Introduction

Landslides can be important erosional processes in upland landscapes with moderate to steep hillslopes1. In particular, landsliding dominates hillslope erosion in rapidly uplifting terrain where high-relief, threshold hillslopes, and relatively narrow river gorges are common1,2,3,4,5. Landslides can introduce large amounts of sediment into river systems and large landslides can dam channels, impounding water and sediment6,7. In turn, an elevated sediment supply can retard river incision by shielding bedrock8. Korup and Montgomery proposed that frequent glacial damming in the eastern Himalayan syntaxis (EHS) during the Quaternary inhibited headward incision of major rivers, thereby retarding dissection into the edge of the Tibetan Plateau2.

While this hypothesis was rooted in evidence for repeated glacial damming of the Yarlung Tsangpo immediately above its gorge through the Himalaya9, similar glacial and landslide blockages occur on other major tributaries upstream of the deeply incised bedrock gorges in the EHS2. Little is known, however, about the frequency with which channel damming events have occurred in the region. The Lulang landslide-dammed lake in the upper reaches of the Lulang River, a tributary to the Parlung Tsangpo, was reported recently to have remained stable from before 24.2 ka bp to around 8.8 ka bp10. This period extends roughly through the last glacial maximum and well into the early Holocene. Like the glacial dam on the Yarlung Tsangpo, the Lulang landslide sits just upstream of the headward extent of the knickzone upstream of the deeply incised-trans-Himalayan bedrock gorge10.

In April, 2000, the giant Yigong landslide dammed the Yigong Tsangpo River with 3 × 108 m3 of sediment that formed a 60 m high dam11. Subsequent failure of the dam sent floodwaters rushing down through the gorge, causing extensive damage for 500 km downstream into India12. Like the glacial dam at the head of the Tsangpo Gorge, and the landslide dam on the Lulang River, the Yigong landslide is also located immediately upstream of the knickzone defining the headward limit of river incision into the edge of the Tibetan Plateau. Here we present evidence for older landslides at the site of the Yigong Landslide, documenting repeated damming of the river at this location over the late Holocene. Our findings show that repeated failures of large landslides at this location both present a recurrent hazard and help retard headward propagation of fluvial knickpoints, thereby affecting the development of river profiles in the EHS13.

Field area

The giant Yigong landslide occurred in Zhamunong Gully, a tributary of Yigong Tsangpo River 59 km upstream of the confluence with the Yalu Tsangpo River (Fig. 1a). The area is underlain by Gangdisi bedrock14, with Himalayan granites exposed in the upper part of Zhamunong gully. The active Jiali-Chayu fault passes through the landslide area, which together with active faulting along the nearby Lulang-Yigong fault left the rock mass in the landslide area highly fractured.

Figure 1
figure 1

Location map of the Yigong landslide section at 30° 10′ 58″ N, 94° 55′ 53″ E. (a) Landsat-7 image obtained on May 4, 2000 (25 days after Yigong landslide occurred (image data download from https://www.gscloud.cn, which is open and free sharing for science study); (b) photograph of Yigong distal debris avalanche deposition zone before the dam failed (provided by Y.P. Yin, taken on April 23, 2000); (c) photograph of Yigong landslide after the 2000 dam broke (photo by Guo in 2014, view to NE). Figure generated with CorelDRAW X7 software (from https://www.coreldraw.com).

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On April 9, 2000, a rock avalanche1 of about 3 × 107 m3 collapsed from a hilltop at 5,500 m in elevation and dropped more than 1,500 m, impacting older detrital materials deposited along Zhamunong gully11. The resulting debris avalanche transformed into a high-speed debris flow that swept both sides of the Zamunong valley as it ran out 8 km in under 10 min before depositing15 (Fig. 1b). The Yigong Tsangpo was completely blocked as the landslide formed a natural dam with a length of 4.6 km, a width of up to 3 km, a thickness of 55–110 m, resulting in a deposit with a total volume of about 3 × 108 m311,16,17. The dammed lake collapsed 2 months later18, producing a catastrophic flood that destroyed the Tongmai Bridge, 17.6 km downstream from the landslide dam. The flood surged downstream through 500 km of Arunachal Pradesh to reach the floodplain of the Brahmaputra River in Assam12, India, destroying 20 bridges and leaving 50,000 people homeless. The flood down the gorge produced sustained high bed shear stresses capable of plucking meter-scale blocks from the riverbed19.

A prior landslide involving collapse of 5 × 108 m3 occurred along Zhamunong gully in 190011,18, and also blocked and dammed the Yigong Tsangpo River, producing a 51.9 km2 lake. One month later, the dam collapsed due to lake overtopping, reportedly releasing about 2.7 × 109 m3 of water20. Investigations and studies on the Yigong landslide concluded that the 2000 landslide eroded portions of the 1900 landslide17,21.

Methods

Geological age dating

Field surveys examined stratigraphic exposures along Zhamunong gully, as well as in riverbank and valley wall exposures. Stratigraphic exposures in the study area were dated with seven radiocarbon (14C) dates on organic materials sampled from the deposits. Field surveys located datable materials at three sites examined in further detail.

Geomorphic analysis

To examine the influence of repeated landslide dam formation on river channel development, we extracted the longitudinal profile of the Yigong Tsangpo River using a 30 m SRTM (Shuttle Radar Topography Mission) digital elevation model (DEM) and the Google Earth image (Landsat data), based on field investigation aided by the interpretation of 1:25,000 topographic maps. The relative steepness of channel reaches was quantified via the empirical relationship

$$ {\text{S}} = {\text{K}}_{{{\text{sn}}}} {\text{A}}^{ - \uptheta } , $$
(1)

where S is channel slope, Ksn is the steepness index, A is upstream area, and θ is the concavity index22. We used θ = 0.45 to calculate Ksn values every km along the river profile.

Results

Exposures at three sites record emplacement of lacustrine sediments and debris flow deposits several times during the Holocene. Sites 1 and 2 are exposures in the eroded walls of Zhamunong gully (Fig. 1a,c). Site 3 is exposed in valley wall sediments several km up valley from the gully mouth (Fig. 1a).

Site 1

Site 1 is located on the west side of the Zhamunong Gully (Fig. 1c), where a series of four debris flow deposits are exposed in a 50 m-thick section consisting of six identifiable strata (Fig. 2). The uppermost layer consists of a 20.5 m thick massive, unsorted debris flow deposit with gravelly and sandy lenses. This unit unconformably overlies a thinner, older debris flow deposit that displays oxidation extending down from its upper surface, indicating a period of ground surface stability and incipient soil formation prior to emplacement of the overlying debris flow. Below this second debris flow unit several meters of blue-gray medium to fine grained sand overlie up to 5 m of grayish-yellow medium to coarse grained sand. These sands are deposited on top of another (third) unsorted debris flow deposit from which two samples were collected for 14C dating, yielding mutually consistent ages of 2450 ± 30 bp (595–411 cal bc) and 2520 ± 30 bp (695–542 cal bc). The lowest unit at the exposure is a fourth debris flow deposit that yielded two consistent 14C dates of 3100 ± 30 bp (1432–1283 cal bc) and 3110 ± 30 bp (1437–1288 cal bc). Hence, this exposure reveals evidence for four pre-historic landslides, with one occurring around 1300 bc, another around 600 bc and two since that time (and prior to 1900).

Figure 2
figure 2

(a) Photograph (note people for scale) and (b) sketch log of positions of dated samples at Site 1. ① Dark-grey debris flow deposit with gravelly and sandy lenses; ② grayish yellow medium to fine grained debris flow deposit; ③ blue-grey medium to fine grained sand with gravel; ④ grayish-yellow medium coarse sand; ⑤ unsorted debris flow, the top of which consists of carbon-rich peat and charcoal, location of 14C samples 2019YG-06 (2520 ± 30 bp) and 2019YG-09 (2450 ± 30 bp); ⑥ debris flow deposit with carbon-rich sandy lenses, location of 14C samples 2019YG-14 (3110 ± 30 bp) and 2019YG-05 (3100 ± 30 bp). Figure generated with CorelDRAW X7 software (from https://www.coreldraw.com).

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Site 2

Two samples of organic debris collected from within the debris flow deposit at Site 2 (Fig. 3) dated at 2760 ± 30 bp (975–830 cal bc) and 2940 ± 30 bp (1225–1045 cal bc). These dates are intermediate in age between the two dated debris flows at Site 1. This deposit likely therefore represents a different landslide event than the two youngest, undated debris flow deposits at Site 1. Hence, there was also at least one pre-historic landslide around 1000 bc.

Figure 3
figure 3

Exposure at Site 2 indicating position of dated samples from the east bank of Zhamunong Gully. (a) Context view showing area from which samples were collected. (b) Location of 14C samples SXZ008-5 (2760 ± 30 bp) and SXZ008-6 (2940 ± 30 bp). Figure generated with CorelDRAW X7 software (from https://www.coreldraw.com).

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Site 3

Site 3 exposes a section about 8 m thick southeast of Tongjia village, Yigong Town. Field surveys revealed that the section at Site 3 is composed of four distinct layers (Fig. 4a). The surficial layer consists of a roughly 0.5 m thick brown colluvial soil. Below this layer a locally more than 1 m thick deposit of lacustrine sediment consisting of light gray clay rests atop a 30 cm thick layer of organic-matter rich black silty clay (Fig. 4b). A sample of charcoal taken from this layer was 14C dated at 4650 ± 30 bp (3517–3396 cal bc). This layer of lacustrine sediment was deposited on top of clay and debris flow sediment, roughly 2 m below the surface.

Figure 4
figure 4

(a) Photograph and (b) sketch log of exposures at Site 3, showing stratigraphy and position of dated samples: ① Soil, ② lacustrine sediment, ③ weathering surface of ④ debris flow sediment. We collected one wood charcoal sample at depths of 1.2 m (2019YG-18) that yielded a 14C calibrated age of 4650 ± 30 years bp (Table 1). Figure generated with CorelDRAW X7 (https://www.coreldraw.com).

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Table 1 14C dating for samples from Yigong landslide-dammed lake sediments.
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Longitudinal profile

The Yigong landslide dam occurs at the downstream end of a 20 km long reach of the river that has a very low Ksn value due to sediment infilling of the valley bottom (Fig. 5). The more than fourfold drop in Ksn values in the reach upstream of the recurrent dam location indicates little potential for river incision, and thereby headward erosion in this location at present.

Figure 5
figure 5

Longitudinal profile for Yigong Psangpo River and the dam created by the 2000 Yigong landslide.

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Discussion and conclusions

Historical records, the most reliable sources of data for reconstructing the landslide reoccurrence, are only available for the two most recent (1900 and 2000) landslides. Geological dating of pre-historic landslide deposits generally relies on either 14C dating, of organic materials or optically stimulated luminescence (OSL) dating methods of mineral grains23. Uncertainties inherent to 14C dating include both the analytical error in the reported date and the uncertainty arising from the nature of dated material24,25. In radiocarbon dating of charcoal, for example, there is uncertainty associated with the potential for it to have been reworked and whether it came from the inner or outer wood of a tree. While it is preferable to radiocarbon date twigs, cones, or needles the outcrops examined in this study yielded charcoal, black carbon-rich peat and clay for dating the Yigong landslide. Hence, the charcoal samples provide limiting maximum ages for the landslide deposit they were recovered from, as the dated organic material may have died prior to the landslide and been incorporated into it. In contrast, the peaty deposits, like the organic-rich clay, may develop after deposition of the associated sediments, and therefore represent minimum limiting ages. However, the reasonable concurrence of the replicate ages for the samples of each of the landslide deposits reported here suggests that the landslides at sites 1 and 2 represent discrete events reasonably well dated by these 14C ages.

The evidence for repeated landslide-damming of the Yigong Tsangpo River at the location of the 2000 landslide and outburst flood indicates that such impoundments and the floods they produced were recurrent events that occurred frequently enough to potentially influence river profile development. In particular, the new dates for prior late Holocene river-damming landslides allow estimating maximum recurrence intervals for such events. Together with prior reports of the 2000 and 1900 landslides, the exposures of six older landslide deposits indicate that there were at least eight large landslide events at the Yigong landslide over the past 5,500 years (Fig. 6). In light of the dating uncertainties, all together our evidence indicates less than millennial recurrence intervals for river-damming landslides at this location. However, the cluster of dated exposures at sites 1 and 2 indicate that the interval between landslides was just over 200 years across the time interval the two outcrops span, and the two most recent, historical landslides occurred just a century apart (in 1900 and 2000). Hence, the known record of past landslides in this location indicates that channel-damming events occurred with sub-millennia recurrence intervals through the late Holocene.

Figure 6
figure 6

Age ranges dating the different landslides at the Yigong location; width of boxes shown corresponding to 14C Calendar calibrated results (this study).

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The uplift of the Tibetan Plateau is the main driving force for topographic development and geomorphic change in this region26,27. In the EHSs, river profile steepness values predict that steep fluvial knick points at the southeastern plateau margin should erode rapidly27, driving a wave of incision back into the plateau2. However, dams that reform or persist at the same location could stabilize knickpoints due to the long-term effects of sedimentation, and recurrent landside dams could stall fluvial kinckpoint retreat, headward incision, and reorganization of the river network7. The recurrent damming of the Yigong Tsangpo at the same location over the late Holocene reported here establishes that the Yigong Landslide repeatedly created river-damming deposits capable of retarding upstream incision and potentially producing highly erosive flooding downstream.

At the head of the knickzones on each of the Yarlung Tsangpo, Yigong Tsangpo, and Lulang rivers, landslide or glacial dams acted to retard headward incision over geomorphically significant time scales. For the Lulang landslide, where a large landslide dammed a small tributary, the blockage persisted for millennia through the last glacial maximum as well into the early Holocene10. In the case of the Yigong landslide, the direct impoundment by the 2000 landslide only lasted several months, but the sediment impoundment upstream of the landslide location was continuous from 1900 to 2000, and likely persisted between earlier landslides documented here as well. On the Tsangpo River, repeated glacial damming at the same location likewise acted to retard river incision. In each of these locations, the repeated blocking of the river at the same location for a large proportion of time would act to inhibit knickzone propagation and thereby retard river incision into the margin of the Tibetan Plateau2.