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# Permafrost Degradation and Re-Aggradation in the Beaver Creek Floodplain

Thomas Dépret, E Gautier, Frédéric Gob, Daniel Fortier, Marc Pessel, Rémi

Lambert, Clément Virmoux, Marie Mousset

► To cite this version:

Thomas Dépret, E Gautier, Frédéric Gob, Daniel Fortier, Marc Pessel, et al. Permafrost Degradation and Re-Aggradation in the Beaver Creek Floodplain. Climat et Impacts, Nov 2024, Saclay, France. hal-05377928

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Submitted on 22 Nov 2025

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# Permafrost Degradation and Re-Aggradation in the Beaver Creek Floodplain (Yukon Territory, Canada)

Thomas Dépret¹*, Emmanuèle Gautier², Frédéric Gob², Daniel Fortier³, Marc Pessel⁴, Rémi Lambert⁵, Clément Virmoux¹, Marie Mousselet²

## I. Introduction

* The **Arctic amplification** (rise in air temperatures more than double the global average, greater precipitation, increase in soil humidity) has led to a cascade of reactions, the central element being the **degradation of the permafrost**.

* Yet, it is only recently that attention has turned to the morphodynamic and sedimentary evolution of rivers draining permafrost regions.

* The main objective of this work is to analyze the way fluvial dynamics control **permafrost distribution at the alluvial plain scale** and to determine the **time needed for permafrost aggradation in undisturbed fluvial deposits**.

<figure><img src="image_2.png" /><figcaption>Figure 1: Study site location</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Beaver Creek watershed</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3: Oblique view on Beaver Creek (towards upstream)</figcaption></figure>

## II. Study site

* **Beaver Creek, a tributary of the White River**, drains a 2308 km² basin (Yukon and Alaska), located in the north of the Kluane Range. The river flows east from a glacier and then north through the Wellesley basin, where the study site is located (Fig. 1 to 3).

* **Middle part of the catchment unglaciated during the last glacial maximum** (Fig. 2). Area corresponding to the extensive discontinuous permafrost zone and the Pleistocene permafrost is ice-rich with networks of ice-wedges.

## III. Material and methods

* Spatial distribution of the permafrost in the different units of the valley bottom (riverbed, bars, abandoned channels, floodplains, terraces) along three cross-sections:

- Detrended LiDAR DEM (Fig. 5).

- Electrical resistivity tomography (ERT) surveys (Fig. 5).

- Permafrost coring (portable core-drill) (Fig. 6).

* Chronology of the various floodplain levels:

- Remote sensing -> river mobility since 1977.

- Dendrochronology (Fig. 4).

- Radiocarbon dating.

- Presence of White River Ash in the soil (1250-1900 AD).

<figure><img src="image_5.png" /><figcaption>Figure 4: Dendrochronology sample</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5: Location of one the ERT profile</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6: Core-drilling</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7: Schematic representation of permafrost distribution in the alluvial plain</figcaption></figure>

## IV. Results

* Absence of permafrost in all units except for the highest terrace (Fig. 7 and Fig. 9).

* Units without permafrost older than 100 years (dendrochronology) (Fig. 7 and Fig. 9).

* Upper part of the permafrost (5-40 cm below the active layer) in the oldest unit younger than 1200 years: formed after the White River Ash deposits.

* Time needed for **permafrost aggradation** thus comprised between more than 100 years and less than 1250-1900 years.

<figure><img src="image_9.png" /><figcaption>Figure 8: White River Ash layer within an alluvial deposit</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9: Successive location of the active bed from 1977 and 2023</figcaption></figure>

## V. Discussion and perspectives

* Permafrost aggradation longer than on other rivers: 30 years in the Lena River (Costard et al., 2022), 200 years for 50% of deposits in the Koyukuk River (Douglas et al., 2024).

* Pending C14 dating to precise the time required for permafrost formation.

* Extrapolation of profile-scale results over several km of valley; combination of topographic data from LiDAR DEM and vegetation data from remote sensing.

* Examining the relationship between permafrost formation rate and regional climate change.

**References:**

Costard F., et al., 2022. Thermal regime variability of islands in the Lena River near Yakutsk, eastern Siberia. *Permafrost and Perigl. Process.* (33), 18-31.

Douglas M. et al., 2024. Permafrost formation in a meandering river floodplain. *AGU Advances*, 5, e2024AV001175.

*¹CNRs - Laboratoire de géographie physique : Environnements Quaternaires et Actuels (LGP), 2 rue Henri Dunant 94320 Thiais - France

*²Université Paris 1 - Panthéon-Sorbonne - Laboratoire de géographie physique : Environnements Quaternaires et Actuels (LGP), 2 rue Henri Dunant 94320 Thiais - France

*³Université de Montréal, Centre d'études Norviques, Université de Montréal, Complexe des sciences 1375 Avenue Thérèse-Lavoir-Roux Montréal (Québec) H2V 0B3 - Canada

*⁴Université Paris-Saclay - Géosciences Paris Saclay (GEOPS), Université Paris Saclay, bâtis. 504 & 510, 91405 ORSAY Cedex - France

*⁵CNRs - Géosciences Paris Saclay (GEOPS), Université Paris Saclay, bâtis. 504 & 510, 91405 ORSAY Cedex - France