THE ROLE OF KNICKPOINTS IN THE WAIPAOA RIVER: BASIN-WIDE COMMUNICATOR OF BASE LEVEL FALL OR CONSEQUENCE OF AN EROSIONAL THRESHOLD?

Knickpoints, which we define as discrete negative steps in the long profiles of river channels, have been frequently associated with the dynamic adjustment of channels following changes in climate, tectonics or base level.  The danger in this association arises from the numerous circumstances, both static (e.g. substrate contrasts) and dynamic (e.g. stream capture) that can create knickpoint morphologies.  Most previous studies of knickpoint retreat have examined only the response of single channels to base level fall, but we propose that the timing and pattern of knickpoint distribution throughout entire fluvial networks must be characterized in order to ultimately understand landscape response times to external forcing and the history of sediment delivery to offshore basins.

Toward these ends, we examined 236 active knickpoints distributed within the fluvial network of the Waipaoa River on the North Island of New Zealand.  A base level fall of 50-100 m on the Waipaoa mainstem was initiated 18,000 years ago in response to a climatically triggered pulse of incision.  Using field work, aerial photo analysis and digital elevation data, we studied the knickpoints’ positions within the network.  We found that ~70 % of the knickpoints are located at drainage areas between 1 x 10⁵ m² and 1 x 10⁶ m² and more than half are < 1 km upstream of tributary junctions.

We compared the results of two end-member models for knickpoint behavior to the observed knickpoint distribution in the Waipaoa.  In the first model, we examined the time-evolution of a knickpoint as it propagates upstream throughout the network at a rate that is a power law function of drainage area.  In the second, we examined if knickpoints form at threshold drainage areas where their fluvial erosive potential, as determined by water and sediment flux, is incapable of incising as rapidly as downstream reaches.  Though neither model addressed along-stream variability in substrate or knickpoint form, surprisingly, both models provided highly accurate fits to the ~70% of knickpoints at drainage areas < 1 x 10⁶ m².  Though the field and modeled results suggest that the present positions of the 236 observed knickpoints are determined by this threshold area behavior, explaining the basin-wide time evolution of a pulse of incision will require further model refinement and field observation.