IT was with interest I read John Hinton's letter (You Say, November 11) about undercurrents in the Severn.

What is the downstream current other than a undercurrent?

The top half-metre of water is a retarded flow, owing to friction with the atmosphere.

The flow's far greater 600mm down than on the surface, which is why the flow of the river's measured at this depth.

M SIMPKINS, Worcester.