People are being urged to avoid Sawcut Gorge and the Waima River valley near Isolated Hill Scenic Reserve after a landslip created an estimated 250m-long river dam.
Marlborough District Council and DOC say the dam may be unstable and further rock falls are possible. Landslide dams can fail suddenly, often within days or weeks of forming.
Recreational users are strongly advised to stay out of the river channel and surrounding area.
