Impact of Climate and a large Landslide in the Tien Shan: Shaping the Naryn Alluvial Valley, Kyrgyzstan

Fluvial terraces are key archives of alluvial rivers responding through aggradation and incision to environmental signals. As a result, terraces are commonly attributed to regional climate, tectonic activity, base-level fall, or site-specific geomorphic events. However, if the effects of the different environmental drivers overlap spatially and/or temporally in the catchment, disentangling their impacts on terrace formation remains a significant challenge. Applicable to a variety of catchments, the same challenge applies to the semi-arid Naryn catchment in the Tien Shan, Central Asia: based on moraine, loess and speleothem records the high-elevation landscape reacts sensitively to global climate impacting very probably sediment and water supply to the channel. Additionally, the probably largest known landslide in Central Asia, the Beshkiol landslide, occurred 25 ky ago in the central Naryn valley, damming a lake that persisted for ~17 ky. Rapid drainage of this lake likely triggered a fluvial aggradation and incision response. Numerous fluvial terraces occur along the main stem and several tributaries. These features provide an excellent location to assess the relative, spatial contributions of regional Pleistocene climate and the Beshkiol landslide to terrace formation and to the long‑profile evolution of the Naryn River and its tributaries.

We first ran numerical models of long-profile river evolution to understand better terrace formation patterns in endmember (climate-only, landslide-dam only) and combined scenarios. Secondly, we combineed a set of 38 cosmogenic nuclide exposure samples (¹⁰Be) with 10 optically stimulated luminescence (OSL) samples to constrain fluvial aggradation and incision phases. Finally, we map terrace profiles along ~250 km of the main stem and its tributaries.

Our geochronology identifies aggradation and terrace abandonment phases matching (1) late‑Pleistocene cold phases and (2) the inverval between 20 and 15 ka (post‑LGM), during which the Beshkiol landslide‑dammed lake formed.

 The numerical models generate two contrasting terrace patterns: (1) limited downstream terrace extent associated with lake‑drainage incision, and (2) a basin‑wide suite of terraces produced by climate‑driven changes in sediment‑to‑water ratio. Fluvial terrace mapping reveals widespread terraces even in the tributaries. Terrace slopes are generally sub-parallel; only in the vicinity of the landslide, the main trunk and the tributaries show concave-up profiles.

We conclude that both the landslide‑dammed lake and regional Pleistocene climate influenced terrace formation. Lake drainage primarily affected the lowermost main stem and adjacent tributaries, whereas regional climate was the dominant driver of alluvial terrace formation throughout the catchment. Our study demonstrates that a multi‑method approach—combining numerical modeling, cosmogenic‑nuclide dating, OSL, and detailed terrace mapping—greatly improves the interpretation of alluvial river archives in complex settings. It also provides a framework for quantifying the relative contributions of competing landscape‑evolution drivers.