Lake Manyara lies in a shallow depression at the base of the Rift Valley.  This picture is taken from the top of the Rift (see a corner of the cliff in the center-right of the photo).  

The Great Rift Valley is 6,500 km long, from the Dead Sea in the north to Mozambique in the south).  It is formed by the division of a HUGE tectonic plate into two plates (Africa and Eurasia).  It is like a seam: over the course of millennia the plates have moved independently, have risen and fallen independently and formed lakes and volcanoes.  This picture is taken from the Eurasian plate (which, at this point is higher), looking down at the African plate, with Lake Manyara.  

Life in the African Savannah depends on the availability of water.  In the rainy seasons everything is green and lush, and the animals are happy.  But in the dry season everything turns brown.  That's when the great migrations take place, as the animals are moving to, quite literally, "greener pastures".  EXCEPT a few lucky areas stay green all year round.  Lake Manyara is one of them.   Lots of animals around Lake Manyara ( I have seen hundreds and hundreds of animals).