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Abstract: This paper examines the geomorphological and sedimentological development of blocked-valley lakes in the Mkuze floodplain on the coastal plain of Maputaland, northern KwaZulu-Natal, South Africa. Blocked tributary valley lakes north of the floodplain become progressively shorter, broader, and less linear toward the eastern (downstream) end of the east–west oriented Mkuze floodplain. Clastic sediment forms surface sedimentary fill in tributary valleys in the west, while peat predominates tributary valley fill in the east. Two contrasting adjacent tributary valleys were examined, the more western Yengweni dominated by clastic sediment at the surface, and the more eastern Totweni with peat. The Mkuze floodplain is characterised by silt with a low organic content. Surface sediments fine downstream and with distance from the main channel. Tributary sediment south of the lakes (adjacent to the floodplain) contains little organic material at the surface, but increases with depth. North (upstream) of Yengweni lake, the tributary valley contains peat up to 1.5m thick, with organic contents up to 30% (generally 10 to 20%). In contrast, north (upstream) of Mpanza lake, peat up to 7m thick is extensive with high organic contents (typically >60% at the surface but decreasing with depth). The thickness and width of the peat deposits increase longitudinally from the head of the tributary valley toward Mpanza lake. The distribution of clastic and organic sediments illustrates that as aggradation of the Mkuze floodplain progresses, tributary valleys initially fill with sediment from the local tributary catchment, lakes form, there is a phase of peat formation and finally, peat is buried by sediment from the Mkuze floodplain. We hypothesise that peat formation in subtropical and tropical settings through these processes is likely to be an important long-term sink for carbon. [Copyright &y& Elsevier]