Roy quotes:
1 quoted line
"This is the Brook Madregot, and it runs from Yesod to Briah."
Roy writes:
1 quoted line
The multiple brooks refers to the brooks that run between the tiered universes. Small Tzadkiel could show him only the one brook, Madregot, because that was where she had been exiled by her larger self.
We don't know that Madregot runs ONLY from Yesod to Briah. And I think there are implications that being at the brook(s) means you are outside space and time, hence outside all universes.
Anyway, as I understand BotNS's time scenario, an event in the past can
lead to multiple branched futures. Thus a linear time traveller would perceive one brook travelling backward in time but multiple brooks travelling forward. This allows Madregot to be both singular and plural.
A superior time traveller would be able to go sideways and diagonally and able to traverse all possible pasts and futures. To use Wolfe's chessboard analogy, normal humans would be pawns able only to move forward in time in small increments. Master Ash, the Green Man, sometimes Severian and probably Juturna are rooks. Tzadkiel is a queen.
-bsharp