What is the mood of "The Negro Speaks of Rivers"?

The mood is solemn and proud.

Consider the first line, which is repeated in the second: "I've known rivers." The speaker's use of the present perfect tense evokes knowledge that was acquired in the past, but has not been forgotten. The speaker's use of anaphora, or the repetition of words at the beginning of two or more lines, emphasizes certainty.

The use of rivers as a metaphor in relation to the black American experience is interesting. Rivers are deep ("My soul has grown deep like the rivers") and run along their course. They always move in one direction—forward, toward the oceans or seas. If Hughes's metaphor is applied to black culture and experience, one could use it to contemplate the depth of that heritage and its broader benefit, not just to American culture, but to world culture—the ocean in which the river releases itself. 

The speaker refers to him or herself as "I" throughout the poem. In this first-person pose, no gender is assigned. "The Negro" could be a man or woman. The speaker's experiences of rivers through time—from the birth of civilization (the Euphrates) to the New World (the Mississippi)—also makes him or her ageless. 

The effect on the reader is to express the richness and depth of black culture and history.