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The Tana delta is the name loosely given to the flood-plain ecosystem of the lower Tana river, a vast wetland complex on the Kenyan coast. The delta is roughly triangular in shape, with its apex at Lake Bilisa (north of Garsen) and its base a 50 km stretch of beach along Ungwana (or Formosa) Bay, stretching from Kipini in the north-east to Mto Kilifi in the south-west. This low-lying area is bounded by higher land to the east and west, and to the south by a dune system bordering the Indian Ocean. It forms the interface between the river and the ocean, with fresh and brackish lakes and streams, freshwater and saline grasslands and wetlands, and successional stages of forest and woodland on the riverbanks and the dune ridges parallel to the shore. The mouth of the river has shifted many times. Today, the main stream of the Tana follows an artificial course, directly into an estuary at Kipini, rather than into the complex system of channels and distributaries leading to its old mouth at Mto Tana. Until recently, some fresh water still flowed into the ‘old’ delta through one of these channels, the Kalota Brook. However, since 1988 a small dam has blocked this, built by Pokomo farmers, who use the tidal bore to push fresh water into their fields and irrigate their crops. Alluvial sediments cover the entire flood-plain in the lower parts. Flooding happens as a result of rain in the river’s catchment on Mount Kenya (IBA KE005) and the Aberdare mountains (KE001). Normally, the major floods occur in April–May, with a smaller, short-rains flooding in October–November. The timing, extent and duration of the flooding vary greatly from year to year. The Tana river delta contains a very wide variety of habitats, including riverine forest, grassland, woodland, bushland, lakes, mangroves, dunes, beaches, estuaries and coastal waters. Small fragments of riverine forest, not nearly as extensive as the forests north of Garsen (see KE023), occur along the present or former river courses. Seasonally flooded flood-plain grasslands cover c.67,000 ha of the delta. West of the flood-plain is a diverse bushland. Wooded bushland or grassland, with fire-resistant tree species, occupies a broad swathe east of the flood-plain, merging into the Boni forest vegetation to the north. Other bushland associations form a complex mosaic with the flood-plain grasslands. Parallel to the coast along Ungwana Bay run lines of high dunes, some as much as 37 m above the sea. These are covered by their own distinctive vegetation, a dense thicket dominated by Dombeya sp. and Grewia similis. In the valleys the thicket mingles with taller trees, including various palms. Palms are prominent in many places. In some areas, especially those cleared and burned in the past, these form substantial tracts of palm-bushed grassland. Tall mangrove forest grows at Kipini in the Tana estuary and along the network of channels further south. As well as seasonal wetlands in the oxbows and flood-plain depressions, the delta contains a number of near-permanent lakes and marshes. Some of these may dry out in certain years, but others, like Lake Shakababo and Bilisa, maintain true aquatic plants and good populations of several species of fish. Luo and Luhya immigrants to the area are responsible for an active and thriving fishery, while Orma pastoralists use the wetlands as dry-season grazing areas for their livestock. Fishermen also camp for days or weeks, while catching, salting and drying fish, on the coral outcrops of Mwamba Ziwayuu, c.10 km offshore from Kipini. The other main ethnic group in the area, the Pokomo, are agriculturalists who cultivate a narrow strip on either side of the river, and around the seasonal and permanent wetlands. As the floods start to recede, rice is planted in the shallow water. Several crops of rice follow the water as its level drops, and other crops, such as maize and sweet potatoes, are planted on the drying mud. There is also an irrigation scheme growing rice in traditional style at Ozi, near Kipini, and a much larger, mechanized one upstream, east of Garsen, that is eventually intended to cover as much as 16,000 ha.
See Box and Table 3 for key species. The Tana river delta is a stronghold for two Near Threatened, restricted-range species, Anthus melindae and Acrocephalus griseldis (probably its main wintering ground). Circaetus fasciolatus is uncommon in riverine forest, but has not been recorded in recent surveys. The wetlands, including the coastline and offshore islets, at times hold exceptional concentrations of waterbirds. Internationally important populations have been recorded here for no fewer than 22 species, making the delta one of the key sites in the country for waterbird conservation. The Tana delta also supports one of the very few breeding sites for colonial waterbirds in Kenya. This heronry is near Idsowe, south of Garsen, on Ziwa la Matomba, a seasonally-flooded lagoon where the birds nest in a thicket of Terminalia brevipes, usually between May and September but also at other times if the lagoon is flooded. Up to 5,000 colonial waterbirds of at least 13 species have been recorded nesting here, including Anhinga rufa (up to 100 pairs), Ardea cinerea, A. purpurea, Egretta ardesiaca, Ardeola ralloides and Nycticorax nycticorax, Casmerodius albus, Mesophoyx intermedia and Egretta garzetta, Anastomus lamelligerus, Threskiornis aethiopicus and Plegadis falcinellus, and Platalea alba. Mwamba Ziwayuu, a small coral platform offshore from the Tana estuary, is a resting site for significant numbers of Sterna saundersi and S. bengalensis that feed in Ungwana Bay. Regionally threatened species include Casmerodius albus; Ephippiorhynchus senegalensis (a regular visitor in small numbers, May to September) and Turdoides squamulatus (local and uncommon).
Non-bird biodiversity: The importance of the Tana river delta lies in the expanse, intactness, variety and productivity of its habitats. The flood-plain is grazed by a number of ungulates, including the restricted East African coast subspecies of the ungulate Damaliscus lunatus topi, with some 30,000 or so in the area. The rivers and channels support large numbers of Hippopotamus amphibius (estimated at 400–450) and Crocodylus niloticus. Ungwana Bay is one of the few places where Dugong dugon (VU), critically endangered in the region, has been recorded recently. The turtles Chelonia mydas (EN), Eretmochelys imbricata (CR) and Lepidochelys olivacea (EN) nest on the sandy beaches. Twenty-two freshwater fish species are recorded from the lower Tana, including three eels Anguilla spp. and a distinct subspecies of Petrocephalus catastoma. The mangroves provide vitally important spawning and nursery grounds for many species of fish and crustaceans. The extensive mangrove forests include the only significant stands in Kenya of the plant Heriteria littoralis, and two other plant species that are considered threatened Xylocarpus granatum and Bruguiera gymnorrhiza. At least 280 plant taxa are recorded for the delta, and many more undoubtedly occur as there has been no thorough botanical survey; of these, 18 are considered rare in Kenya or globally.
BirdLife International (2020) Important Bird Areas factsheet: Tana River Delta. Downloaded from http://www.birdlife.org on 20/10/2020.