泛滥之谜 21

The rain-sodden earth can hold no more water.

It spills out across the mountainsides, creating a host of tiny streams.

As they merge, the cargo of fertile soil driven downstream increases.

Streams become rivers.

Finally these rivers flow into the mountain bowl that is Lake Tana.

The lake is filling up all the while, creating marshland on its shores.

Here, weaver birds turn the flower heads of papyrus into building material

and they set about constructing a woven city over the growing lake.

Papyrus is one of the Niles great gifts.

Growing along much of the river's length

it provides a strong waterproof building material

for expert craftsmen such as these.

The people of the Nile have also found it useful for building.

The Ancient Egyptians even used it to make the paper

on which they recorded the story of their great civilisation.