Stabilizing the soil on the slope is the answer. Soil in these areas is usually lacking microorganisms and nutrients or suffering from compaction. There are ways to control a slope:
Baffles or barriers are obstruction devices that slow down or divert water from flowing directly downhill. They consist of partially buried stone or timbers (laid parallel to the slope). These barriers work best for lesser slopes.
Riprap is rough, loose stone (at least 6-8 inches wide each). Usually granite, the stone is imbedded into or spread loosely onto the slope. Riprap slows and diverts flowing water. It is effective but can appear stark or harsh in some landscape designs. To soften the impact, you can plant the areas between the stones with a variety of ground covers or rock garden plants.
Terraces stair-step up the slope. The flat surfaces allow you to plant on the terraced levels. Terraces allow water to soak in instead of running off. Use timber, stone, concrete or precast concrete block to build the retaining walls. Start at the bottom and fill in the level above with soil from the area just leveled (a technique called "cut and fill"). Terraces should slope a bit (about 2% is recommended) to prevent water from collecting at the back of the terraced portion.
Remember that mortarless retaining walls can only reach a certain height (usually around 2 feet). Check the specifications of the product you are using for height restrictions. Also remember to backfill the area behind the wall with crushed rock to ensure drainage. If you build a terrace around existing trees, make sure the soil level is not raised. Covering the roots too deeply can damage the tree.
Plants can help control slopes. You can plant any of the slope control methods above or you can use plants alone. When plants are established, the roots help anchor the soil. However, getting them established on a slope can be difficult. Seeds and mulch wash away and planting holes erode before the plant gets established. Wildflowers, clump-forming ornamental grasses or other perennial native plants usually adapt quickly to slopes and unimproved soil. You can also adapt rocky soil for a rock garden.
If you choose to provide irrigation for a planted slope, make sure that the system's water pressure is adequate to water the entire area. Some recommended landscape plants for slopes includes ajuga, barberry, cotoneaster, daylilies, forsythia, ivy, juniper, liriope, pachysandra, sedum and vinca.
Temporary remedies include: plastic sheets, straw bales, mesh, silt fences and mulches. These are short-lived, as most will biodegrade or wash away over extended periods of time.
Turfgrass can control erosion on minor slopes if the grass is healthy. A grass such as annual rye can germinate quickly and help stabilize soil while perennial grasses get established. Compacted soil is a major contributor to runoff. Aerate if possible, and add organic matter such as compost to promote a healthy stand of grass. A covering of straw can help secure the seed and reduce water runoff. An excellent method of starting turfgrass on slopes (or anywhere) is by using a seed germination blanket. The blanket is a combination of biodegradable material and wood fiber that holds seed and fertilizer in place until grass seedlings can take root.
Getting grass seed to stay in place until it germinates can be tough on steeper slopes. On some slopes you may need to resort to hydroseeding by a landscape contractor. Severe slopes, especially those that lead downhill towards a structure, require the attention of an engineer or landscape architect. Check local building codes before starting a major project.