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Carson Sink

The Carson Sink is an endorheic and basin in the Carson Desert of northwestern , , functioning as the terminus for the , which originates in the and discharges into it without outflow to the sea, leading to evaporation in the arid environment. Located primarily in Churchill County near the city of Fallon, the sink features extremely flat terrain with central elevations varying by less than 10 feet, underlain by alluvial, eolian, and lacustrine deposits. As a remnant of the prehistoric , the Carson Sink has experienced episodic expansions into shallow lakes during wetter climatic phases, including a notable high stand in 1862 reaching approximately 1,300 square kilometers in area. These paleohydrologic fluctuations provide empirical evidence of climate variability in the , with lake levels influenced by regional precipitation and river diversions rather than solely anthropogenic factors. The region supports the Fallon National Wildlife Refuge and has been studied for geothermal potential due to subsurface structures.

Geography

Location and Topography

The Carson Sink is a playa located in Churchill County, northwestern Nevada, United States, within the northeastern portion of the Carson Desert. It serves as the terminal basin for the Carson River in an endorheic system characteristic of the Great Basin. The approximate central coordinates are 39°56′00″N 118°18′51″W, situated about 10 kilometers south-southeast of the city of Fallon. The surface elevation averages around 1,184 meters (3,885 feet) above sea level. Topographically, the Carson Sink features a broad, nearly flat dry lakebed with minimal elevation variation, typically less than 3 meters (10 feet) across its central expanse, reflecting its origin as a remnant of the prehistoric . The basin is bounded by mountain ranges, including the West Humboldt Range to the west and the Stillwater Range to the east, which rise sharply from the surrounding desert floor. Subsurface geology includes sedimentary deposits extending to depths of at least 2.5 kilometers below the surface in the southern portion near Fallon, with surface materials dominated by evaporites, clays, and alluvial sediments. The Carson Sink spans an area encompassing variable extents of the , with historical flooded extents reaching up to 1,300 square kilometers during high-water events such as in , though the dry basin core is more compact, approximating a 32-kilometer-wide feature. Its flat terrain facilitates episodic flooding from river inflows, forming shallow, temporary lakes that evaporate rapidly in the arid climate, leaving salt flats and mud cracks.

Hydrology and Climate

The Carson Sink functions as an , receiving inflows primarily from the , which originates in the mountains of eastern California and flows eastward for approximately 131 miles before terminating in the sink near . Additional water enters via drainage canals from the Truckee-Carson Irrigation District, diverting flow from the and Lahontan Reservoir, with historical surface water inflows to the broader Carson Desert area averaging about 370,000 acre-feet per year from 1975 to 1992. These inputs support upstream but diminish before reaching the sink, where remaining surface water spreads across the , undergoing rapid or subsurface infiltration without external outflow. Hydrologic conditions in the Carson Sink are characterized by episodic flooding during high river flows or regional storms, temporarily forming shallow lakes that recede quickly due to ; the southeastern margin hosts persistent wetlands fed by canal discharges. The basin's closed nature concentrates dissolved minerals, resulting in expansive salt flats as exceeds and infiltration removes water from the surface. Groundwater interactions occur, with regional aquifers contributing minor , though surface dynamics dominate during wet events. The climate of the Carson Sink region is classified as a cold desert (Köppen BWk), featuring low annual averaging 4.8 to 5 inches (122 to 127 mm) near Fallon, with most falling as winter rain or snow from Pacific storms. High rates, estimated at 50 inches (127 cm) or more annually in comparable settings, far outpace inputs, maintaining the sink's predominantly dry state. Temperatures exhibit wide diurnal and al ranges, with summer highs often exceeding 100°F (38°C) and winter lows dropping below 0°F (-18°C), amplifying during the warm .

History

Indigenous and Prehistoric Use

The Carson Desert, encompassing the Carson Sink, preserves evidence of prehistoric human occupation dating to the early , approximately 10,000–8,000 years ago, following the recession of pluvial Lake Lahontan. Archaeological surveys, particularly those intensified after wet periods in 1982–1984 and 1986 that temporarily reflooded and then exposed sites upon receding, reveal clusters of artifacts including stemmed projectile points, ground stone tools, and faunal remains indicative of economies focused on lacustrine and resources such as fish, waterfowl, and aquatic plants. These assemblages suggest seasonal occupations tied to hydrological fluctuations, with human adaptation to the post-Lahontan landscape emphasizing mobility and exploitation of episodic patches rather than permanent settlement. Bioarchaeological analysis of human remains from the adjacent Stillwater Marsh, recovered from contexts dated 2,000–1,000 years ago, indicates a dominated by marsh-dependent like tui chub fish and cattails, supplemented by hunting, supporting reconstructions of resilient subsistence strategies amid . studies from nearby Hidden Cave further document dietary shifts from in the terminal Pleistocene to intensified gathering of seeds and small fauna by the middle , reflecting broader patterns of resource intensification. Claims of occupation, such as at Leonard Rockshelter, remain contested due to stratigraphic ambiguities and lack of corroborating radiocarbon dates. Northern bands, particularly the Toi Ticutta ("tule eaters"), historically utilized the Carson Sink and surrounding marshes for seasonal camping, hunting waterfowl and , and gathering edible plants like cattail roots and seeds, which formed staples of their diet and for basketry and mats. Ethnographic accounts and archaeological correlations confirm the sink's role as a key node in territorial ranges, with oral histories emphasizing sustainable harvest of lacustrine resources amid variable . groups overlapped in the broader Carson Basin, sharing access to sink-margin wetlands for similar activities documented in 19th-century observations of semi-permanent encampments. These practices persisted into the protohistoric period, predating Euro-American contact, though disrupted by 19th-century hydrological alterations from and ranching.

European Exploration and Early Settlement

In January 1844, during his second expedition to map the , , guided by frontiersman , traversed the Carson Desert and documented the Carson Sink as a desolate where an unnamed river vanished into the arid ground. Frémont named the river the in honor of his guide, recognizing its northward flow terminating at the sink before evaporation or subsurface loss. The expedition's arduous winter crossing highlighted the region's extreme aridity and alkali terrain, with Frémont's party suffering from thirst and exhaustion en route to the via . Frémont's published reports from influenced emigrant traffic, leading to the adoption of the by 1848 as an alternative route that skirted the sink's most impassable sections by following the upstream, though travelers still contended with and mirages in the surrounding desert. This path facilitated thousands of overland migrations through the 1850s, with Mormon traders establishing way stations along the river by 1849, including bridges in Carson Canyon to ease wagon passage. Permanent settlement at the Carson Sink remained sparse owing to its inhospitable conditions, but transient infrastructure emerged with the in March 1860, when Bolivar Roberts and crew erected a fortified frame house within walls near a perennial spring, serving as a critical relay for mail riders until the line's end in October 1861 following the transcontinental telegraph's completion. Initial homesteads appeared nearby at Carson Lake, a seasonal in the sink's southern arm, claimed in 1860 and 1861 for limited grazing, supplemented by a over the to support ranching access. These outposts underscored the area's role as a logistical rather than a hub for sustained or population growth.

Modern Infrastructure and Water Management

The Newlands Project, one of the earliest federal reclamation initiatives authorized in 1902, forms the backbone of water management infrastructure supplying the , diverting flows from the Carson and Truckee Rivers to irrigate roughly 57,000 acres of farmland in the while directing surplus tailwater to the endorheic basin. The (TCID), established in to assume operations and maintenance from the U.S. Bureau of Reclamation, oversees daily diversions, canal systems, and delivery schedules under a 1926 contract, prioritizing senior water rights holders including federal entities and the Pyramid Lake Paiute Tribe per the 1990 Truckee-Carson-Pyramid Lake Water Rights Settlement Act. Central facilities include Lahontan Dam on the , completed in 1915 with a capacity of 295,000 acre-feet, which regulates releases for downstream , and the approximately 5 miles below the reservoir, channeling water into the T-Line Canal and other distribution networks that convey up to 80% of annual surface flow for agricultural use. The 30-mile , operational since 1905, enables interbasin transfers from the , supplementing flows during deficits, though it has historically contributed to seepage losses exceeding 20% in unlined sections. Excess return flows, after percolating through soils, converge in the Carson Sink, supporting intermittent wetlands like Stillwater Marsh and Carson Lake before evaporation or in the terminal basin. Contemporary management emphasizes efficiency amid variable hydrology and legal mandates, with Operating Criteria and Procedures codified in 1988 dictating seasonal allocations to minimize spill to the Sink during low-flow years while complying with endangered species protections for cui-ui fish in Pyramid Lake. TCID's ongoing modernization, including canal lining initiated in phases since the 2010s, aims to curtail seepage by up to 15,000 acre-feet annually, conserving water for upstream priorities and reducing unintended hypersalinity buildup in the Sink. These efforts integrate real-time monitoring via automated turnouts and SCADA systems, though challenges persist from climate-driven runoff declines, with Truckee River inflows dropping 10-20% since the 1980s baseline.

Ecology

Wetlands and Biodiversity

The wetlands of the Carson Sink, located at the of the in the , consist primarily of seasonal alkali marshes and shallow ponds, including the historic Stillwater Marsh spanning up to 80,000 acres during high-water periods. These features form a terminal delta environment with water levels fluctuating based on river inflows, supporting hypersaline to brackish conditions that sustain halophytic plant communities such as alkali bulrush (Schoenoplectus maritimus) and saltgrass (Distichlis spicata). Prior to extensive water diversions in the , these wetlands covered approximately 85,000 acres, maintained by natural flows from the Carson and Humboldt Rivers. Biodiversity in the Carson Sink wetlands is concentrated in avian populations, with the area functioning as a key stopover along the for migratory shorebirds and waterfowl. The Stillwater National Wildlife Refuge, which manages much of the wetland complex, is designated as a Shorebird Reserve and supports peak concentrations of species such as western sandpipers (Calidris mauri), least sandpipers (Calidris minutilla), and eared grebes (Podiceps nigricollis), with surveys from 1986 to 2019 documenting up to 25,000 shorebirds during spring migration—representing a substantial portion of totals for certain taxa. Waterfowl diversity includes Canada geese (Branta canadensis) and various ducks, while resident and breeding birds encompass piscivorous species like American white pelicans (Pelecanus erythrorhynchos), though populations have been impacted by contaminants such as mercury. Invertebrate communities, including (Artemia spp.) and alkali flies (Ephydra spp.), provide essential forage, underpinning the for these birds. Mammalian and reptilian biodiversity is lower due to the arid, saline setting, featuring species adapted to wetland edges such as kit foxes (Vulpes macrotis) and western pond turtles (Actinemys marmorata), though fish populations like tui chubs (Siphateles bicolor obesus) have experienced episodic mass die-offs from hypersalinity events, as recorded in 1986–1987 when over 7 million individuals perished. Overall, the wetlands' ecological value lies in their role as one of the few remaining playa systems fostering high seasonal avian abundance, with restoration efforts by entities like aimed at sustaining these habitats amid variable hydrology.

Paleoenvironmental Record

The paleoenvironmental record of the Carson Sink is documented through lacustrine sediments, shoreline morphology, tufa deposits, and fossil evidence spanning the late Pleistocene to the Holocene, reflecting major hydrological fluctuations tied to regional climate shifts. During the late Pleistocene, the area formed a subbasin of pluvial Lake Lahontan, which achieved its final highstand around 13,000 years ago at elevations reaching 1,335 meters (4,380 feet) above sea level, prior to rapid recession. This phase deposited thick sequences of clay (up to 18 meters), gravel, sand, silt, and diverse tufa forms, including dendritic varieties at 1,255–1,278 meters and cellular types at 1,250–1,326 meters, alongside fossils such as gastropods (Parapholyx nevadensis), ostracods, fish remains (Siphateles bicolor obesus, suckers), and mammal bones (Equus sp., Camelops). Post-highstand desiccation around 11,000 years ago initiated arid conditions, marked by extensive eolian that excavated basins 3–18 meters deep and redistributed over 0.5 cubic miles of into dunes and sandhills oriented east-northeast. The Turupah Formation records this hyperarid interval (~7,500–4,500 years ago), dominated by eolian sands, , and alluvial deposits, with minimal soil development until dune stabilization during the Toyeh interval (~5,000–4,000 years ago). Holocene lake levels in the Fallon Formation (~4,000 years ago–present) document five shallow cycles with maximum depths of 5–26 meters and elevations up to 1,202 meters (3,948 feet), preserved in alternating lake sands, silts, clays, and eolian veneers, accompanied by mollusks (Anodonta californiensis), (Rhinichthys osculus), and vertebrate fossils. Late expansions, evidenced by radiocarbon-dated shorelines and sediments, occurred at 3.6–3.1 ka, 2.1–1.8 ka, and 1.1–0.8 ka, when lakes grew to exceed 1,000 km²—far larger than historic maxima like the 1862 stand at 1,186 meters covering 1,300 km²—necessitating inflow increases by a factor of four sustained for decades, likely from enhanced effective or Walker River diversions via paleochannels. These proxies, including ostracode assemblages and from cores, indicate wetter intervals linked to strengthened Pacific winter storms, contrasting drier phases with deposition and marsh . Geoarchaeological sites buried in post-Lahontan wetlands further constrain landscape evolution, showing stable environments supporting early human occupation amid fluctuating hydrology.

Human Utilization and Economic Role

Agriculture and Water Diversion

The Newlands Project, authorized by the U.S. Reclamation Act of 1902 and sponsored by Representative Francis Newlands, marked the beginning of systematic water diversion from the Carson and Truckee Rivers to support agriculture in the arid Lahontan Valley surrounding Carson Sink. Initial infrastructure included the Truckee Canal, completed in 1905 to enable transbasin diversion of approximately 1,300 cubic feet per second from the Truckee River into the Carson River basin, supplementing natural flows insufficient for large-scale farming due to average annual rainfall of about six inches in the valley. Lahontan Dam, constructed between 1911 and 1915 with a storage capacity of 295,000 acre-feet, captures and regulates these waters for downstream irrigation, transforming desert lands into productive farmland primarily growing alfalfa, grains, and other row crops. The Truckee-Carson Irrigation District (TCID), established in to operate the federal facilities, delivers water through 391 miles of canals and 340 miles of drains, irrigating roughly 57,000 acres of farmland in the Fallon area. Annual diversions average about 300,000 acre-feet, with comprising the dominant crop due to its suitability for hay production and water-intensive requirements in the region's hot, dry climate. These diversions, prioritized for agricultural use under the project's water rights, result in minimal surface inflows to Carson Sink, which historically received the full discharge but now primarily accumulates only excess floodwaters or subsurface seepage, exacerbating the sink's intermittent conditions. Agricultural demands have prompted ongoing modernization efforts, including and efficiency improvements to reduce seepage losses estimated at 20-30% in unlined channels, though such measures must balance farm viability with downstream ecological needs in the . Recent investments, such as a 2024 USDA allocation of up to $15 million to TCID, aim to conserve up to 50,000 acre-feet annually across 250,000 acres of irrigated land through technologies like precision irrigation, reflecting pressures from and competing uses without altering the core diversion framework established over a century ago.

Mining and Resource Extraction

The Carson Sink Mining District, located in , has historically focused on the extraction of s, particularly from deposits formed in the prehistoric basin. production in the Carson Sink and adjacent Teels Marsh districts contributed to Nevada's industrial mineral output, with operations targeting surface and near-surface and crystalline layers accumulated through episodic flooding and cycles. Specific sites like the Parran Salt Deposit exemplify these resources, where (rock ) forms the primary commodity, though extraction volumes have remained modest compared to Nevada's metallic mining sectors. No significant locatable metallic mineral deposits, such as or silver, have been identified or exploited within the Carson Sink district itself, distinguishing it from upstream areas like the . Resource extraction activities have been limited by the district's environment, characterized by low-relief sediments and intermittent hypersaline waters, which constrain large-scale open-pit or underground . Recent assessments indicate moderate potential for extraction from subsurface brines in the Carson Sink, driven by fault-controlled fluid isolation and geothermal influences that may concentrate -bearing waters. Exploratory efforts, including geophysical surveys, have targeted extensions of structures like the Desert Queen Fault, but as of 2023, no commercial production has occurred, with development hindered by water rights issues and environmental regulations. Geothermal resource evaluation has also featured in the region, with gravity and subsurface mapping revealing potential for energy extraction via enhanced geothermal systems, though no operational facilities exist within the proper. These activities underscore the shift toward non-traditional resources in arid basins, prioritizing and heat over conventional ores.

Environmental Impacts and Controversies

Mercury Contamination from Upstream Mining

The mercury contamination in the Carson Sink stems from historic gold and silver mining operations in the upstream district near , where elemental mercury was used in the amalgamation process to separate precious metals from ore between approximately 1863 and 1900. from over 200 mill sites, laden with unrecovered mercury, were discharged directly into tributaries of the , initiating downstream transport toward the terminal Carson Sink. This process released an estimated 7,500 metric tons (approximately 15 million pounds) of mercury into the river system, with the majority lost through inefficient milling practices during the peak Comstock era from 1859 to 1880. The , flowing through contaminated reaches from Carson City to the Sink, delivers mercury primarily as sorbed particulates in sediments, which settle in the low-gradient, evaporative basin of the . Sediment cores and surface samples from the Sink reveal persistent elevated total mercury concentrations, often exceeding background levels by factors of 10 to 100, due to the basin's role as a depositional lacking outlet drainage. formation, driven by microbial activity in anoxic sediments, amplifies risks in the Sink's sparse food webs, though levels in columns remain lower than in upstream river segments owing to sedimentation trapping. The Mercury Site, designated a site by the EPA in 1990, encompasses the contaminated corridor from upstream mills to the , highlighting the Sink's inclusion in long-term exposure pathways for and potential contact via dust or . Remediation efforts have focused primarily on riverbank stabilization and source removal upstream, but the Sink's sediments serve as a legacy , with modeling indicating minimal remobilization under current hydrological regimes dominated by and sporadic flooding. Ongoing by the USGS documents stable but elevated mercury fluxes into the Sink, underscoring the enduring causal link to 19th-century inefficiencies rather than modern sources.

Water Scarcity and Ecological Die-Offs

Diversions of water from the Carson and Truckee Rivers for in the , primarily through the Truckee-Carson established in the early , have substantially reduced inflows to the Carson Sink, resulting in chronic . Historically, the sink supported extensive wetlands such as Stillwater Marsh and Carson Lake, but wetland acreage has declined by over 90% since pre- eras due to these appropriations, with remaining marshes experiencing frequent drying during low-flow years. This scarcity intensifies during droughts, as evidenced by level drops of up to 10 feet in the Carson and rising from without replenishment. Ecological die-offs have been directly linked to these low water conditions, which concentrate salts and legacy pollutants like mercury in shrinking pools, leading to hypoxic events and toxicity. In February 1987, approximately 7 million tui chub fish perished across a 30-mile shoreline in the within the Carson Sink, accompanied by the deaths of about 1,500 migratory birds, attributed to dissolved oxygen depletion in stagnant, shallow waters following variable inflows. Similar oxygen-starved die-offs occurred in 2001 during a severe , with hundreds of trapped and suffocating in desiccating Carson City wetlands fed by remnants. Fluctuating levels also strand fish during recessions, pushing them beyond tolerances and causing mass mortality, as documented in tui chub populations native to the sink's marshes. Broader die-offs affect avian species reliant on the , where reduced extent—now often limited to managed refuges—curtails and , exacerbating vulnerability during . Persistent scarcity has further promoted playa , generating dust storms that degrade air quality and indirectly stress ecosystems through abrasion and burial of vegetation. These impacts compound with upstream mercury legacies, but causal primacy lies in diminished freshwater delivery, which prevents dilution and flushing of contaminants. efforts under the 1990 Truckee-Carson-Pyramid Lake Water Rights Settlement Act have allocated modest flows, yet agricultural priorities continue to constrain terminal basin replenishment amid climate-driven flow reductions.

Debates on Development vs.

The central tension in the Carson Sink revolves around balancing upstream agricultural water diversions with downstream needs, as captures over 80% of the Carson River's annual surface flow, severely limiting inflows to the sink and contributing to wetland shrinkage and degradation. This conflict has persisted since the early 20th-century Newlands Reclamation Project, which prioritized farming expansion in the Carson Desert by storing river water in Lahontan Reservoir and diverting it for crops, often at the expense of terminal wetlands like those in the Fallon National Wildlife Refuge. Agricultural proponents, including irrigators in Churchill County, maintain that water-intensive farming—particularly and other export crops—drives the local economy, generating jobs and revenue in a region where alternatives are limited, and view mandates as prioritizing non-productive uses over human needs amid chronic . They argue that enforced fallowing or water leasing to refuges disrupts farm viability, potentially leading to land abandonment without commensurate economic benefits from preservation. In contrast, conservation advocates, supported by federal agencies like the U.S. Fish and Wildlife Service, stress the empirical value of the sink's playas and marshes as critical stopover sites for migratory shorebirds and habitats for species like the , with data showing a 90% decline in surface water extent from 1984–1999 to 2000–2018 correlating to losses. These groups cite hydrological models indicating that reduced diversions could restore ecological functions, such as and flood mitigation, while challenging claims of wetland "waste" by quantifying services against agricultural outputs in water-scarce basins. Resolution attempts, such as the Truckee-Carson settlements, have facilitated transfers—committing to at least $9 million in purchases for protective measures benefiting refuges and fisheries—yet implementation sparks disputes, as farmers resist permanent rights retirement amid droughts that already strain supplies. Recent proposals for banking, aimed at retiring unused rights to augment environmental flows, have drawn farmer opposition over fears of market distortions and economic contraction, underscoring unresolved trade-offs between development-driven growth and imperatives.

Current Management and Future Prospects

Protected Areas and Restoration

The National Wildlife Refuge, established in 1948, protects over 79,000 acres of wetlands, playas, and uplands in the Carson Sink portion of the , serving as a key habitat for migratory waterfowl on the . Managed by the U.S. Fish and Wildlife Service, the refuge features shallow freshwater and brackish marshes that depend on inflows from the and Truckee-Carson District diversions. The Fallon National Wildlife Refuge, part of the Stillwater Complex, encompasses more than 15,000 acres of and intermittent wetlands at the Carson River's terminus within the sink. This area floods during high river flows, supporting diverse avian species, though water availability is limited in dry years due to upstream agricultural demands. The Carson Lake Wildlife Management Area, operated by the Nevada Department of Wildlife since 1955, spans 30,040 acres around Carson Lake, immediately upstream of the sink proper. It functions as a buffer for wetland-dependent wildlife, with management emphasizing seasonal inundation for breeding and staging grounds. Restoration initiatives target viability through infrastructure enhancements and water allocation. has supported ditch cleaning, capacity expansions, and new excavations totaling several miles to improve water distribution across the Marshes. Canals and ponds constructed in prior decades aid in marsh revival, countering historical drainage for . collaborates on and Carson Lake , prioritizing habitat connectivity amid arid conditions. Water rights acquisitions by refuge managers ensure periodic flows, with holding the county's largest holdings as of 2021 to sustain ecological functions.

Recent Hydrological Trends

In the Carson River Basin, which feeds the Carson Sink, USGS gauge data from 1940 to 2019 reveal statistically significant declining trends in average instream flows at multiple sites, including a reduction of 36.5 cubic feet per second (cfs) near Carson City and 14.3 cfs near Fort Churchill, with p-values ranging from 0.00 to 0.01 indicating high confidence in the downward trajectory. These declines correlate with observed climatic shifts, such as annual precipitation decreasing by 0.0364 inches per year and temperatures rising by 0.0263°F per year in Carson City over the same period, contributing to reduced snowpack and earlier runoff timing across 70% of monitored Sierra Nevada sites since the 1950s. For the Carson Sink itself, a terminal receiving primarily return flows, excess spills from Lahontan Reservoir, and episodic , hydrological conditions exhibit high variability rather than monotonic decline, with coverage often limited to shallow, transient ponds during wet periods. High-flow events, such as those in 2017 when emergency canals diverted overflow to avert flooding in nearby Fallon, temporarily inundated portions of the Sink, while prolonged droughts from 2012 to 2016 and below-normal persisting into early 2025 minimized inflows and amplified evaporative losses. Observed increases in air temperatures—0.74°C to 1.07°C from 2000 to 2020 relative to long-term averages—have heightened evaporative demand across watersheds, including the Sink's HUC12 unit (160502031000), where recent demand rose notably from 1980 to 2020, further constraining water persistence in this . Return flows from upstream , governed by the Alpine Decree and Newlands Project allocations, provide a baseline input to the Sink but have not offset basin-wide flow reductions, leading to episodic and expansion amid overall . Monitoring at nearby USGS sites, such as below Lahontan Dam, underscores this variability, with post-2010 peaks during events (e.g., 2023) contrasting multi-year lows that limit ecological inundation.

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