BOTANICAL ENCOUNTERS: The Okavango Delta - The Wetlands

Text and photos by Ida K. Rigby, for Let’s Talk Plants! October 2022.


Botanical Encounters 4


Before we make the transition from our last column, which featured the dry mopane woodlands, to the Okavango wetlands, we’ll begin with photos that remind us of the savannah we visited. The airport consisted of a ramada and an emergency fire station. Before we taxied for takeoff, elephants crossed the airstrip (usually it’s zebras that have to be chased off before landings and departures).

As you fly into the Okavango Delta, lagoons, channels and islands appear beneath you. The land turns from tan and gray to green and blue.

It’s all still the Kalahari sands but submerged by a seasonal flood originating in the Angolan highlands. The contrast between the above top two photos (taken in 2016) and the bottom two (taken in 2019) demonstrates the difference a few years of drought have made. The thin lines crossing the plains are well trodden elephant paths leading to water holes.


I visited the Delta each August from 2014 through 2019; over the course of those years the effects of global warming became starkly visible. The “island” on which our lodge was situated was no longer an island. Where there were six feet of water surrounding it only two years before, there were only inches of water and a dry flood plain. We had always approached our camps in the heart of the Delta via motorboat. Each year the boats became smaller with flatter and flatter bottoms, motorboats were replaced by boats that had to be poled until, in 2019, we arrived overland by four-by-four. Our hostess reported that she had encountered a lion in the main area of the lodge a few weeks before as she started her day at 4 a.m. Lions do swim from island to island but are not inclined to (elephants freely swim between islands). She saw the lion, retreated, but thought she was just sleepy and had been seeing things. She returned down the trail thinking “there are no lions on this island,” forgetting that this year it was not an island. He was still there. Fortunately, he was more interested in finding a lioness who was also in camp, so ignored her. All activity stopped as the lion presided over the main gathering area well into the morning.


In 2019 three events conspired to alter the ecology of the Delta. First there were sparse rains in Angola’s western highlands where the Okavango River originates. Ordinarily on its journey southward towards the sea the Okavango joins other rivers in Namibia (the Caprivi Strip) and Botswana but is blocked by a long fault. Its waters back up and spread out creating a wide delta with ever shifting wetlands. After a few months, the waters subside as they retreat northward and sink into the Kalahari sands. By September, the flood plains become grassy open spaces. The second event was the February 2019, 6.2 magnitude earthquake. Botswana is covered by a deep layer of Kalahari sand, which absorbed the shock waves so there was no damage. The main fault, however, lifted and tilted the land northward, so water could not flow as far south as before the quake. Third, there was the ongoing severe drought in southern Africa, which can be attributed to climate change.


There are certain sounds that belong to a place. In the mopane woodlands it’s the call of the Cape turtle dove (BOTsWAna, BotsWAna or Work HARder, Work HARder, or Drink LAger, Drink LAger). In the Delta it’s the cry of the African fish eagle.

Fish eagles perch high in dead trees, and their huge nests make large silhouettes against the sky.


The best way to experience the Delta is silently and by mokoro. Mokoros are traditional dugout canoes.

They used to be carved from large logs; now they are molded from fiberglass. The poler’s pole is forked for a better grip in the sand. On our morning excursions we passed low-lying islands, some had joined together as they grew, and others were now joined because the water was so low.

The core of each island is a termite mound (termitaria).

(Note the large fish eagle on a nest in the bare tree to the right of the termite mound.)


As we peer into the core of an island from our mokoro, we look across water grasses, to the ferns on the margins of the island, into low-lying shrubs and date palms on the island’s edges, and between the water berry trees to the central termite mound. These “common termites” cultivate fungus gardens. Passing birds and mammals, including elephants and baboons, deposit seeds, around them. The seeds germinate, and small ecosystems develop around the expanding termite mound. Island trees include strangler fig, jackal-berry, water berry, African ebony, sausage tree, knob thorn acacia, sycamore fig, water-fig, African mangosteen, and various palms. Tall, spreading jackal-berry trees become “mother trees” as do the white barked sycamore figs. They are home to birds, snakes (which eat birds’ eggs and chicks), baboons, monkeys and insects. In the shallows beneath their spreading branches crocodiles lurk waiting for chicks ft fall from nests. Bream waits for falling fruit. Crocodiles snap up the bream.


The water berry trees, Syzygium cordatum, have many uses. Monkeys, birds, bush-babies, and human children enjoy their acidic fruit. Human adults make an alcoholic drink from the berries. Powdered water berry bark is used to poison fish. The bark, leaves and berries are used to treat many ailments including intestinal, fungal and respiratory illnesses, sores, burns, tuberculosis and malaria. The jackal-berry, Diospyros mespiliformis, is used to make canoes, spoons and as an antibiotic to kill internal parasites. Its fruits are eaten fresh or dried. Young twigs are used as toothbrushes.


Our mokoros followed the channels made by hippos and elephants whose movements through the Delta create drainage channels.

Elephants are a keystone species. They open up the dens of papyrus and reed islands and drop palm nuts to create palm tree highways. The elephant and hippo activity is necessary to maintain a healthy ecosystem (much like beaver’s dam building in North America) and keeping water flowing. A knob thorn acacia (Senegalia — or Acacia—nigrescens) blooms at the right of the hippo channel. Sedges, Cyperus papyrus, ...

... the giant, perennial Phragmites australis (termed a grass/reed on the PlantzAfrica website), bulrushes, Typha capensis, and grasses, Miscanthus junceus, towered over us as we navigated the channels.

Indigenous people used reeds to weave mats and for the sides of houses. Grasses were used for thatch. Phragmites australis helps filter water and holds soil and sand from washing away in flood times. Its invasive rhizomes spread quickly to create large islands, which offer protection for birds and animals. It’s used for thatching roofs.


The rhizomes of the Cyperus papyrus create impenetrable, island-like rafts loosely anchored by roots. Papyrus is remarkably fast growing, about three months from shoot to seed to death. The tangled dead stems form part of the raft, and the nutrients from decaying, matted stalks feed the new shoots. Papyrus also has two adaptations to help it grow so tall and in such large masses in the nutrient poor sands of the Delta. Papyrus stalks have a nitrogen fixing bacteria between their scale-leaves and also have a very efficient pathway for photosynthesis to compensate for the lack of soil nutrients.


Navigating the Delta in a mokoro gives you a water level view of the tiniest phenomena. Water chestnut, bladderworts and water lettuce, Ottelia ulvifolia, snow lilies, Nymphoides snow lily, ...

... and the pink vlei ink flower, Cycnium tubulosum, a straggly, hemiparasitic plant growing out of the edges of papyrus mats ...

... all pass by at arm’s length. The tiny painted reed frog ...

... about an inch and a half long makes a sound like a tinkling bell. The umbels of the papyrus attract the diminutive malachite kingfisher ...

... and a papryrus stalk is a good perch for spotting minnows.


Ida Rigby.
Dragonfly larvae ascend water lilies when they are ready to molt.

The waters of the Delta are home to both day and night water lilies. The day lily, Nymphaea nouchali caerulea, blossoms first open violet blue and turn white as they age.

The blossoms are insect pollinated. Water lily rhizomes are a nutritious carbohydrate, ...

... and the hollow stem of the blossom lends itself to being carved into a necklace.

On one mokoro excursion our water level elephant viewing started peacefully enough. Sequestered in the reeds and grasses we watched a breeding herd cross a shallow lagoon (which in other years would have been many feet deeper). Suddenly, the matriarch mounted a charge to drive us away.

She advanced with flaring ears and stomping feet, splashing up fountains of water. Junior thought this a great opportunity to strut his stuff too, so he made himself large and thrashed about. The matriarch decided to flee and led the herd in a mad dash along the edge of an island across the channel, and they all disappeared into a low area to seek tasty grasses and shrubs ...

... away from prying human eyes. We retreated into a narrow hippo channel.


The end of most days found us somewhere on a lagoon watching the changing colors behind the silhouetted Delta islands reflected in the darkening waters.


 

Ida Rigby is a past SDHS Board member and Garden Tour Coordinator. She has gardened in Poway since l992 and

emphasizes plants from the northern and southern Mediterranean latitudes. Her garden received the San Diego Home/Garden Magazine Best Homeowner Design and Grand Prize in their Garden of the Year contest in l998. Her travels focus on natural history.