Langston Hughes's poem "The Negro Speaks of Rivers" connects the African-American race to rivers, especially ancient rivers, to show the value of the African race. Rivers fertilize soil and offer fresh drinking water, making rivers a valuable source for sustaining life. All rivers he names in the poem are famous for the fertility and even economic prosperity they provide. Most of the rivers he names are also associated with Africa or the African-American race. Hence, Hughes is using the ancient rivers as symbols of prosperity connected with Africa to show the value of the African race.
The connection between the rivers and the African race is especially portrayed in the lines, "I have known rivers: / Ancient, dusky rivers." As mentioned earlier, most of the rivers named in the poems have been known of and valued since ancient times, including the Euphrates, the Congo, and the Nile. Just like the rivers are ancient, so is the African race. Plus, just as the rivers are valued because they are essential for human life and provide abundance and prosperity, so should the African race be valued. Hence, Hughes refers to the rivers as "ancient" to draw a parallel between the ancient rivers and the ancient African race.
The term dusky in the phrase "dusky rivers" can be translated as meaning "dim," "shadowy," or "dark" in color (Random House Dictionary). Depending on the viewpoint, river water can appear deep blue, deep grey, or even deep brown. River water can especially look deep brown when it becomes polluted with substances like mud, just as the Nile and the Mississippi rivers are often polluted with mud during flooding seasons. Since river water is dark in hue, we can see Hughes is relating the water color of the ancient rivers to the skin color of those of the African race in order to draw a further connection between the value of the ancient rivers and the value of the ancient African race.