The name dambo comes from central Africa, but almost every region south of the Sahara has its own word for the same idea. In Mashonaland they say matoro. In East Africa, mbuga. In South Africa, vlei. In Nigeria, fadama. In Sierra Leone, bolis. French colonial scientists called them bas-fond. Germans proposed Spültal. The fact that so many cultures developed names for this kind of ground tells you something: grass-covered, seasonally waterlogged, rimmed by forest and branching like a river - dambos are one of the defining features of an entire continent. Seen from above in Zambia's Luapula Province, a headwater dambo looks exactly like a river delta with the water turned down low, the grass tracing what the water would do if there were more of it.

Where a river hurries, a dambo waits. Rainfall soaks into the hills around a shallow depression and then moves slowly, sometimes taking years to percolate through the rock and emerge as a spring or a soggy patch of sedge at the dambo's centre. This lag is why hydrologists studying Lake Mweru Wantipa and Lake Chila near Mbala have sometimes been puzzled by water levels that do not match the previous season's rain - the answer was buried in the hillsides, running on dambo time. During the wet season the grass is inundated but rarely submerged above its tops, and by the end of the dry season the surface may be cracked grey soil or black clay. Yet the lowest drainage lines stay damp the whole year. The dambo never fully empties. It is a sponge with a long memory.

For the people who live around them, dambos have always been the reliable part of the landscape. When the rains fail and the upland fields wither, the dambo's deeper soils still hold enough moisture to grow vegetables. Rushes cut from the margins become thatch and fence. Clay from the edges is dug for bricks and earthenware. Bitter cassava is soaked in ponds hollowed out of the wet ground to leach away its poisons. Fish traps line the small central streams. Hunters come for birds and small antelope that drink here. More recently, fish ponds and upland rice have been added to the list. The science of farming dambos is still catching up with the practice - their soils and hydrology turn out to be stubbornly variable - but for drought years, they remain what they have always been: insurance written in grass.

Flying over the plateau south of Mansa, the contrast is almost diagrammatic. Cleared farmland and charcoal-burning have opened up the surrounding country, but one protected forest reserve still holds its miombo canopy intact, and through it a dambo winds like a pale green vein. Headwater dambos tend to branch like the fingers of a hand, each arm roughly the same width, each joining the next at a gentle angle. Further west, about 100 kilometres northwest of Mulobezi, a different kind called a pan dambo sits like a dark eye in the grassland - a closed basin where water collects and evaporates, the centre burnt black by late-season fires. Five recognised types have been mapped across Luapula Province alone: upland, valley, hanging, sand dune and pan. Each one does something slightly different with the water, and together they are the reason so much of Zambia is green when it has no business being green.