Erosion occurs when soil detaches and moves downhill. This is often caused by flowing water, wind, gravity, or moving ice. Erosion can also occur when soil isn’t well covered. It is critical to prevent erosion, as it can take about 200 years to form one inch of soil! Loss of soil can be devastating to the natural landscape when it occurs on steep hillsides or along a stream. Such movement of soil can also cause damage around a home, compromising foundations, clogging drains, and dislodging entire gardens. Depending on where the soil goes, it can also lead to liability issues.
In Portland, the most easily eroded areas are in the West Hills. Most of the soil there was formed during the Ice Age following events known as the Missoula Floods, which would have covered Portland in as much as 400 feet of water, carrying soil and rocks from distant parts of Washington, Idaho, and Montana. After the flood water receded, mostly sand and silt remained, which was then carried by wind up into the West Hills.
Silt increases the likelihood of erosion in two main ways. First, silt is the most erosive type of soil due to its very small particles which are easily carried away by water or wind. These particles don’t bond together chemically, as do small clay particles. The windblown particles, or loess, that made their way to the West Hills contain more silt than soil, compared to other areas of Portland or the Willamette Valley. Second, the material blown into the West Hills buried all the soil that had been there before the Missoula Floods, creating a layer above the old soil that engineers call a slip plane. Rain and irrigation water seeps through the newer soil but the older soil is denser and less permeable, so the water saturates the new soil above the old soil. If there is enough water and not enough support (from roots or other erosion control measures) the slope can fail and slide downhill. These types of landslides have occurred for thousands of years. In modern days, we have seen these landslides cause homes in the hills to slide into one another.
One of the best ways to stabilize soil and hillside slopes is by planting native grass, shrubs, and trees. Their root systems, and the fibrous mycorrhiza fungus that attach to them, literally hold the soil in place. The roots can also create holes, known as pores, which allow water to seep deeper into the ground so that it doesn’t collect on the surface and wash soil away. The plants pull water they need from the ground, preventing soil in steep areas from getting too saturated and heavy. Find more information on plants to use for erosion control.
The roots of plants also pump organic matter, formed from the breakdown and composting of living material, deep into the soil. It is literally the glue that holds soil together. Organic matter also helps water seep deep into the soil while providing nutrients for crops, trees, and other plants in your garden. While mulches, composts, and other amendments help add organic matter to soil, plants are the most efficient way to get these nutrients deep into the soil and to help prevent erosion.