Imagine you are in the middle of a yard project when a heavy rainstorm rolls in, washing away some of the loose soil you had laying around. You see it travel down a nearby storm drain, which you know leads straight to a local waterway. It’s just soil, which is natural, so it’s not really a problem, right? Not quite! Sediment (loose particles of soil, sand, and clay) is considered a pollutant, and is the most common pollutant in US waterways according to the Environmental Protection Agency. Sediment often comes from construction projects, including small home projects, but it is also the result of erosion. Read on to find out how erosion and sediment runoff affect streams and rivers, and how trees can help!
Sediment may seem harmless, but it can negatively impact a waterway in a variety of ways. Sediment runoff often carries with it nutrients such as nitrogen and phosphorus, which can lead to harmful algal blooms in water bodies. Sediment pollution results in murky water, making it difficult for vegetation to grow, including beneficial algae that water organisms feed on. It can reduce visibility for fish, and end up clogging their gills. High levels of sediment change the entire structure of a waterway, with lower levels and altered flow. It can be costly, too – if the waterway is used for drinking water, sediment will cause water treatment costs to increase.
Along with seeing sediment wash down storm drains, you may have been walking near a stream and noticed the streambank falling away, maybe with some tree roots hanging over the edges. This is called erosion, and is a naturally occurring process as the pressure of running water wears away at the soil along its banks. Unfortunately, erosion can be accelerated by human factors such as increased stormwater runoff and replacing vegetation with grass. In addition to the erosion adding lots of sediment to the waterways, land that is often used for parks or agriculture is lost, and nearby structures such as bridges, roadways, and even homes can be put at risk as the erosion continues.
So, how can trees help with sediment pollution? They play a couple of important roles! Trees absorb rainwater, decreasing the amount of stormwater runoff that can transport sediment to the waterways. Less runoff going into the stream also means a lower risk of flooding, which causes erosion when water levels get too high. A mature White Oak tree can absorb over 3,600 gallons of rainwater per year, avoiding over 700 gallons of runoff! Imagine how much an entire forest can absorb. Trees have the added benefit of preventing erosion – as roots work to hold the tree in place, they also hold soil in place, working to stabilize stream banks and prevent them from being eroded.
Creating riparian buffers alongside waterways is a common form of green infrastructure used to help minimize erosion and pollution. This is frequently used for waterways near agricultural lands, where nutrient runoff from fertilizers is high, and there is a great deal of soil disturbance that can worsen erosion and sediment runoff. These buffers often consist of several different sections of forest. Mature, unmanaged forest is created alongside the banks of the waterway and extends at least fifteen feet. Mature trees help to create shade to lower water temperatures, stabilize banks with their roots, and provide habitat for native wildlife. The second section, which extends at least 60 feet, consists of managed forest. This area can be rows of trees and shrubs that are grown for nurseries and replaced after removal, and works to absorb runoff and filter out pollutants. The final section, of about 20 feet, contains grass or smaller plants that may be grazed or mowed. This portion serves to spread out concentrated streams of runoff, making it more usable for plants.
Obviously we don’t all have waterways on our property where we can create our own riparian buffers. But there are still ways you can help at home! Use mulch for gardens and around trees to help prevent any loose soil from washing away, and try to avoid doing any projects that disturb the soil if you know there’s a rainstorm coming. Install a rain barrel to reduce the amount of stormwater runoff from your roof. Sweep your sidewalks instead of hosing them off, disposing of any sediment in a trash can or compost pile. And of course, plant a tree! Trees will help to keep soil in place, and absorb stormwater so there’s less of it running off your property and taking sediment with it. Nearly everything we do has an impact on our waterways, so it’s important to make sure we’re doing what we can to keep them clean and healthy!