LANDSCAPE DISEQUILIBRIUM ON 1,000-10,000 YEAR SCALES, MARYSANDI RIVER, NEPAL, CENTRAL HIMALAYA

In actively deforming orogens, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵-10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. In order to interpret long-term denudation properly, the extent to which climate modulates short-term rates of landscape evolution must be quantified.

At time scales >10⁶ years ⁴⁰Ar/³⁹Ar dates indicate that the vertical incision rates in the Marsyandi River catchment in the central Nepal Himalaya is 1.5-2 mm/yr (Brewer, 2001). On 10,000-year timescales the Marsyandi has oscillated between as much as 7 mm/yr bedrock incision and >100 m of alluviation, in response to changes in monsoon intensity. Fill terrace deposits reveal a major climatically induced alluviation, probably coincident with a monsoonal maximum, 40-60 ky BP. ¹⁰Be and ²⁶Al exposure ages of polished fluvial surfaces show an alluviation and reincision event 6-8 ky BP, also at a time of strong South Asian monsoons. Maximum bedrock incision rates calculated from the lowest dated surfaces are 7 mm/yr in the Greater Himalaya and 1.7 mm/yr in the Lesser Himalayan Mahabarat Range during the Holocene and Late Pleistocene, respectively.

We propose a model in which changes in monsoon precipitation lead to out-of-phase variations in river and peak elevations. Increases in climate-driven hillslope erosion lead to shielding of the bedrock channel by alluvium in the valley bottoms as the sediment supply outstrips the Marsyandi's transport capacity. This shielding should cause the river-channel elevation to rise as rock uplift continues in the absence of incision. Concurrently, the increased hillslope erosion lowers the hillslope elevations. Subsequent removal of these valley-bottom deposits re-exposes the bedrock channel and bedrock incision rates are predicted to accelerate beyond the long-term mean as the river profile is adjusted downward towards a more "equilibrium" profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.