Erosion occurs when soil particles detach and move around-usually caused by water, wind, gravity and even ice. It’s usually because the soil isn’t covered well enough. Erosion is something to avoid, especially when you consider that it takes about 200 years to form one inch of soil! Erosion can be devastating to the natural landscape when it occurs on steep hillsides or along a stream and it can cause damage around a home as well, compromising foundations, clogging drains, and dislodging whole gardens. Depending on where the soil goes – it can also lead to liability issues.
In Portland, the most erosive 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 is as much as 400 feet of water carrying soil and rocks from distant parts of Washington, Idaho and Montana. After the flood water receded, it left mostly sand and silt, which was then carried by wind up into the West Hills.
Silt causes two main problems. First, silt is the most erosive type of soil because the particles are so small that it’s easily carried away and it doesn’t bond together chemically like small clay particles. The windblown matter, or loess, blown to the West Hills has more silt than soil in Portland or the Willamette Valley. Second, the material blown into the West Hills buried all the soil that was already there and the layer between the old and new soil created what engineers call a slip plane. Rain and irrigation water seeps through the newer soil but the older soil is less permeable so the water collects at the slip plane. If there is enough water and not enough support (from roots or other erosion control measures) the slope can fail. These types of landslides have occurred for thousands of years and most of us have seen the media pictures of homes sliding into each other in the West Hills.
One of the best ways to stabilize soil and your slope is by planting 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 into the ground so that it doesn’t pond on the surface and wash soil away. The plants pull the water they need from the ground, preventing soil in steep areas from getting too saturated and heavy. For more information on plants to use for erosion control, click here, and check out our Library for other erosion information.
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 to seep deep into the soil while providing nutrients for crops, trees and even ornamental plants in your garden. While mulches, composts and other amendments help add organic matter to soil, plants are the most efficient way to get it deep into the soil and help prevent erosion.