Silty Soil is usually found in river delta areas...that area of the river just prior to entering a larger body of water, such as a lake or sea. These are the areas that are most likely to flood at regular intervals. Over the years the river, slowing it's onward rush to the ocean, will deposit much of the solids being carried in the water over these areas. As the fine soils and minerals are layered over time, this makes for very rich and fertile land.