Recurrent dammed-lake outburst superoods in the Yigong river, southeastern margin of the Tibet

Landslide dam outburst oods have a signicant impact on landform evolution in high mountainous areas. Historic landslide dams on Yigong River, southeastern Tibet, generated two outburst superoods of > 10 5 m 3 /s in 1902 and 2000 AD. One of the slackwater deposits, newly found immediately downstream of historic dams, has been dated to 7 ka BP. One-dimensional backwater stepwise method gives an estimate of 225,000 m 3 /s for the peak ow related to the paleo-stage indicator of 7 ka BP. The recurrence of at least three huge landslide dam impoundments and super-outburst oods at the exit of the Yigong lake during the Holocene greatly changes the morphology of the Yigong river. Recent reports of giant landslides elsewhere on the southeastern Tibetan Plateau indicate repeated landslide dams at the same location, which counters the traditional viewpoint that landslide damming is spatially random, unlike glacial damming that persists at the same location. Repeated landslide damming may be a persistent source of outburst oods as well as impede the upstream migration of river knickpoints in the southeastern margin. Zhamu deposits estimated with radiocarbon and optically stimulated luminescence (OSL) dating methods. We calculate the paleoood peak discharge related to these deposits by using 1-D step-backwater method and discuss the impact of the three ood events on the Yigong river’s evolution. Such rare direct observations and records of repeated outburst oods at the same river reach are very valuable in verifying paleoood hydraulic reconstruction and better understanding the role of landslide dam outburst oods in the landscape evolution of the Tibet plateau’s eastern margin.


Introduction
Large landslide or glacier dams and associated outburst oods have profound impacts on alpine landscape evolution around the Tibetan plateau [1][2][3][4][5] . The impacts have two side effects. On the one hand, natural long-lived dams as stable knickpoints protect upstream channels from river incision and hence retard headward erosion into the plateau interior. On the other hand, the dammed lake outburst oods may control long-term valley evolution such as erosion of bedrock canyons in the Himalaya. Among them, the so-called super oods play a key role in bedrock plucking, boulder mobilization, coarse grain comminution that moderate ows have poor competence in 6-9 . However, super ood events are infrequent and rare direct observations are made. Most information of super ooding process comes from sedimentary records of outburst ood deposits at the downstream of breached dams 10,11 . Records of modern and ancient super ood events in a river are useful for veri cation of the ood geomorphic effects and hydraulic hypothesis.
The Yigong River, a tributary of Yarlung Tsangpo River, southeastern Tibetan Plateau, experienced two well documented modern super oods of > 10 5 m 3 /s in 1902 and 2000 AD [12][13][14][15] (Fig. 1). Both events were generated by failure of landslide dams at the same location, the Zhamu Creek con uence (Fig. 1B). In this study, we report the ndings of old lacustrine and slack water deposits (SWDs) downstream of the Zhamu landslide during the recent eld survey (Fig. 1C). Age of the deposits is estimated with radiocarbon and optically stimulated luminescence (OSL) dating methods. We calculate the paleo ood peak discharge related to these deposits by using 1-D step-backwater method and discuss the impact of the three ood events on the Yigong river's evolution. Such rare direct observations and records of repeated outburst oods at the same river reach are very valuable in verifying paleo ood hydraulic reconstruction and better understanding the role of landslide dam outburst oods in the landscape evolution of the Tibet plateau's eastern margin.

Study Area
The study area is at downstream of Yigong river, a tributary of Parlung Tsangpo which is ow into Yarlung Tsangpo river through the Grant Tsangpo Gorge in the southeastern margin of Tibetan plateau ( Fig. 1). It is surrounded by numerous summits > 6000 m a.s.l. in Nyaiqentanglha range on the north and Himalayan range on the south. Local topographic relief is up to 6 km in the gorge between Namche Barwa (7782 m) and Gyala Peri (7294 m) massifs. Warm and humid Indian monsoon reaches further to upstream of Yigong river through the deep gorge. The average annual rainfall descends from ~ 3000 mm at the town of Medog to ~ 1100 mm at the town of Yigong. Abundant precipitation and high relief make it the most active region for monsoonal temperate glaciers with the total area of 2490 km 2 in China.
The Yigong region is in highly tectonically active mountain belts where the rock uplift rate is as high as 10 mm/yr. The eastern syntax of Himalaya, a northeast protrusion between India and Asia plates is about 30 km to the south. The Jiali fault, a large WNW-ESE trending strike-slip fault extends from the Yigong to the Parlung Tsangpo, traversing the southeast of the Tibetan Plateau. Due to active tectonic movement and high relief, frequent strong earthquakes happened and triggered numerous geological hazards, e.g.
the Great Assam earthquake of 8.6 magnitude in 1950, and the Ms 6.9 Milin earthquake in 2017. The Yarlung Tsangpo suture zone is a distinct boundary of Tibetan source and Himalayan source rocks (Fig. 1A). The Tibetan source block completely contains the study area. Primary outcropped rocks in the area are Proterozoic gneiss, Pre-carboniferous schist, and Carboniferous slate and sandstone.
Widespread quaternary deposits including laterofrontal moraines, debris fans, and ood deposits distribute along the river's valley.
The Yigong River has a drainage area of 13,500 km 2 and mean annual discharge of 378 m 3 /s. Two largemagnitude landslides took place in 1902 and 2000 AD at the Zhamu Creek next to the exit of Yigong lake Three charcoal samples were collected from the ne sand unit and one from the coarse sand unit.
Radiocarbon ages of the charcoal pieces range from 6.7 ka to 7.1 ka BP by radiocarbon dating at the Beta Analytic laboratory, USA (Table 1). Four samples were also collected for OSL dating at the OSL laboratory of the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences. The OSL dating results are shown in Table 2. Additionally, OSL dating of samples from each of the lower three units was performed on a Lexsyg Research automatic TL/OSL instrument at the Institute of Mountain Hazards and Environment, CAS. The sample from the silt unit gave an OSL age of 7.6 ± 0.7 ka, which is consistent with the radiocarbon age. The other two samples gave ages of 18.5 ± 1.7 ka ( ne sand) and 15.3 ± 1.4 ka (coarse sand) (Fig. 2D); the difference may be due to their larger grain size, or lack of bleaching in a catastrophic ood event. Combining the OSL and radiocarbon dating results gives a probable age of ~ 7 ka BP for the Shuangyu SWD.  In addition, we found lacustrine deposits (N 30°09'05", E 94°59'42") at the mouth of Dayi creek, a Yigong's tributary, 2 km upstream of the Shuangyu SWDs (Fig. 1C). The 2.2m parallel lacustrine lamination with yellow-brown silt is elevated c. 40.0 m above the adjacent oodplain. It is capped by 4.3 m debris-ow accumulation composed of angular and sub-angular gravels and boulders. A 1.5 m no-bedding mixture with sand and gravels is exposed under the lamination (Fig. 3A). Two pieces of charcoal were taken from the silt deposit and radiocarbon dated at Beta Analytic laboratory, USA (Fig. 3B). The 14 C ages are determined as 6.3k Bp and 6.6k Bp, which means the Dayi lacustrine deposits are a little later than the Shuangyu SWD. The sediment sequence is located at the mouth of the branch Dayi creek on the right bank of the Yigong river. The bank is a concave bend behind a narrow reach, where the transport capacity of the 7k Bp super ood decreased and large volume of sediment that carried by the super ood stopped here. We speculate the paleo ood deposits jammed the Dayi's outlet, forming a small temporary dammed lake in the creek. In ow silt had deposited in the dammed lake and produced the lacustrine lamination.
The upper debris-ow accumulation on the lacustrine deposits implies that it is very likely a large-scale debris ow happened in Dayi and broke out the dammed lake.

The modern ood SWDs
Another SWD was found on the opposite bank of Jiazhong village, a hydraulically sheltered area 3.9 km downstream of the 2000AD breached dam (Fig. 1C). The Jiazhong SWD is located within a cove on the river right bank, and its surface is a grassland about 6.0 m higher than the oodplains on the both banks ( Fig. 4A). The excavated section is a 1.2 m thick sequence of ne to medium-grained sand capped by a ~ 10 cm thick sandy loam at ~ 21.0 m above the river level. The dark gray ne sand is interbedded with three units of ~ 10 cm thick grey medium sand and mingled with sub-angular gravels (Fig. 4B). The 0-70 cm upper part is nearly parallel laminated, but the lower part inclines to the river at an angle of ~ 5°, indicating an original local gradient and ow direction. A piece of charcoal collected in the middle of the SWD section was dated at the Beta Analytic laboratory. The measured percent modern carbon is 100.12 ± 0.37 and the radiocarbon age ranges from 1880 to 1956 AD with 87.3% probability. Monsoon seasonal oods of ~ 2100 m 3 /s peak unlikely reach the location of Jiazhong SWD. The upmost sandy loam is very thin, which means it formed in a relatively short period. Moreover, the SWD's elevation is close to the 2000AD ood deposits next to it (Fig. 4A). Therefore, we interpret the Jiazhong SWD as the product of the 2000 outburst ood. The Jiazhong SWD's level can be used as the ood stage indicator of the 2000 event.

Peak ow reconstruction
The well documented 2000 AD ood can be used as a benchmark to aid reconstruction of historic events.
The peak 2000 AD discharge at Tongmai Bridge has been estimated as 126,400 m 3 /s 13 , 120,000 m 3 /s 12 , and 124,000 m 3 /s 20 , with a peak water depth of 52 m. We compare peak discharge calculations for the 2000 AD ood using 15 empirical formulas and know data on breach depth, dam height, volume of released water, and impounded water volume (Table 3) Table 4). The resulting discharge of ~ 168,000 m 3 /s is more than 50 times that of a large monsoon season ood.

Discussions
The landslide dams and outburst oods greatly changed the landscape of the Yigong river. Delaney and Evans 15 plotted a speculative pre-existing pro le derived from SRTM-3 data. They proposed it could be the river valley pro le before 1900 event (i.e. 1902 event in this paper). The intersect point of the preexisting channel with the present is about 5.8 km downstream of the breached dam (Fig. 6). The channel slope from the dam to the intersect point is 16.5‰, double of the pre-channel slope. This reach lled up by quick deposits of the landslides and the outburst oods. The sediment supply from the landslides keeps the bed stable under such a steep slope. From the intersect point to the Tongmai bridge, the channel slope is 8‰ which is probably a natural slope without the disturbance of landslide damming.
It is observed that the lake and braided stream system had developed as early as in 1973 from the KeyHole-9 satellite image (Fig. 7A). The lake's level and uvial deposits show little change even after 23 years (Fig. 7B). We estimate the trapped sediment in the lake is about 0.26 billion m 3 with 1 km of the lake's average width. Considering the low sediment concentration of the Yigong river, it should take more than thousand-year span to trap such a huge volume of sediment. It is reasonable concluded that the Yigong lake likely formed as early as 7ka ago and the 1902 landslide accumulation overlapped on the previous residual dam. The height of the dam associated with the 7 ka Bp event was at least 170 m based on the peak discharge of 225,000 m 3 /s (Fig. 6). The stable knickpoint induces backwater aggradation and protects upstream river channel from incision. We conjecture that the 7 ka Bp dam may be caused by an ancient landslide at Jiazhong village or debris ows in Zhamu or Bailong catchments from the remained site-speci c alluvial fan or landslide terrace (Fig. 8).
Glacial and landslide dams in Tibetan-Himalayan rivers contribute greatly to the long-term stability of river knickpoints in the Himalayan syntaxes and, hence, impede incision into the interior of the Tibetan Plateau 3,4 . Previous research suggests that landslide damming is spatially random, unlike glacial damming repeatedly formed at the same location because of periodic advance of glaciers 4,26 . However, recent large-scale landslides triggered by earthquakes or ice-rock avalances at the entrance of the Tsangpo Gorge and on the Parlung River demonstrate that repeated blockage of the rivers by landslide dams at the same location may be common near the eastern syntax of Himalaya 27,28 . Our ndings provide a real case of landslide negative feedback to river incision over geological timescale. The Yigong's three large damming events near Zhamu creek demonstrate that recurrent landslide dammings played a primary role on the stability of some knickpoints when glacial or moraine dams cannot reach to the river trunk.

Conclusions
Dammed lake outburst oods are common in the margin of the Tibetan Plateau. In this paper, two slack water and one lacustrine deposits near the 1902 and 2000 AD Yigong landslide dams are reported in detail. From these sediment records, a paleo ood event of ~ 7 ka BP is identi ed by the radiocarbon and OSL dating tests. The 1-D backwater stepwise method gives a 225,000 m 3 /s of peak ow for the paleo ood. The peak discharge of the 1902 AD ood is about 168,000 m 3 /s by comparing 15 empirical models with the 2000 AD ood. The river pro le was strongly in uenced by the dams and associated outburst oods. It is roughly estimated that a volume of 0.26 billion m 3 sediment or even more has been aggraded upstream of the 2000 AD dam. The slope of the channel immediately after the dam is twice of that before the 7 ka BP event due to rapid settling of the ood sediments. Moreover, at least three super oods of > 10 5 m 3 /s happened on the Yigong since 7 ka BP indicate that the recurrence interval of Holocene outburst oods on the southeastern margin of the Tibetan Plateau is much shorter than that of monsoon oods with the same magnitude. The dominant effect of outburst oods should be accounted for in long-term landscape evolution models of the southeastern margin.    * Volume was calculated using (Delaney and Evans, 2015), where A is the area of impounded water †Peak discharge was calculated using (MacDonald and Langridge-Monopolis, 1984), where H is barrier lake water depth and V is lake volume. Figure 1 (A) Location of the Yigong river and the Yarlung Tsangpo Gorge. The eastward owing Yigong River turns southward and ows into the gorge after con uence with the westward owing Parlung River at Tongmai.     The location of Bailong creek and the ancient landslide at Jiazhong village. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.