The Great Blue Lake is the large lake that feeds the Rain Wild River. It is an immense lake from which the opposite shore is not visible. In ancient times in particular it was a source of enormous fish which did not venture far down the river. Downstream are the Elderling cities of Cassarick and what is now called Trehaug. Upstream is the city of Kelsingra. Most of the lake is currently shallow, often no more than waist deep.
In ancient times the Elderlings fished the lake heavily, keeping a few small settlements on its banks and numerous summer homes for the wealthy.