escarpment and perennial springs below the rift wall support tall, evergreen groundwater forests dominated by Trichilia roka with Broad-Leaved Croton (Croton macrostachyus), sycamore fig (Ficus sycomorus), quinine tree (Rauvolfia caffra) and forest toad-tree (Tabernaemontana ventricosa). Trees are densest in gorges, along the edge of streams and in areas where springs emerge. Growing at the edge of the groundwater forest are dense stands of yellow fever trees (Acacia xanthophloea) and the palm Phoenix reclinata. The flora of Lake Manyara National Park is diverse, with over 670 flowering plant and fern species documented. Most are widespread species; there are few rare or endemic species. Woodlands on the western shore of the lake dominated by Acacia and Commiphora species are also supported by groundwater. Trees are spaced irregularly; densest in zones of seepage and along drainage channels. The dominant tree, Acacia tortilis, grows to about 10m tall with a flat-topped canopy. There is a patchy and diverse understory shrub layer. Extensive swamps have formed where the Simba River and its tributaries flow into the north end of Lake Manyara and smaller swamps have formed elsewhere, associated with the minor rivers flowing off the walls of the rift valley. A number of aquatic species have been documented, including two species of water lily (Nymphaea caerulea and N.lotus) and extensive mats of water lettuce (Pistia stratiotes). Cattails (Typha sp.) forms pure stands in the northern swamps and various Cyperus species are found around the edges of the swamps. Cyperus immensus forms dense clumps in areas with flooding by fresh water. Cyperus laevigatus is prominent along the lake shore in areas where the alkaline water table is high, in areas of shallow lagoons enclosed behind sand ridges and flanking river deltas. Grasslands are dominated by Cynodon dactylon in dry locations, or by Sporobolus spicatus, often in association with Sporobolus consimilis on the alkaline lake flats. Sporobolus spicatus is dominant on exposed lakebed and can be very extensive during low lake levels, becoming restricted to a narrow zone above the high water mark when lake levels are high. There are extensive areas of Psilolemma jaegeri grasslands outside the park, along the eastern shore of the lake. Vegetation on the escarpment is characterized by Ruellia megachlamys and African baobab trees (Adansonia digitata). Birds Lake Manyara National Park is known for flocks of thousands flamingos that feed along the edge of the lake in the wet season. In 1991