What is stormwater runoff?
Stormwater runoff is rain, melted snow, hail, or sleet that doesn’t soak into the ground or get absorbed by natural materials, and instead runs across the land. The problem gets more complicated in developed areas where hard surfaces like roads, buildings and parking lots prevent water from soaking into the ground and being absorbed. Stormwater systems are in place to help funnel that excess water into a natural system that can accept larger amounts of water like streams, rivers and lakes, including Lake Superior.
What systems are in place to handle runoff in Minnesota?
The stormwater systems in some areas of Minnesota, like in St. Paul and parts of Duluth, have been around for a really long time. Prior to the Clean Water Act in 1972, people were typically routing all of the water away from their homes as quickly as possible to avoid flood damage without treating it first. Many old storm drains are funneled directly to lakes, rivers and streams. Newer developments are required to install pre-treatment practices like stormwater ponds, allowing heavier materials to settle, and capturing or filtering some of those nutrients, excess fertilizers, and pollutants coming off of the street before sending cleaner water downstream. Stormwater ponds and other stormwater management practices like these also help reduce flooding and erosion by reducing the volume of water sent downstream and slowing it down. With those older systems, most people don’t realize that what goes in that storm drain on the street in front of their home goes right to the river, or in Duluth, right into the lake.
What happens during the spring thaw?
When you look at a watershed, everything is drained toward that centralized body of water. Before we developed these areas, a lot of that excess water was getting captured and absorbed by things like the soil, plants and trees. Trees have a lot of surface area on their leaves, stems, and trunks that capture rainfall and snow, which is then absorbed or evaporated. Tree roots are also soaking up that water as rainwater and snowmelt filters into the ground. However, once an area is developed and has a large amount of impervious surfaces like buildings, streets and parking lots, there are fewer natural systems in place to absorb or slow the water. Instead, oftentimes it runs off the houses, streets and roads and gets routed directly into the stream or lake all at once through street stormwater systems. In newer developments, those treatment systems not only clean, but also capture and retain the water for a little bit. This means it can be more naturally absorbed and recharge the groundwater, and there isn’t a huge surge of water downstream.
What do near-record snowfalls mean for the spring thaw? What can folks do about that?
Heavy winter snowfall means more water will go into our water bodies when it all melts. A rough estimate is that 10-12 inches of snow will result in one inch of meltwater, depending on how light or heavy the snow is. That is quite a bit of water coming downstream, and any street debris that hasn’t been removed will be going with it. The debris might clog up storm drains, along with clumps of snow, and ice. If that debris freezes or otherwise clogs storm drains, heavy rain or snow melt can cause localized flooding. So it’s important to make sure the stormwater drains on your street are free of debris as things freeze and thaw. The same is true for the gutters on your home. A lot of times they can become clogged with snowpack or ice, so make sure that your gutters and downspouts are clear.
Why is street sweeping important after the spring thaw?
Street sweepers work really well at keeping debris and harmful materials out of our stormwater drains and local waterways. Snowmelt carries dirt, leaves, salt, and other debris with it, including material from overhanging trees whose canopies cover the roads and will often shed their flowers and bracts in the spring. So much of that ends up in the street gutters and the drains, then flows with the stormwater into the stream, river or lake. As the leaves decompose they release a lot of phosphorus. This can be problematic for the stream or lake because too much phosphorus often results in too much algae or excess plant growth. A variety of local and statewide studies have shown that street sweeping can be a really cost effective way of reducing the amount of phosphorus in stormwater and protecting our waterways.