Rowan is a man (strongly prefer it for males) or a very mannish woman, likely to be white; I picture someone plain, and smart.
River's either sex, but I prefer it as a male name. It seems a little celeb-styled to me, especially on females. White; young, maybe a little eccentric or airheaded. Not that I really have that expectation of real people with the name, just that various associations make me picture a
River as sort of superficial and dreamy.