Initiation and recession of the fluvial knickpoints of the Island of Tahiti (French Polynesia)

In this paper, we study the origin and evolution of the 42 knickpoints spanning the Island of Tahiti, a large extinct shield volcano in the South Pacific Ocean (French Polynesia), by combining DEM analysis and numerical modeling. These knickpoints are located along rivers (107 in total) with a total length exceeding 6 km and with a total drainage of > 3 km2. The knickpoint locations do not correspond to lithology, tributary confluence, or uplift. We argue that these knickpoints have been initiated by a sudden sea level drop of 135 m 20 ky ago, and that the littoral cliffs circling two-thirds of Tahiti are the result of marine erosion that took place 7 ky ago from a stand level that was 5 m higher than now. The head-to-toe height of the knickpoints increases with respect to the knickpoints' distance from the ocean. The major process controlling the knickpoints is plunge-pool incision and the n = 2 stream-power model works well for modeling the profile form. The mean retreat rate of the knickpoints corresponds very well with a drainage-area dependant model with velocities ranging from 0.17 to 1.2 m/y.