Flood Control

Wetlands are a valuable part of flood control. Wetlands can soak up and hold a lot of water after heavy rains.

There are two ways wetlands hold water…

bowl31. THE SHAPE
Many wetlands are found in low spots left in the ground by glaciers that covered the land 15,000 years ago. These “kettles” are like big shallow bowls in the earth, and this shape is what keeps the water in Lincoln Marsh and other wetlands.

Underneath most of these bowl-shaped kettles are layers of hard packed clay, a very tight soil that doesn’t let water move through and down into the ground and rock below. Wetlands hold large amounts of water above and below the soil surface until plant growth and warm weather draws the water out through evaporation and transpiration. Some wetlands become dry in late summer.

soils2. THE SOILS
Wetlands are FULL of plant life! And each year, most of these plants die back for the winter. That’s when all the leaves and stems of the plants fall to the bottom of the wetland and start decomposing. This has been happening every year for thousands of years. So, the soils in wetlands, called muck or peat, are made from 10,000 years worth of dead plants!

What makes a wetland soil different from a regular backyard soil?
Well, there really isn’t such a thing as a “regular backyard soil.” Scientists are familiar with more than 7,000types of soils, and these can be mineral soils or organic soils.

soilproMineral soils are made mostly of ground up pieces of rock, with some organic matter mixed in the top layer. Organic soils are made almost entirely of decomposed plant material. Wetland soils have O and A Horizons that are very deep. Often when you dig way down in a wetland soil, you will find stems and leaves of plants thousands of years old. They are still there because there is no oxygen to support decomposition.

At left, a soil profile showing the different layers of soils at an undisturbed site. The darkest layer at the top is the O Horizon which contains very high amounts of organic matter. The A Horizon is usually organic matter, from dead plants and roots, mixed in with the mineral soil. The B Horizon has low amounts of organic matter and the mineral soil is usually oxidized. The C Horizon is made of the soil “parent material.”

(Illustration from the USDA, Natural Resources Conservation Service.)

For more interesting facts about dirt, visit Dirtland at the Microbe Zoo!
(A Michigan State University website)

Particle Sizes
If you compared clay particles to sand particles it would be like comparing a BB or a tiny glass bead to a very large beach ball; there is a very big difference in particle sizes, and this affects how water behaves on top of or inside the soil.

Mineral soils can be classified according to the size of the mineral particles. This is called soil texture. The largest particles of a mineral soil are called sand. The medium-sized particles are called silt, and the smallest particles are called clay. Mineral soils that have different amounts of sand, silt and clay can be very easy to tell apart from each other. Sandy and silty soils are often loose, soft and crumbly. Clay soils can be as hard as a rock.