{{Use American English|date=February 2025}} {{Infobox river | name = Big Lost River | name_native = | name_native_lang = | name_other = | name_etymology = <!---------------------- IMAGE & MAP --> | image = | image_caption = | map = Lostidahomap.png | map_size = | map_caption = Map of the lost streams of Idaho including the Big Lost River | pushpin_map = | pushpin_map_size = | pushpin_map_caption= <!---------------------- LOCATION --> | subdivision_type1 = Country | subdivision_name1 = United States | subdivision_type2 = State | subdivision_name2 = Idaho | subdivision_type3 = | subdivision_name3 = | subdivision_type4 = | subdivision_name4 = | subdivision_type5 = Cities | subdivision_name5 = Mackay, Arco, Atomic City <!---------------------- PHYSICAL CHARACTERISTICS --> | length = {{convert|135|mi|km|abbr=on}} | width_min = | width_avg = | width_max = | depth_min = | depth_avg = | depth_max = | discharge1_location= near Arco<ref name=USGS>{{cite web |url=https://wdr.water.usgs.gov/wy2009/pdfs/13132500.2009.pdf|title=USGS Gage #13132500 on the Big Lost River near Arco, ID |publisher=U.S. Geological Survey|work=National Water Information System |date=2009 |accessdate=2010-11-21}}</ref> | discharge1_min = {{convert|0|cuft/s|m3/s|abbr=on}} | discharge1_avg = {{convert|86.3|cuft/s|m3/s|abbr=on}}<ref name=USGS /> | discharge1_max = {{convert|2500|cuft/s|m3/s|abbr=on}} <!---------------------- BASIN FEATURES --> | source1 = Confluence of North Fork and East Fork Big Lost River | source1_location = Custer County, Idaho | source1_coordinates= {{coord|43|56|01|N|114|06|38|W|display=inline}}<ref name="GNIS">{{cite gnis|id=400591|name=Big Lost River|entrydate=1979-06-21|accessdate=2010-11-21}}</ref> | source1_elevation = {{convert|6860|ft|abbr=on}} | mouth = Big Lost River Sinks | mouth_location = Butte County, Idaho | mouth_coordinates = {{coord|43|47|39|N|112|50|26|W|display=inline,title}} | mouth_elevation = {{convert|4790|ft|abbr=on}} | progression = | river_system = | basin_size = {{convert|1867|sqmi|abbr=on}}<ref name="basin1">{{cite web|url=http://www.deq.idaho.gov/water/data_reports/surface_water/tmdls/big_lost_river/big_lost_chap1_pages1to18.pdf|title=Subbasin Assessment – Watershed Characterization|publisher=Idaho Department of Environmental Quality|work=Big Lost River Subbasin Assessment and TMDL|date=2004-05-06|accessdate=2010-11-21|archiveurl=https://web.archive.org/web/20101122101935/http://www.deq.idaho.gov/water/data_reports/surface_water/tmdls/big_lost_river/big_lost_chap1_pages1to18.pdf|archivedate=22 November 2010|url-status=dead}}</ref> | tributaries_left = North Fork Big Lost River, Thousand Springs Creek | tributaries_right = East Fork Big Lost River, Antelope Creek | custom_label = | custom_data = | extra = }}
The '''Big Lost River''' is a major river in the U.S. state of Idaho, about {{convert|135|mi}} long.<ref name=NHD>U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. [http://viewer.nationalmap.gov/viewer/ The National Map] {{webarchive |url=https://web.archive.org/web/20120329155652/http://viewer.nationalmap.gov/viewer/ |date=2012-03-29 }}, accessed May 4, 2011</ref>
==Description== The river starts in the Rocky Mountains and flows in a generally southeast direction into the Snake River Plain. True to its name, the Big Lost River's surface flow does not reach any larger river but vanishes into the Snake River Aquifer at the Big Lost River Sinks, giving the river its name.<ref name="basin1"/> The river is one of the Lost streams of Idaho, several streams that flow into the plain and disappear into the ground.
It rises at the confluence of the North Fork and East Fork Big Lost River deep in the Pioneer Mountains, a subrange of the Rockies, in Custer County, south-central Idaho. It flows northeast, then turns sharply southeast at the confluence with Thousand Springs Creek, which comes in from the left and into Butte County. The river is dammed to form Mackay Reservoir near the town of Mackay, then continues south through an agricultural valley, passing Arco. After Arco, the river begins flowing east, northeast, and finally due north. The river terminates at the Big Lost River Sinks, a patch of marshland where its water drains into the ground.
Even though its surface flow is lost (hence its name) a short distance out of the mountains, the river is hydrologically connected to the Snake River, the largest river of Idaho by discharge, via the Snake River Aquifer and various springs along the course of the Snake in its journey through the plain.
==See also== {{stack|{{portal|Rivers}}}} * Little Lost River * List of rivers of Idaho * List of longest streams of Idaho
==References== {{reflist|22em}}
==External links== {{commons category-inline|Big Lost River}}
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Category:Rivers of Idaho Category:Rivers of Custer County, Idaho Category:Rivers of Butte County, Idaho Category:Rivers of Bingham County, Idaho