The shady trees (or, the lotus trees) cover him with their shadow (see the comment on Job 40:21); the willows of the brook compass him round about. The "willow of the brook" (Leviticus 23:40) is probably the Saliz Aegyptiaca, or safsaf, which grows plentifully in the Nile valley, fringing the course both of the Nile itself and of the many streams derived from it.
The Saliz Babylonica, or "weeping willow," is less likely.