A short-lived flashflood in Rishi and Dhauli Ganga rivers on 7th February 2021, Uttarakhand Himalaya, killed 65 people with 141 reported missing (official estimate) and devastated two hydropower projects. Geomorphological observations supported by meteorological data suggest that the flood was triggered by a combination of avalanche and debris flow. The Dhauli Ganga valley has preserved ponded sedimentary sequences (laminated sand and silty-clay), suggesting that the valley is prone to episodic mega foods in the recent geological past. Considering that the receding glaciers in the higher Himalaya have left behind enormous sediment, unusual weather events are likely to generate such disasters more frequently as the climate becomes warmer. Thus, the study calls for not only incorporating the disaster risk assessment in the developmental planning of the Himalayan region but also recommends routine monitoring of the potential areas of structural failures in the glaciated valleys along with supra-glacial lakes.

The geomorphic changes over the earth’s crust are influenced by tectonic activities. These geomorphic changes are remnants of deformation that occurred in the recent geological past. Geomorphic features can be quantified to assess relative tectonic activity and response of landscape to active tectonics, regional structures, lithology and climate. To achieve the objectives, we evaluated the relative tectonic activity of the Garhwal synform, for which six major river basins were selected. The relative tectonic activity of all the basins is computed based on quantitative analysis of geomorphic indices. Quantitative analysis of each geomorphic parameter has been carried out, and a combined product of relative tectonic activity index (TAI) was derived for each basin. The TAI is classified into three classes based on their relative tectonic activity; basins having TAI value ${\leq}$1.75 (basins I, II and III) are placed in very high tectonic activity class, basin with a value ranging >1.75 to ${\le}$2.0 are categorised as moderately active basins (basin ‘IV’), while basins having values >2.0 are less active (basins V and VI). A relative tectonic activity map of the area surfaces for the prioritisation of each basin based upon their TAI. Furthermore, analysis of the longitudinal profile of rivers for knickpoint, precipitation and temperature variability over the last 100 years and seismic events since the last 100 years have been studied to interpret the tectonic regime and their influence on landscape evolution. The regional seismicity data suggest that the area falls in a seismic gap and has not experienced a great earthquake in recent history but have received seismic events of moderate intensity in the past. We opine that the Garhwal synform is tectonically active, and thus, significant steps should be taken for seismic risk assessment along with preventive measures. We also suggest that the influence of tectonic activities in the southeastern part of the Garhwal synform comprised by basins V and VI is relatively less than the rest of the basins. Finally, the six basins were prioritised based on their relative tectonic activity.

$\bf{Highlights}$

$\bullet$ Assessment of geomorphic indices from 30 m shuttle radar topography mission-digital elevation model (SRTM-DEM) in six drainage basins of the Garhwal Himalaya.

$\bullet$ Six river basins categorised under relative tectonic classes based on the calculation of geomorphic indices.

$\bullet$ Correlation of the large-scale geological setting and drainage basin dynamics contemplated with field evidence and regional seismicity.

$\bullet$ Quantification of relative tectonic activity index (TAI) of six river basins in the Lesser Himalaya of the Garhwal Himalaya.

We present millennial to centennial-scale monsoon variability during the Late-Pleistocene to Holocene using multi-proxy data from 100 cm thick peat deposit in the Baspa Valley, northwest Himalaya. Based on high-resolution mineral magnetism, carbon isotope, and total organic carbon data supported by radiocarbon dating, four climatic phases of alternating strengthened and weakened Indian summer monsoon (ISM) are identified for the last 20 kyr in the higher central Himalaya. Periods of strengthened ISM are dated to ${\sim}$15 to ${\sim}$14 ka, ${\sim}$10 to ${\sim}$7 ka, ${\sim}$2.4 to ${\sim}$1.3 ka, and 243 yr BP to present, which is ascribed to the post-Older Dryas associated with an increase in solar insolation. The phases of weakened ISM are bracketed between ${\sim}$20 and ${\sim}$15 ka, ${\sim}$14 to ${\sim}$10 ka, ${\sim}$7 to ${\sim}$2.4 ka, and ${\sim}$1300 to ${\sim}$243 yr BP. These phases are attributed to global cooling events, i.e., the Last Glacial Maximum (LGM), Younger Dryas (YD), and the Middle to Late Holocene. They govern by changes in the solar insolation.

The June 2013 disaster in Kedarnath hit the Mandakini–Alaknanda river valley, resulting in devastating floods. This study deals with the pre- and post-flood event changes in the channel morphology and aims to investigate the geomorphological processes under river management in the Alaknanda valley, NW Himalaya and how the hydroelectric reservoir may have impeded the natural impact of the disaster and created geomorphic discontinuity. This work analyses the spatio-temporal variations in channel morphology over the last decade 2010–2020, discussing the impact of 2013 extreme event; the role of gradient in morphological patterns in river basin system. It highlights how the channel parameters like the thalweg shifts, active channel width, and area under sedimentation responded, from headwaters to lower gradient Lesser Himalayan zones to the 2013 event and suggests that any positive changes in these parameters diminish soon after the reservoir. The study implies that the capability of the reservoir to adjust the sediment load of the event in its upstream is an immediate short-term effect, but brings out the fact that it creates a geomorphic disconnect in the channel between upstream and downstream channel reaches of the reservoir. This disconnect may have a negative impact on sediment storage and sediment–water routing of the river and should be factored into the dam design ensuring natural continuum of geomorphic processes. Further, the study argues that the terrain north of the Main Central Thrust (Higher Himalaya) should be kept free from major human interventions to reduce flood hazards.