The diverse nature of iron-using societies in the semi-arid tropics of West Africa is illustrated by the very different sequences of growth and change in the floodplains of the Middle Senegal Valley and the Inland Niger Delta during the first millennium A.D. The Inland Delta is characterized by a pattern of rapid population growth over the course of centuries-long occupation in very large, high-density settlement mound clusters. The Middle Senegal Valley has low-density occupation in fairly regularly spaced small villages. These differences are due, in part, to differences in the size, distribution of critical resources, and nature of subsistence scheduling conflicts and opportunities in the two floodplains. Faced with similar risks to agriculture posed by chaotic variation in the timing and extent of annual flooding, societies on both floodplains have responded in broadly similar ways historically by developing subsistence specializations that are integrated through exchange. Despite the large scale and considerable integration of first millennium societies in the Inland Niger Delta, the clustered settlement pattern that persisted even through the period of maximum population density at around A.D. 1000 indicates that centralizing pressures did not succeed in producing an effective administrative hierarchy at that time. Rather, various models of heterarchical organization provide a better “fit” for the archaeological data currently available. In the Middle Senegal Valley, on the other hand, the small and relatively dispersed nature of settlement around A.D. 900 should not immediately be assumed to indicate simple, autonomous, village-level organization. Here, the possibility that complex territorial units composed of overlapping territories of subsistence specialists had already emerged in the first millennium deserves consideration. As archaeology works to counterbalance its long-standing focus on vertical hierarchies as the primary modality of complexity, the archaeological sequences in these two African floodplains offer insights into the various ways that specialization, trade, population heterogeneity, and even urban growth may have emerged and flourished relatively unconnected to the development of social stratification and centralized power.