HR: 0800h
AN: H31A-1263    [Abstracts]
TI: Bedrock River Incision Following Aggradation: Observations from the Waipaoa River Regarding Tributary Response to Mainstem Incision and the Role of Paleotopography
AU: * Crosby, B T
EM: mountain@mit.edu
AF: Dept. Earth, Atmospheric, and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Whipple, K X
EM: kxw@mit.edu
AF: Dept. Earth, Atmospheric, and Planetary Science, MIT, Cambridge, MA 02139 United States
AB: Following a period of valley-filling aggradation, the form and extent of subsequent alluvial and bedrock incision is governed by the pre-aggradation topography and the position of the channel at the time of incision. We present findings from an extensive along-stream survey of the Waihuka Stream, a tributary to the Waipaoa River on the North Island of New Zealand. Never glaciated, this basin aggraded 5 to 25 m of coarse alluvial sediment during the last glacial period, creating an extensive and distinctive valley-fill surface. ~18 ka, aggradation ceased and a subsequent pulse of fluvial incision abandoned the aggradational surface. At present, the river gorge is incised 25 to 60 m into alluvial fill and mudstone bedrock beneath the top of the aggradational surface. Using a laser range-finder and GPS-enabled GIS surveying tool, we surveyed ~17 km of the Waihuka Stream (4.8×10⁶ m² to 6.3×10⁷ m²). We collected a longitudinal profile and the relative elevations of fill and strath surfaces exposed in channel banks. We also surveyed a total of ~9 km in 14 tributaries to the Waihuka. Drainage areas of tributaries ranged between 5×10⁵ m² and 9.9×10⁶ m². In the Waihuka, we find that the amount of bedrock incision depends on whether the incising channel locally lowered through alluvial fill to reoccupy the preaggradation channel or whether it locally had to cut an entirely new valley into bedrock. Reaches dominated by bedrock incision were observed where alluvial fan deposits laterally shifted the mainstem channel out of the paleovalley and against the opposite bedrock hillslope. The along-stream variation in bed erodability forced by whether the re-incising channel encountered bedrock or alluvium had a significant effect on the propagation of the incision signal into tributaries. Where the channel dominantly re-incised alluvial fill, tributaries have stepped but not dramatically over-steepened longitudinal profiles that appear to be adjusting to the new base-level. In reaches where the mainstem had to incise a new valley in bedrock, the tributary basins are often left as hanging valleys above the mainstem, divorced from the trunk by a series of near vertical waterfalls. These observations lead us to conclude that incision into aggraded valleys is a non-uniform process, highly dependant on the substrate below the incising channel. This field example emphasizes that when examining landscapes incised into a relatively uniform paleosurface, the magnitude of bedrock incision below that surface may be highly irregular.
UR: http://www.mit.edu/~mountain/work/meetings
DE: 1637 Regional climate change
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005