The most productive lands in the world are flood plains. At every period
of high water, a stream brings down mantle rock from the higher grounds,
and deposits it as a layer of fine sediment over its flood plain. A soil
thus frequently enriched and renewed is literally inexhaustible. In a
rough, hilly, or mountainous country the finest farms and the densest
population are found on the "bottom lands" along the streams. The flood
plain most famous in history is that of the river Nile in Egypt. For a
distance of 1500 miles above its mouth this river flows through a rainless
desert, and has no tributary. The heavy spring rains which fall upon the
highlands about its sources produce in summer a rise of the water, which
overflows the valley on either side. Thus the lower Nile valley became one
of the earliest centers of civilization, and has supported a dense
population for 7000 years. The conditions in Mesopotamia, along the Tigris
and Euphrates rivers, are similar to those along the lower Nile, and in
ancient times this region was the seat of a civilization perhaps older
than that of Egypt. The flood plains of the Ganges in India, and the Hoang
in China, are the most extensive in the world, and in modern times the
most populous. The alluvial valley of the Mississippi is extremely
productive of corn, cotton, and sugar cane.

--Dryer: _Lessons in Physical Geography_.


Notice that the first sentence gives the topic statement, flood plains are
productive. The second and third sentences tell why this is so, and the
rest of the paragraph is given up to illustrations.

In preparing this paragraph for recitation the pupil should have in mind
an outline about as follows:--