Geomorphic Characteristics of Drainage Ditches in Southern Minnesota, and the concept of a Two-Stage Ditch Design (Brad Hansen, Bruce Wilson, Joe Magner, and John Nieber)
«Archived copy». Arxivat de l'original el 2007-08-01. [Consulta: 14 octubre 2007].
Geomorphic Characteristics of Drainage Ditches in Southern Minnesota, and the concept of a Two-Stage Ditch Design (Brad Hansen, Bruce Wilson, Joe Magner, and John Nieber)
«Archived copy». Arxivat de l'original el 2007-08-01. [Consulta: 14 octubre 2007].