Excellent Mystery - Know Only the Ending - Which Book Is It?

The book I'm trying to remember is a wonderful mystery, possibly written by
Ruth Rendell or her incarnation Barbara Vine. The plot revolves around the
memories of a girl or young woman. A main plot point is that she remembers
sitting by a flowing river and overhearing or witnessing damning evidence.
As it turns out, the river flows in the opposite direction, which means that
her memories are a dream (?). (This might lead to the discovery of a victim
under the newly built barbecue site, but I'm not entirely sure that's the
same book.)

Who can help me identify this book and the author?