Ian Watson 
Book of the River 
Black Current Book 1

The river cuts right across the known world, from the impassable Far Precipices to the sea. The people on one bank are cut off from those on the other, for the black current has the power to stop them crossing.Only women can travel repeatedly up and down the river, but when Yaleen joins the boating guild and becomes a riverwoman, she is singled out to follow an even more extraordinary path…