« Sustainable Urbanism | Main | The Chicago Green Alley Handbook »

Forests and Water Quality

(note: This is the unedited and un-shortened version of an article I worked on for the Spring 2008 issue of "Streamline", a newsletter for residents of the Lester-Amity watershed published by the Natural Resources Research Institute; this and other issues are on the Lake Superior Streams website.)

Quick: how many trees are in the Lester-Amity rivers watershed? Give up? Me too, but if the first thing that came in to your head was “a lot!�?, you’re on the right track. Most of the land in the watershed is forested, with trees covering approximately 71% of the Amity Creek watershed’s 10,533 acres and 63% of the Lester River’s 22,773 acres (that’s almost 22,000 acres of forest), and this is clearly the dominant land cover. As it turns out, that’s a pretty good thing for water quality in the creek, too.

Trees play a critical role in protecting and maintaining water quality all around Lake Superior, and do many things that we don’t always recognize. During rainfall, a forest canopy can capture up to 30-50% of a typical rain, and hold it on their leaves or needles until it evaporates. The floor of a forest, with all the undergrowth and decaying leaves, can hold a significant amount of water as well, trapping 3 times more water than a grass lawn can. The roots of trees also help break up the soil (particularly important with the clay soils so common in this area), and this helps water infiltrate down into the ground. During the spring, coniferous forests reduce and delay snowmelt runoff due to the shade their needles provide. All of these things help keep water on the land, and protect the stream from higher runoff volumes, which erode the banks and pull sediment into the creek.

Trees along the banks of the stream play a few other roles, as well. The shade provided by trees to the stream helps keep the water cool for the Brook Trout, which do best when the water temperature is between about 52-61 deg. F, and can’t survive when the water temperatures exceed 75 degrees F. The roots of trees along the banks help prevent erosion as well, keeping the water clean. The trees also directly provide food and habitat to stream organisms as well; leaves falling into the stream are eaten by many invertebrates, and can be the main source of food entering the stream. Trees falling into the stream are also both a source of food (some critters feed on the wood as it decomposes), and create habitat for fish by blocking high flows, creating pools and eddies, and providing cover from predators.

Research has actually found that if you remove too many trees from a watershed, peak stream flows increase, the amount of sediment in the water increases, and water temperatures increase. Trees are definitely a good thing for water quality.

So, along with rain barrels, rain gardens, and other things, add planting trees to the list of things you can do to help out water quality in the Lester River and Amity Creek. As part of the Weber Stream Restoration Initiative, NRRI planted hundreds of native conifers in the Amity Creek watershed. Coniferous trees are probably most helpful, but many varieties suitable for our area are available each spring through the South St. Louis Soil and Water Conservation’s annual tree sale (info online here: http://www.southstlouisswcd.org/tree.html); orders are due by April 15 this year. As you may know, Deer will kill many tree seedlings, but a good 5-6’ high fence can keep the deer away until the trees are tall enough to be out of reach. The best time to plant a tree is 40 years ago, of course, but the 2nd best time is today!