In the novel Dead Man's Folly, the River Helm is a river i Devon. The town Helmmouth is located on the mouth of the river. The boathouse belonging to Nasse House is located down by the river. Not far from there, is the location of the Gooseacre Rock.
Since Nasse House is based on Greenway, the home of Agatha Christie, the River Helm is based on the River Dart.