Rock stack that kills aquatic insects
A tower of dried out river rocks that kills aquatic insects. credit: Elize Jekabson, City of Ypsilanti

Humans like moving rocks and like blocking moving water. It is something we all do as kids, and probably something that many people never grow out of!  And it is fine to stack a few rocks a little above the water line when you play in the water and see how you can get them to balance on each other, provided you knock them over when you are done having fun.

However, when you pull rocks out of the water and and stack them and leave them to dry, you are killing aquatic insects.  There can be 10 insects on every small rock in the river, and larger rocks can have hundreds.  When a stack gets as big as what we see in our picture, then we are talking about thousands of aquatic bugs that have died from suffocation or from being squished. River rocks are the primary homes for a variety of aquatic life: mayflies, stoneflies, and caddisflies in particular.  This is because such rocks are considered stable habitat; in an environment where getting washed downstream could mean death by fish, an insect wants to hold on and hide.

Erosion is also brought up as a reason why these rocks should be left alone, but honestly in a river situation, any rock a human can lift will be knocked over and brought back to the river bottom after the first storm or high flow event.  So let’s concentrate on the river life.  There is a place for the death of river bugs if we are talking about the food web or for limited scientific studies, but we shouldn’t be actively harming them in the name of fun.

If you see rocks stacked in the river, knock them over so others can start to get the hint!  To be clear, we are not saying that people should knock over rock cairns anywhere. There are ancient rock cairns that are graves and others that served as way finding structures. These ancient cairns are inland, not on rivers and should not be disturbed.

Bug Count Coming up

We do two River Roundup events per year and one is coming up on October 12. All are welcome to tap into their inner scientist by joining us in the field, searching for aquatic insects. This is a kid-friendly volunteer event and a fun way to get outside and explore wildlife. Learn more and sign up for our fall 2019 River Roundup here.


Check out the list of all the bugs we look for in our river and creeks

Curious about bugs? Here’s a fun blog about dragonflies by Paul Steen