Incredibly rich farm land, flood plains appear along river banks and, therefore, produce one extra Food.
Units stationed here can be damaged by disease, as can the population of cities, until the discovery of Sanitation.
Deserts are often very rich in their soil composition; it is only their lack of rainfall that prevents their soil from being used. When rivers run through such areas, the periodic flooding usually carries the nutrient rich soil up into the flood plains surrounding them, depositing the nutrients and the water into these areas and making them unparalleled in value. Many great cultures such as those in the Nile river basin or those dependent on the flooding of the Tigris and Euphrates rivers have harvested the bounty of these lands, plains which transform lifeless wastelands into lush oases of life.