Aquifers in the sand alluvium of the broad riverine flats (fadama) of the southern Sahel and northern Sudan climatic zones of West Africa are a major source of water for irrigation essential to crops in bad years. Annual recharge is from rivers flowing seasonally into the Sahel from the wetter south. Landforms which evolved in the mid Tertiary control the modern drainage. From the Pleistocene onwards rivers vigorously eroded the crystalline highlands of the Jos and Air massifs and carried the coarse alluvium far downstream. The resulting aquifers are often several kilometres wide and mostly over ten metres thick, sometimes much thicker, and are capped by late Pleistocene loessic soils. Aquifer properties are typical of the mostly coarse unconsolidated sands. The alluvial groundwater is reached and abstracted by methods appropriate in scale to family size farms. The system of farming and irrigating is socially beneficial and economically viable. The paper concludes with case studies of three rivers of Bauchi State in Nigeria. The alluvial aquifers are evaluated and related to irrigation potential, expressed as the percentage of land which may be irrigated using only the groundwater beneath it.