River, up to very close to its mouth, rushes with the sound of a great storm. It tumbles and gurgles over rocks, capturing oxygen from the air; it scours gravel beds and cuts swirling eddies and creates undercut banks for foraging fish. The habitat is complex and life thrives here. Green algae cling to smooth boulders as water shears over them into pools of bubbling water. Water striders skate on the water surface in calmer backwaters.

A cursory sampling of rocks in the river revealed a diversity of macro-benthic organisms. I spotted several species of mayfly, including rock-clinging Heptagenids (flat-headed mayflies) and the stone-building caddisfly Glossoma, all indicators of well-oxygenated turbulent flowing waters.chironomid larva

About 500 m from where we had sampled in the Credit River, we investigated a small tributary in the forest that led into the river. The creek obviously drained storm water runoff from the streets above; and, while the water was clear and contained riffles with a good flow, I found no macro-benthos on the rocks. Only blue-green algae populated the shoals. This was not surprising, given that storm water and street runoff generally contain contaminants (e.g., chlorides, heavy metals, organics, and oxygen-depriving materials) that susceptible organisms can’t tolerate.

What struck me was the deceptive nature of this contamination. Most of us, when we think of polluted water, envision a turbid stagnating watercourse with visible garbage, bubbling with toxic algae. The pollution in this tributary was invisible; so was the life. It reminded me that the face of pollution varies and ranges from the obvious—as with most organic enrichment—to the insidiously subtle—as with heavy metal contamination or acid rain.

black fly larvaWater holds many secrets; some good, some not so good.

Water is an introvert.



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Water Is-pBook COVERThis article is an excerpt from “Water Is…” (Pixl Press) by Nina Munteanu

On sale worldwide May 10, 2016




nina-2014aaNina Munteanu is an ecologist, limnologist and internationally published author of award-nominated speculative novels, short stories and non-fiction. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit for the latest on her books.

3 thoughts on “Secrets of the Credit River

  1. Great illustrations by Kerste! Have you read Thomas Hardy’s “Return of the Native”? He refers to the adult stages of some of these insects as “ephemeron,” especially short-lived mayflies.

    Liked by 1 person

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