Symbolism, Imagery, Allegory
Wollstonecraft uses the image of a stream several times in this book, but it's really more of a trope than a symbol, because she never really uses it in the same way twice.
She originally introduces the image when she writes, "[W]hen men neglect the duties of humanity, women will follow their example; a common stream hurries them both along with thoughtless celerity" (4.41). In this case, the stream refers to the force of social habit that tends to make people just "go with the flow" and not get too worked up about anything, even if it's totally unjust.
Later in the text, though, Wollstonecraft reverses the current of her image (see what we did there?), and uses it to symbolize the opposite of unthinking conformity. Instead, she says, "From the clear stream of argument, indeed, the supporters of prescription, of every denomination, fly" (11.11).
In this case, the "clear stream of argument" shows that the power of clear and rational thinking has the power to overwhelm anyone who's being unfair or prejudiced.