Impact of glacial retreat in Himalayas on Hydrology: A Comprehensive Review Often referred to as the “Third Pole,” the Himalaya and Hindu Kush, are the largest concentration of glaciers outside the polar regions. Himalayan glaciers are critical of freshwater in Asia, feeding major river systems like the Indus, Ganges, Brahmaputra, etc. Supporting about 1.9 to 2 billion people. With climate change and global climate change, growing rate of glacial retreat is becoming a major issue which is leads to major issues related to water and food security, river discharge change, underground water recharge, the risk of floods and droughts across the region (Bolch et al., 2012; Kulkarni et al., 2021). This literature review examines the hydrological impacts of Himalayan glacier retreat and explores its subsequent socio-economic implications. Glacier Retreat in the Himalayas Rising global temperatures has accelerated Himalayan glaciers retreat over the past few decades. According to studies up to 67% of Himalayan glaciers has been lost in last decade itself, and projections predict more paid decline with current temperature rise (King et al., 2021). Some recorded examples are Gangotri Glacier has been retreat at an average rate of 18.5 meters per year from 1963 to 2006 (Ramanathan, 2011) and Samudra Tapu Glacier receded by 862 meters during the same period (Kulkarni et al., 2007). The annual rate of glacial shrinkage in the Indian Himalayan region ranges from 0.2–0.7% for 11 river basins between 1960–2004, with a mean extent of 0.32–1.4 km² (Kulkarni et al., 2011; Bolch et al., 2012). The Himalayan glaciers are retreating at rates ranging from 10 to 60 meters per year, with many small glaciers already disappearing (Dasgupta & Sen, 2020). Satellite imagery shows the Zemu Glacier in Sikkim has significantly receded since 1935 (Singh, 2016). This trend is attributed to rising temperatures, reduced snowfall, and black carbon deposition that decreases the reflective capacity of ice, accelerating melt (Lee et al., 2021). The rate of glacier retreat is influenced by various factors , including rising temperatures, changes in precipitation patterns, and the deposition of black carbon, which reduces the albedo effect and accelerates melting. The rate of glacier retreat is not uniform across the Himalayas; it varies due to regional climatic conditions, glacier size, and topographical features. Research shows that smaller glaciers are retreating faster compared to larger ones due to their lower ice mass and surface area (Bolch et al., 2012). Hydrological Impacts  1. Variation in River Runoff Glacier melts significantly impact on the rivers water discharge originating from the Himalayas. Major rivers like Indus, Ganges, Brahmaputra etc are glacier-fed water, especially during dry seasons. Glacier retreat initially cause increase in river discharge due to accelerated melting, which raises the risk of floods and soil erosion (Immerzeel et al., 2010). But, with continued glacial retreat, there is reduction in river flow due to decreased discharge which leads to water availability for agriculture, drinking, and industrial use (Prakash, 2020).  Studies predict that the upper catchments of these rivers will experience an initial increase in streamflow due to accelerated melting, followed by a sharp decline as glacial storage is exhausted (Singh et al., 2013). For instance, the Indus Basin, heavily reliant on glacial melt, may see its water availability significantly impacted by mid-century (Lutz et al., 2014). Seasonal variations in runoff patterns are also becoming more pronounced. The shift from glacial to rainfall-dominated river systems can result in more erratic flow regimes, increasing the frequency of both floods and droughts. This has significant implications for water resource management, particularly in regions that depend on consistent river flows for irrigation and hydropower (Immerzeel et al., 2010). 2. Variability in Seasonal Flow Observed shift in period of peak flow from glacier to earlier period of the year , which was originally observed during summer months (Rasul & Molden, 2019). This has lead to decreased water availability during critical agricultural periods, affecting crop irrigation and also affects hydropower generation, during summers which is period of high energy demand (Pathak et al., 2017). This shift in seasonal flow patterns can lead to mismatches between water supply and agricultural demand. For example, earlier peak flows may coincide with periods when water demand for irrigation is low, while reduced flows during the actual growing season can negatively impact crop yields. Moreover, changes in flow timing affect aquatic ecosystems, disrupting the life cycles of fish and other aquatic organisms dependent on specific flow conditions (Pathak et al., 2017). 3. Groundwater Recharge Initial increase in river runoff assist in groundwater recharge especially during dry months , (Kulkarni et al., 2007). But as glaciers retreat, there is reduction in meltwater flow which leads to declining groundwater levels (Rasul & Molden, 2019). When this situation persists for long period it far reaching impacts. Groundwater systems are particularly vulnerable to changes in glacial meltwater input. In many Himalayan regions, groundwater serves as a critical buffer against seasonal water shortages. The decline in recharge rates due to reduced meltwater can lead to the over-extraction of groundwater resources, exacerbating water scarcity issues. Additionally, changes in groundwater recharge can affect water quality, as lower recharge rates reduce the dilution of pollutants (Rasul & Molden, 2019). 4. Glacial Lake Outburst Floods (GLOFs) Glacial lakes which are formed due loose moraine or ice left by glacier retreat that are highly vulnerable to sudden breaches, causing catastrophic Glacial Lake Outburst Floods (GLOFs) (Mool et al., 2001). GLOFs can lead to widespread flooding, causing the destruction on large scale. More than 32 GLOF events have been recorded in the Himalayas, with increasing frequency in recent years. (Dasgupta & Sen, 2020)  5.Floods and Droughts During period of initial glacier retreat due to high river discharge, risk of seasonal flooding increases particularly during monsoons (Immerzeel et al., 2010). But as glaciers continue to shrink, the long-term reduction in meltwater will lead to water scarcity and an increased risk of drought, particularly affecting agriculture and drinking water supplies (Prakash, 2020). 5. Sediment Transport and River Morphology Glacier retreat also changes the sediment transport dynamics. Initially, increased meltwater