The Loess Hills of western Iowa are made of wind-blown soils that are highly susceptible to erosion unless protected by a patchwork of conservation practices, so farmers place multiple practices in specific areas of their farms to reduce soil loss. Terraces are built on hillsides to shorten slopes, slow rainwater, capture it, and reduce its erosive power. In western Iowa, almost all terraces are constructed on the contour around a hill, a practice unique to the Loess Hills because the deep loess soil allows captured water to soak away quickly and prevents crop loss from standing water, unlike terraces elsewhere in Iowa that typically use intakes and underground outlets to move water downslope through drains. Iowa has four types of terraces: broad base, narrow base, grassed back slope, and basin. Broad-base terraces usually carry crops on both sides and are built on fairly flat slopes of 6 percent or less. Narrow-base terraces, also called A-frames, have steep sides, are the most popular style with farmers, are seeded to grass on both sides, and are never farmed. Grassed back slope terraces are farmed on the upper side and seeded to grass on the lower side. Basin terraces are built on non-cropland such as pastures where livestock graze and are entirely seeded to grass. Farmers also plant grassed waterways where water flow concentrates into rills or gullies, using grassed, cup-shaped channels in western Iowa to carry concentrated water safely from fields. Other conservation practices include water and sediment control basins, dams, and contour buffer strips that slow rainwater, catch sediment, and help protect water quality. In the Harrison County Soil Conservation District, organized in 1942, farmers used plows to build 15 miles of terraces, some of the first in Iowa, and 15 years later conservationists used heavy equipment to build 263 miles of terraces within the district. On December 7, 1953, the Harrison County Soil Conservation District Commissioners, other agricultural agencies, and area farmers organized the Harmony Creek Watershed Association, and four years later the Harmony Creek watershed became Iowa's first federal watershed project under Public Law 566. Since 1957, conservation construction has expanded to hundreds of dams and many thousands of miles of terraces adapted for modern 16- to 24-row farm equipment, while adoption of no-till and other advances helped reduce erosion from 60 to 70 tons of soil per acre per year in 1942 to 4 to 5 tons per acre per year.