How a Bag of Rocks Becomes a Valuable Scientific Tool

At a Glance

In 2018, as part of our water quality monitoring services contract with the Long Creek Watershed Management District (LCWMD), GZA took on the task of biomonitoring in Maine’s Long Creek. The Creek, which flows through South Portland and Westbrook, has experienced impacts on aquatic life and water quality due to runoff from impervious surfaces. Part of our work was assessing habitat and taking samples of the creek’s macroinvertebrates (bugs) to determine whether water quality standards are being met. Biomonitoring must be conducted in accordance with the Maine Department of Environmental Protection’s Methods for Biological Sampling and Analysis of Maine’s Rivers and Streams1 , which loosely translates to placing a rock bag in the stream to simulate the habitat these macroinvertebrates like to colonize.

The macroinvertebrates that colonize the rocks in the bags are used to measure the health of the stream.  Certain macroinvertebrates are highly associated with intact and unaltered streams, while others have a high tolerance for disturbed stream conditions.  The State has developed a statistical model that measures whether an assessed stream is meeting aquatic life use criteria by characterizing the macroinvertebrate community living in the stream as assessed through rock bag data. 

Not just any bag or any rock will do, however. A bag of rocks may sound easy to make, but the wrong rocks will impede wildlife and the wrong bag will make it impossible for them to make themselves at home. The bag must have a 1” aperture, a secure closure, and be able to hold 15-17 pounds of rocks, while the rocks have to be 1.5 to 3 in diameter and smooth. And as we quickly discovered, there’s no vendor making complete rock bags, so we were going to be doing it ourselves.

Luckily, thanks to GZA’s extensive relationship with Maine DEP, we were put in touch with Tom Lawrence of Lawrence Enterprises in Seal Harbor, ME. Tom, it turns out, makes these bags by hand to help get these surveys done. 

Next, we had to find the rocks, 384 pounds of them. We began calling landscape companies asking for these specific rocks, and, after many phone calls, found one that had outdoor bins of just the smooth rocks we needed. You can just imagine the strange looks we got as we brought out a scale, ruler, and some five-gallon buckets, setting up a weigh station beside the outdoor bins of smooth rocks. We hand-picked smooth river rocks and weighed out 16 pounds of smooth rocks 1.5-3” in diameter, dumped the rocks in a five-gallon bucket, then repeated this 23 more times – all the while the workers there wondering just why we are sitting in this pile of rocks, hand-picking each one.

Since the bags were delivered to the Portland office and the rocks were collected by the Bedford office, when the team met in South Portland on site, we quickly used a backseat station to re-weigh rocks to create our rock bags. The rock bags were deployed and a month later were carefully retrieved and the colonized macroinvertebrates (bugs) were collected and sent for taxonomic identification. It’s a good reminder that a scientific tool doesn’t have to be complicated, but it does have to be thoughtfully considered.


1Davies, Susan, P. and Tsomides, Leonidas (2002), Methods for Biological Sampling and Analysis of Maine’s Rivers and Streams, Maine Department of Environmental Protection, Bureau of Land and Water Quality, January 1987, revised August 2002.

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