In the week following July 5 and 6, local communities began inventorying the damage to streams, stormwater systems, and construction sites caused by 4-7 inches of rain falling in less than 24 hours. Some weather stations in southeastern Washington County reported 6 inches of rain on July 6, while Lake Elmo, Stillwater and Woodbury reported 4-5 inches each. In River Falls, WI, a kayaker attempting to paddle in the swollen Lower Kinnickinnic River spent an hour perched on a tree branch in the center of the stream before she was eventually rescued by a sheriff’s deputy and a River Falls firefighter in an airboat Monday evening. The river was roaring at least twice as wide and three feet higher than average that day, and witnesses say the woman fell out within minutes of launching on the river. Smaller streams in Washington County experienced similar flash-flooding as well.
Throughout the day on July 6 and 7, water monitoring field staff from the Washington Conservation District surveyed and reported on the post-rain carnage around the county – washed-out culverts, eroded gullies, and streams running chocolate brown. At a construction site in Lake Elmo, about 100,000 gallons of untreated stormwater overflowed into a newly installed pipe, discharging to an intermittent stream that eventually connects to Wilmes Lake in Woodbury. Trout Brook, which flows through Afton State Park to the St. Croix River, was swollen and muddy on Monday, and monitoring staff could see that the water had been even higher earlier before they arrived. The one spot of good news among the week’s inspection reports were the many newly installed raingardens around the county that were full but functioning as designed.
Why do heavy rains cause so many problems and why are streams affected more than lakes and wetlands? The answer has a lot to do with where the rain goes once it lands on the ground. In a natural system like a forest or prairie, about 40% of the rain that falls is either captured by plants and trees before it lands or taken up by their roots. Approximately 50% of the water soaks into the ground, some filling shallow groundwater resources that feed wetlands, lakes and streams, and some percolating deeper down into aquifers. Only 10% runs off the land and into rivers, lakes and streams. In contrast, our built environments have much more impervious surface – places like rooftops, driveways and roads where there are no plants and water can’t soak in. Additionally, ditches and stormwater pipes carry water off the land more quickly and into streams and waterways.
Depending on how densely an area is developed, we often see 20%, 30% or even 50% of the rain running off instead of soaking into the ground or being captured by plants. More runoff leads to more frequent flash flooding and more erosion in rivers, streams and gullies where the water is concentrated and flowing fast. Wetlands and lakes tend to be more resilient because there is a larger area for the rainwater to slow down and spread out and more opportunity for the water to either evaporate or soak into the ground.
There are many ways we can reduce the damage caused by heavy rains and plants play a central role in most of these strategies. A buffer of deep-rooted plants along a river, lake or stream helps to stabilize the soil so that it doesn’t erode when the water rises. Preserving or recreating natural floodplains is important as well to spread out and slow down the water. Plants also help to stabilize the soil in highly erodible locations such as steep hillsides, farm fields, and construction sites. One common best management practice for agricultural areas is to create grassed waterways by planting natives or hearty perennials instead of crops along the drainage pathways that rain naturally follows. On construction sites, temporary seed mixes can be used to quickly cover bare soil until building begins. Grading smaller areas at a time also reduces the risk of major washouts.
Climatologists warn that our weather patterns in Minnesota have been changing and larger storms with heavier rain are becoming more common. Finding ways to adapt to these deluges will help us to protect the health of our water resources and reduce the human impacts from flash flooding as well.