Terraces

Terraces are like speed bumps in a field. They effectively cut the slope in half to slow down water flow. Terraces control water runoff by either directing it to a grassed waterway, underground drainage system, or back into the soil. They are most effective on long, uniform slopes and are often installed in parallel to maximize water collection and movement.

Types of Terraces

There are two primary types of terraces: storage and gradient. Storage terraces capture runoff, allowing water to infiltrate into the soil or be drained through underground pipes, reducing phosphorus loss. Gradient terraces act as channels, guiding water to a stable outlet like a grassed waterway. Other common terrace types in Iowa include:

  • Broad base – Fully farmed and suitable for slopes of 8% or less.
  • Narrow base – Steep on both sides and planted with grass.
  • Grass backslope, farmable front – The downhill side is seeded with grass, while the uphill side is farmed.
  • Grass front, farmable backslope – The uphill side is grassed, and the downhill side is farmed, best for slopes of 6% or less.

Benefits for Water Quality and Soil Conservation

Terraces reduce slope length, minimizing the erosive power of runoff and limiting soil loss. By preventing erosion, they help keep sediment and phosphorus from reaching waterways. According to the Iowa Nutrient Reduction Strategy, terraces reduce phosphorus losses by an average of 77%. When used alongside no-till/strip-till and cover crops, they offer even greater benefits.

Advantages and Considerations

Terraces provide effective erosion control and eliminate the need for grass waterways, which can be difficult to maintain. They can also reduce localized flooding. However, they require an upfront investment and ongoing maintenance to prevent failures that could lead to gully erosion. Farmers can consult local Soil and Water Conservation District staff to determine if terraces are the right solution for their fields. Iowa State University Extension and outreach has done great work on this topic. Learn more about their work here.