Dissertation Defense-Tianyue Qu

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CLIFF EVOLUTION THROUGH FEEDBACK BETWEEN DRAINAGE DIVIDE MIGRATION AND WATERFALL RETREAT: MECHANISM, RATES, AND TIMING

Cliffs are steep slopes that separate low-relief highland plateaus from incised lowlands, forming topographic barriers that influence the routing of water and sediment and the spatial patterns of climate and biota. Cliffs whose tops align with drainage divides, where large rivers do not cross the cliff, are expected to retreat slowly and uniformly.

However, such cliffs are often associated with deep embayments (i.e., alcoves) that destroy the cliff front even in landscapes with slow vertical erosion rates. However, the processes that produce and sustain these embayments remain largely unexplored. In such cliffs, embayments typically occur along reversed channels that once flowed away from the cliff but now drain toward it, forming a waterfall (i.e., knickpoint) where the reversed channel crosses the cliff, and a valley-confined drainage divide called a windgap. In this dissertation I explore the hypotheses that (1) a feedback between knickpoint retreat and windgap migration toward the highland can sustain embayments, and (2) that in these settings, the rate of waterfall and cliff can be used to estimate the cliff age and initial location. Topographic analyses and numerical simulations in the Negev Desert show that this feedback is feasible and can reach a dynamic equilibrium. 10Be-inferred migration rates in the post-orogenic Appalachian Mountains show that windgaps, knickpoints, and cliffs generally migrate toward the highland, with their lateral migration rate faster than vertical erosion rates in these areas. Faster knickpoint retreat relative to cliff migration progressively deepens embayments and increases cliff sinuosity. Reconstruction of cliff initiation age and position suggest that the studied Appalachian cliffs began developing tens of millions of years ago, even at the local scale. Results suggests that autogenic feedback between drainage divide and knickpoint migration can sustain cliff embayments and dissect cliffs.

Such feedback can drive substantial topographic change even in slowly eroding post-orogenic landscapes over tens of millions of years. This dissertation provides a quantitative framework linking localized drainage reorganization to the long-term evolution of cliffs and post-orogenic landscapes.