The Tana River is the longest river in Kenya. From its sources on Mt. Kenya and the Aberdares to its present mouth at Kipini on the Indian Ocean is a straight distance of 480 km and 800 km following the major directional curves. But a measurement along the numerous meanders of the lower course would yield a length at least double this (see Fig. I). In its upper course the Tana flows north, but soon turns south by south-east, which is its direction until it reaches the sea. The limit of the Upper Tana is taken as the Hargazo Falls, which are situated at about the point where the river turns south. It is this part of the river which receives all the tributaries, the last to enter the river being the MacKenzie, which joins it 58 km above the Falls. Only during high flood years does water from other rivers, such as the Tiva, enter the Tana. Thus the river gains no new water in its middle and lower courses, but loses water continuously through evaporation. In its lower reaches the Tana flows through a broad flood plain. The entire area is covered by recent alluvial sediments brought down and deposited during the annual floods of the river. The flood plain is primarily grass-covered but there are numerous patches of forest and woodland that are apparently edaphic in origin, depending either on the flooding or on the high water table in the flood plain or both.