Postglacial topographic response

Earth’s topographic form is controlled by interactions between tectonics, climate, and surface processes. Under constant conditions, rivers and hillslopes tend to adjust their gradient so that erosion rates approximately balance tectonic uplift rates. However, short-term perturbations in tectonics, climate, or surface processes may create a temporary imbalance between erosion and uplift.

Repeated glacial-interglacial cycles during the Quaternary have acted as such perturbations in mid-latitude mountain ranges worldwide. Since the Last Glacial Maximum (LGM, ~25 thousand years ago), the transition from glacial to fluvial and hillslope geomorphic processes has induced progressive adjustments of topography in deglaciated landscapes, resulting in erosion rates that exceed or lag tectonic uplift rates. For example, rockfalls and landsliding of steep U-shaped glacial valleys grade hillslopes towards gentler, gradually eroding V-shaped fluvial valleys, and, in some places, progressive fluvial incision evacuates glacial sediments and dissects glacial terraces.

Understanding the timescales over which these adjustments occur before a new quasi-equilibrium between erosion and uplift is reached is a key question for understanding how Earth’s topography responds to periodic climatic perturbations. If landscape response times exceed typical durations of Quaternary interglacial periods (~20-50 thousand years), then mid-latitude mountain ranges may be persistently adjusting and never reach an equilibrium before the next glaciation occurs. On the contrary, if their response times are shorter than interglacial periods, then Quaternary glaciations may only fleetingly disrupt long-term tectonically driven landscape evolution.

POSTCOLD - Understanding the influence of sediment dynamics on postglacial landscape evolution

This project is funded by a Marie Skłodowska-Curie Postdoctoral Fellowship.

In recently deglaciated landscapes, the transition from glacial to fluvial/hillslope processes have induced progressive topographic adjustments, and the large amounts of sediments inherited from glacial periods and generated through landsliding of oversteepened glaciated topography may act as a fundamental control on the incision of postglacial rivers. These sediments can enhance fluvial incision rate by providing more tools for erosion, or inhibit incision by armoring the river bed. Characterizing when and where sediment enhances or inhibits fluvial incision in postglacial landscapes is critical for understanding the changes in postglacial landscape evolution rates over time and quantifying the response times of mountain ranges to deglaciation.

In this project, I propose to develop a landscape evolution model to account for the complex impact of sediment dynamics on fluvial incision in postglacial landscapes. I will utilize this model to investigate the response of fluvial incision to changes in sediment supply and assess the effects of sediment on postglacial landscape evolution. I will apply the model to quantify response times of the deglaciated European Alp, leveraging the rich observational datasets in this region. The proposed work will provide a quantitative understanding of postglacial fluvial incision histories, which is critical for ecosystem management and natural hazard assessment in recently deglaciated mid-latitude mountain ranges and perhaps in high-latitude mountain ranges where continued climate change may eventually lead to deglaciation.

Postglacial fluvial network expansion in glaciated plains

Landscapes of the US Midwest were repeatedly affected by the southernmost portions of the Laurentide Ice Sheet during the Quaternary. Over much of the Midwest, depositional glacial processes diminished pre-glacial relief by filling valleys and left constructional landforms including low-relief till plains and high relief moraines. As the ice retreated, meltwater collected in subglacial or proglacial lakes leaving fine-grained, sorted sediment in the form of glacial lake plains. Glacial lakes episodically drained in outburst floods, carving deep valleys.

However, meltwater valleys did not provide a landscape-wide integrated drainage network. Instead, a significant fraction of the area of low-relief till plains and glacial lake plains was occupied by closed depressions and remained unconnected to external drainage networks. This area is referred to by hydrologists as non-contributing area (NCA) because it does not typically contribute surface runoff to stream networks. NCA becomes integrated into external drainage networks over time.

By combining numerical simulations with field observations, our research found that rates of postglacial fluvial incision and drainage network expansion are controlled by the hydrological connection between the poorly-drained postglacial plains and the growing river channels (Lai and Anders, 2018, JGR), and the hydrological connection can be achieved via filling-spilling of small lakes or groundwater (Cullen, et al., 2021, ESPL).

Sediment Dynamics Control Transient Fluvial Incision - Comparison of Sediment Conservation Schemes in Models of Bedrock-Alluvial River Channel Evolution

Jingtao Lai, Kimberly Huppert, and Jean Braun

ESS Open Archive, Nov 2023

Preprint submitted to Journal of Geophysical Research: Earth Surface, in review

DOI
Asymmetric Glaciation, Divide Migration, and Postglacial Fluvial Response Times in the Qilian Shan

Jingtao Lai, and Kimberly Huppert

Geology, Jul 2023

Abstract DOI

Glacial-interglacial cycles have repeatedly perturbed climate and topography in many midlatitude mountain ranges during the Quaternary. Glacial erosion can move drainage divides and induce fluvial adjustments downstream, yet the time scale over which these adjustments occur remains unclear. We examined landscape evolution in the northwest-southeast–trending Qilian Shan, where the contrast in solar insolation between north- and south-facing slopes has generated larger glaciers on the northern range crest. Our analyses suggest that this asymmetric glaciation has caused southward migration of the main drainage divide, prompting river channels below the extents of ice on north-facing slopes to become oversteepened for their drainage area and channels on south-facing slopes to become analogously understeepened. These changes in steepness should accelerate or slow down postglacial fluvial incision, even in the regions where topography has not been directly modified by glacial erosion. Numerical modeling suggests these discrepancies persist for millions of years, much longer than the duration of recent glacial-interglacial cycles, implying a widespread and enduring influence of intermittent glaciations on landscape evolution in glaciated mountain ranges during the Quaternary.
Numerical Modeling of Groundwater-driven Stream Network Evolution in Low-relief Post-glacial Landscapes

Cecilia Cullen, Alison M Anders, Jingtao Lai, and Jennifer L Druhan

Earth Surface Processes and Landforms, Nov 2021

DOI
Modeled Postglacial Landscape Evolution at the Southern Margin of the Laurentide Ice Sheet: Hydrological Connection of Uplands Controls the Pace and Style of Fluvial Network Expansion

Jingtao Lai, and Alison M. Anders

Journal of Geophysical Research: Earth Surface, May 2018

Abstract DOI

Landscapes of the U.S. Central Lowland were repeatedly affected by the Laurentide Ice Sheet. Glacial processes diminished relief and disrupted drainage networks. Deep valleys carved by meltwater were disconnected from the surrounding uplands. The upland area lacking surface water connection to the drainage network is referred to as noncontributing area (NCA). Decreasing fractions of NCA on older surfaces suggest that NCA becomes drained over time. We propose that the integration could occur via (1) capture of NCA as channels propagate into the upland or (2) subsurface or intermittent surface connection of NCA to external drainage networks providing increased discharge to promote channel incision. We refer the two cases as “disconnected” and “connected” since the crucial difference between them is the hydrological connection of the upland to external drainage. We investigate the differences in evolution and morphology of channel networks in low-relief landscapes under disconnected and connected regimes using numerical simulations. We observe substantially faster rates of erosion and integration of the channel network in the connected case. The connected case also creates longer, more sinuous channels than the disconnected case. Sensitivity tests indicate that hillslope diffusivity has little influence on the evolution and morphology. The fluvial erosion coefficient has significant impact on the rate of evolution, and it influences the morphology to a lesser extent. Our results and a qualitative comparison with landscapes of the glaciated U.S. Central Lowland suggest that connection of NCAs is a potential control on the evolution and morphology of postglacial landscapes.

POSTCOLD - Understanding the influence of sediment dynamics on postglacial landscape evolution

Postglacial fluvial network expansion in glaciated plains

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