The National Policy Statement for Freshwater Management 2020 (NPS-FM) requires councils to determine whether river sites of interest to the community are naturally hard-bottomed (consisting mostly of larger stones and gravel) or soft-bottomed (mainly covered in silt and mud).
GNS scientists utilized a geological model based on borehole data together with pre-human (relict) river channels derived from a digital elevation model to determine the location of the natural (pre-human) transition zone from hard-bottomed to soft-bottomed river channels within the Wairau Plain.
Significant modifications to the natural network of waterways to allow economic development of the Wairau Plain have resulted in changes to river flows, sediment supply and channel morphology. As a result, for some waterways the transition zone between a hard- and soft-bottomed riverbed has shifted. Examples are the Ōpaoa River and Wairau River. However, for other rivers the current transition zone is still within its natural bounds.
