Suggest improvements for the highlighted problem areas:
The Amazon drains water from a region that is large, tropical and flat. This region is 13 times the size of France but little of the river water comes from mountain snow melt. Most of it's water comes from rain in the vast lowlands. This rain forest, located on the equator experiences the equivalent of two summers each year. The result is two seasonal peaks of rainfall and streamflow. In an area as flat as the Amazon basin, these peaks can cause widespread damage. Floods are common in the city of Manaus, which is 1200 kilometers inland but only 26 meters above sea level.
- Comma for an Interjection -
Another important rule of punctuation is shown by a mistake in the middle of our opening paragraph:
This rain forest, located on the equator experiences the equivalent of two summers each year.
Commas must enclose interjections. These interjections aren't essential to the thought or structure of the sentence, but they do provide variety. We should emphasize this variety by signaling to the reader the presence of something different and interesting:
This rain forest, located on the equator, experiences the equivalent of two summers each year.
Interjections can also be set off by parentheses or by dashes:
This extensive region (13 times the size of France) is mostly rain forest.
Large fluctuations in water level -- e.g., 10 meters near Manaus -- are common on the Amazon.
Commas should also enclose words that modify the entire sentence, such as: however, therefore, and on the other hand:
The river, therefore, floods twice each year.