When he explains that the "muddy bosom" of the river (or, of the life of the black culture) turns "all golden in the sunset," that is a sweet transition for a culture, and nothing less than mystical, magical and wonderful. Turning mud to gold is the miracle of survival through all the chaos, carnage, and brutal injustices done to black people over the centuries. In "Mother to Son" the poet is using the genre of family, the power structure in the nuclear family, to make a statement to the whole culture of African-Americans. While the first poem uses broad brushstrokes on a global canvas -- pointing to the past, and the black culture's survival as it moves forward into the future like the flow of a river -- in "Mother to Son" Hughes brings his message home, using the stairs as a metaphor for human struggles and the strength it requires to stay strong in those struggles. "Boy" in this sense is a metaphor for the culture, perhaps specifically the youthful black culture that may have become frustrated with institutional racism in America. "Don't you set down on the steps / 'Cause you finds it's kinder hard" he writes; those passages are the poet's way of saying, if you think you've got it bad, you have no idea what your forbearers have gone though so you can have and advantages you have today. Earlier in the poem "Mother" is saying, without complaining, how miserable life can be with bare floors. Using "Bare" as one line says a great deal...

The images portray a life without the "bare" necessities, with stairs that are steep and sudden curves and corners thrown at the culture.
What seems like a mother explaining her trials (softly scolding her son, "Don't you fall now…") is actually a mother saying, I've been through it all and I keep on moving up those stairs to find peace and safety, so don't you dare tell me you can't make it. You have only begun to understand the challenges that your parents and grandparents have faced, she is saying through the poet's lines.

Conclusion

The "Negro Speaks of Rivers" is a more hopeful poem than "Mother to Son" because notwithstanding all that history behind the culture, with all the misery (embracing the name "Abe Lincoln" brings up the chains of slavery and how they were removed) in the past, that ancient culture / river is not only still flowing, but when the sun's angle hits the muddy river just right, it turns to gold. "Mother to Son" is also a positive poem in a way, but it seems more along the lines of challenging the youthful culture with wisdom from the past, and implying that the steps must continue to be climbed. The difference in terms of positivity, in the opinion of this writer, is, the river will continue to flow (and rivers almost always flow downhill) but steps are all uphill. You have to get to the top before you retreat down the steps. But rivers flow forever.