Explain what a "swale" is. What's the definition, and how is this word used in "The Short Happy Life of Francis Macomber"?

A swale is a moist depression in an area of land, often between two ridges. Because a swale collects surface water, it often contains marsh-like vegetation.
The word "swale" is used twice in "The Short Happy Life of Francis Macomber." The first instance is on page 9, when Wilson and Macomber's car drives "through the swale of high grass." This means that they drive through a depression in the ground which is full of grass. The grass is higher in the swale than elsewhere because, as mentioned above, swales collect surface water, which allows vegetation to grow abundantly.
The second instance of the word "swale" occurs on page 36. Here, the author describes an "island of bushy trees" which runs along "a dry water course that cut[s] the open swale." The "dry water course" is essentially a dry riverbed, or probably something a little smaller in this instance. This "course" widens out at some point into a larger depression, or hollow in the ground, and this larger depression is what's called the swale.