The river it goes right on: Post-glacial landscape evolution in the upper Waipaoa River basin, eastern North Island, New Zealand

Abstract

Post-glacial incision created 50 to 120m of relief in the headwaters of the Waipaoa River basin, but it is not known how the signal of incision propagated along the trunk streams. We addressed this issue by imposing previously determined long-term rates of rock uplift, that vary between 0.6 and 4mmyear−1, on the preaggradation longitudinal profile. Acting over the past 27.1±2cal.kaBP, these rates are sufficient to passively elevate the Waipaoa-1 terrace to its present level. But during the Last Glacial Maximum, rates of vertical incision failed to match those of the uplifting rocks. Thus, by the time favorable conditions for incision were reestablished after the Antarctic Cold/New Zealand Late Glacial reversal we estimate that the channel would have been elevated ≤55m above the datum the river should otherwise have maintained. The channel incised rapidly after the transition from glacial to interglacial conditions and reacquired its steady-state form in the mid-Holocene (5.5±2cal.kaBP). Thereafter, we suggest rates of incision matched rates of uplift. No knickpoints are found on the upper Waipaoa and Mangatu Rivers because incision commenced simultaneously along the entire length of the trunk streams, and the pattern of incision is consistent with the downstream decline in the rate of rock uplift. We also used hypsometric analysis to show how post-glacial incision affected the surface morphology of low order (0.1–4.3km2) drainages in the headwaters of the Waipaoa River basin. Values of the hypsometric integral are higher for proximal basins, which the incision signal did not permeate, than for low-order basins bordering the upper Waipaoa River and the lower reaches of tributaries that possessed enough erosive capacity to keep pace with incision along the mainstem. Hillslope adjustments were focused on the lower and mid-sections of these basins so that the most pronounced changes to the hypsometric curve occur in the vicinity of the toe, whereas the entire form of the hypsometric curve changes once mass wasting processes encroach onto hillslopes in the middle and upper sections of a basin.