For Tambu, the river Nyamarira represents the few joys of the homestead and her life in poverty. It's a life-giving river, allowing everyone a place to bathe, the women a place to do laundry, and those tasked with carrying water a place from which to fetch it. Tambu's love for the river doesn't change throughout the novel, suggesting that in some ways she is doing better than Nhamo did in her endeavor to not look down on life at the homestead—she can still find joy and happiness in being at home, even with her education. However, the fact that the river itself changes as the local village grows and expands (the swimming holes once used by children become a thoroughfare) positions Nyamarira as a symbol of change, while Tambu's continued respect for the river indicates that it's possible to find a middle ground between the colonial system as represented by England and the dire provincial poverty of the homestead itself.
The timeline below shows where the symbol Nyamarira appears in Nervous Conditions. The colored dots and icons indicate which themes are associated with that appearance.
...fields where friends work, through fruit trees, and by the river, which is called the Nyamarira. Tambu explains that there are several spots where children can swim, and others where women... (full context)
...feels extremely important and tells Tambu about it as she waters vegetables with water from Nyamarira. He insists that there's nobody else to go to the mission and speaks horribly about... (full context)