Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Abstract

Abstract. Large boulders, ca. 10 m in diameter or more, commonly linger in Himalayan river channels. In many cases, their lithology is consistent with source areas located more than 10 km upstream, suggesting long transport distances. The mechanisms and timing of “exotic” boulder emplacement are poorly constrained, but their presence hints at processes that are relevant for landscape evolution and geohazard assessments in mountainous regions. We surveyed river reaches of the Trishuli and Sunkoshi, two trans-Himalayan rivers in central Nepal, to improve our understanding of the processes responsible for exotic boulder transport and the timing of emplacement. Boulder size and channel hydraulic geometry were used to constrain paleo-flood discharge assuming turbulent, Newtonian fluid flow conditions, and boulder exposure ages were determined using cosmogenic nuclide exposure dating. Modeled discharges required for boulder transport of ca. 103 to 105 m3 s−1 exceed typical monsoonal floods in these river reaches. Exposure ages range between ca. 1.5 and 13.5 ka with a clustering of ages around 4.5 and 5.5 ka in both studied valleys. This later period is coeval with a broader weakening of the Indian summer monsoon and glacial retreat after the Early Holocene Climatic Optimum (EHCO), suggesting glacial lake outburst floods (GLOFs) as a possible cause for boulder transport. We, therefore, propose that exceptional outburst events in the central Himalayan range could be modulated by climate and occur in the wake of transitions to drier climates leading to glacier retreat rather than during wetter periods. Furthermore, the old ages and prolonged preservation of these large boulders in or near the active channels shows that these infrequent events have long-lasting consequences on valley bottoms and channel morphology. Overall, this study sheds light on the possible coupling between large and infrequent events and bedrock incision patterns in Himalayan rivers with broader implications for landscape evolution.

Influence of Rarely Mobile Boulders on Channel Width and Slope: Theory and Field Application

“…However, whether those boulders were delivered to the Liwu River tributaries recently or if they were placed a long time ago requires further research. Field evidence from other tectonically active sites such as Himalayan rivers (Huber et al., 2020) and steep channels draining into the Dead‐sea, Israel (Haviv, 2007), demonstrates that boulders may last in rivers for periods of tens of thousands of years.…”

“…A series of recent studies explored the morphological effects of large, rarely mobile boulders, a few meters or more in diameter, which are observed in bedrock rivers (Cook et al, 2018;Shobe et al, 2020Shobe et al, , 2021 draining high-relief mountainous terrains. Here, the term "large, rarely mobile boulders", or "large boulders" in short, refers to those grain sizes that are rarely mobile for prolonged durations with expected substantial impacts on long-term geomorphic processes (Haviv, 2007;Huber et al, 2020;Shobe et al, 2021). The emplacement of large boulders is associated with glacial lake outburst floods (Cook et al, 2018), rockfalls, debris flows, landslides, and glacial erratics (e.g., Jouvet et al, 2017;Polvi, 2021).…”

Particle size dynamics in abrading pebble populations

“…Beyond the evolution of smooth size and shape distributions, there is yet another common phenomenon in fluvial geomorphology where the interaction of transport and attrition could be far from trivial. The often observed presence of isolated large boulders in rivers (Huber et al, 2020) may be explained solely by transport, as these large pieces are often not carried by the river; rather, they move by a different process (e.g., landslide or debris flow). On the other hand, these large rocks could also be interpreted as outliers emerging spontaneously in a pebble size distribution on which collisional abrasion certainly has strong impact in upper reaches of rivers.…”

Suyya’s Flood: Numerical Models of Kashmir’s Medieval Megaflood and Ancient Lake Kerewa Drainage Events

“…Hostilities in Kashmir have prevented ground inspection of these boulders to ascertain their petrology or provenance, or to estimate their date of deposition (c.f. Huber et al, 2020). Godwin-Austen, 1859 documents massive granite boulders mantling the 60 m high-river terraces at Gujar Bandi (34.25°N,73.85°E), and the narrow river terrace between Gingal Fort (34.135°N, 74.113°E) and Uri (a combined downstream distance of 26 km) that were derived from the Kaj Naj porphyry in the mountains 15 km NE of Gingal.…”