The ephemeral rivers – A passage to the coast

Text Dr Conrad Brain, Environmental Scientist

Namibia’s ephemeral rivers from the Kuiseb northwards all have one thing in common – actually a few more, but for the sake of this article, I am concentrating on the one common denominator that has been overlooked. It is neither through ignorance nor lack of research that this one feature has barely received mention; it is probably because it is so obvious. The rivers all end at the sea.

Tsauchab River

The Namib coastline is arguably one of Africa’s most inhospitable, barren and impenetrable barriers, both from a seaward and an inland perspective. Indeed the Namib Desert, while coast-hugging and narrow, is home to some of the most hyper-arid adapted life forms, specifically of the plant, insect and reptile varieties. This hyper adaptivity only highlights the extreme conditions that exist in this narrow belt of desert. A phenomenally steep climatic gradient sweeps across this belt so that in close proximity to life forms adapted to live on minute droplets of fog are creatures that drink 200 litres of water in one go. The seaward side of this gradient could hardly be more different than the inland side and yet the life forms of the two extremes are linked. Occasionally these two extremes merge in such spectacular fashion that when photographically displayed, the images elicit gasps of appreciation.

However, the link between the two, between hyper-arid, windswept, fog driven and the relative opposite is complex, catchment dependent, narrow and has a gradient of its own. This physical gradient starts west of the escarpment and ends in the Atlantic Ocean. Drainage lines converge so that eventually catchments feed into extremely narrow gorges that incise and traverse the Namib. These incisions defy the surrounding desert and within their confines temper the extreme climatic gradients of the desert. Below their usually dry sandy surfaces course large quantities of life-giving inland waters. This water is taken up by the roots of plants that are not really desert plants at all. This phenomenon has given rise to the appropriate name of linear oases for these narrow incisions in the desert.

Wildlife in linear oases

It is from these linear oases that we see emerging into the desert the most unexpected creatures. From within these confined yet diverse ecosystem we see wildlife venturing, albeit briefly, into the oldest desert in the world and its westward boundary, the Skeleton Coast – lions on the beach, a kudu in the waves, giraffe and zebra against the Atlantic, ostriches in ocean spray and an elephant with the cold Benguela Current as backdrop. These are brief excursions into another zone before they must retreat to the shelter of the oases that enabled them to venture so far in the first place.

These creatures from the oases bring new and unexpected diversity to a coastline seemingly unfit for any form of non-specialised and adapted life. Their forays into the desert and onto the coast are recorded by the most ancient and most recent of human inhabitants along this unforgiving coast. The animals themselves have on occasion left their marks; elephant footprints in the hardened silt, rhino remains in the gorges and leopard lairs in the ocean mist.

The more recent of these recordings through international exposure have impressed the world to such an extent that this section of coast is a most sought-after global destination for the adventurous and enquiring tourist and scientist: It enables them to witness a process of unusual proportions, one that has been happening for millennia, and that is under serious threat.

Subsurface water

The temporary movement of wildlife from the confines of the ephemeral rivers into the coastal desert is indeed totally dependent on the watercourse itself. While periodic surface flow charges briefly down these sandy rivers, it is actually the subsurface water that is vital to sustaining a riparian flora. Surface water flow acts as a brief recharge for subsurface water and can in some ways act as an indicator of actual river health. But it is the plants themselves in these linear oases that are the true indicators. When the subsurface water table subsides, so the roots of the plants are left high and dry and the river dies. Once continuous oases retreat into pockets of life and then disappear completely.

Massive subsurface water abstraction, far beyond recharge capacity, cause vast sections of ephemeral river vegetation to simply die and disappear and with this retreats and dies the dependent ecosystem. Because water is supplied to the ephemeral water system from the east, so will an overextension of what the river can supply be noticed first on the opposite pole, the western extremes. Likewise, obstruction of periodic surface flow by whatever means will impact the furthest reaches of the river most, again the western regions. Fresh-water wetlands formed deep in the desert or even on the coast, fed by ephemeral surface or subsurface flow can become critical indicators of water overextension of the river and their decline is but a symptom of an entire system in collapse. One other symptom is the retreat of those unlikely creatures from the coast – unable or unwilling to make those ever-longer treks into desert and coast. With them retreats one of the most fascinating phenomena of our Skeleton Coast: the appearance and interaction of creatures in an environment in which by all counts they should never occur.

So while ephemeral rivers undoubtedly form many more important links with our coastal environment, their integral role in supporting and supplying an unusual cohort of wildlife on our northern coast is significant – for diversity, as part of a predator-prey system and for sheer unparalleled beauty. But on occasion, even the lion on the beach spectacle may be eclipsed. The water itself, the chocolate-coloured and textured flow of life, bursting through the desert and emptying into the sea, is a creature in itself, living, writhing with a soul and a purpose – sweeping a channel, dispersing seeds, stimulating germination and feeding the coast with sand, silt and nutrients. A must-see rejuvenating experience.

This article appeared in the 2009/10 edition of Conservation and the Environment in Namibia.