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{{short description|Lake in Montana, United States}}
{{Use mdy dates|date=July 2023}}
{{Infobox body of water
|name = Flathead Lake
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|caption_bathymetry =
|coords = {{coord|47|54|6|N|114|6|15|W|region:US-MT_type:waterbody|display=inline,title}}
|location= [[Lake County, Montana|Lake]] / [[Flathead County, Montana|Flathead]] counties, [[Montana]],
|type = [[Moraine-dammed lake]]
|inflow = {{Plainlist|
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}}
'''Flathead Lake''' ({{lang-fla|člq̓etkʷ|label=[[Montana Salish|Salish]]}}, {{lang-kut|yawuʔnik̓ ʔa·kuq̓nuk}})<ref>{{cite journal |first1=Adam N. |last1=Johnson
The lake is a remnant of the ancient, massive glacial dammed lake, [[Glacial Lake Missoula|Lake Missoula]] of the era of the last [[interglacial]].<ref>{{Cite web |url=
==Geography==
Located in the northwest corner of the state of [[Montana]], {{convert|7|mi|km}} south of [[Kalispell, Montana|Kalispell]], it is approximately {{convert|30|mi|km}} long and {{convert|16|mi|km}} wide, covering {{convert|197|sqmi|km2}}. It is a similar size as Minnesota's [[Mille Lacs Lake]], but smaller than [[Red Lake (Minnesota)|Red Lake]]. It is about half the area of [[San Francisco Bay]] (main bay). It is larger in surface area than [[Lake Tahoe]], but it is much smaller in volume due to Tahoe's depth.<ref name="leeden">{{cite book |last= Van der Leeden |first= Frits |author2=Fred Louis Troise |author3=David Keith Todd |title= The Water Encyclopedia |year= 1990 |publisher= CRC Press |isbn= 978-0-87371-120-3 |url=https://books.google.com/books?id=ByiE-6bEi1oC |page= 185}}</ref> Flathead Lake has a maximum depth of {{convert|370.7|ft|m|abbr=on}},<ref name="flbs">
[[File:Flathead Lake by Sentinel-2.jpg|thumb|left|Flathead Lake from space, Aug 2018|alt=]]
The lake is bordered on its eastern shore by the [[Mission Mountains]] and on the west by the [[Salish Mountains]]. The Flathead valley was formed by the [[Ice jam|glacial damming]] of the [[Flathead River]] and sustains a remarkably mild [[climate]] for a region located this far north and inland; the [[Pacific Ocean]] is almost {{convert|400|mi|km}} to the west. The mild climate allows for [[cherry]] [[orchard]]s on the east shore and [[vineyard]]s for [[wine]] production on the west shore. There are also [[apple]], [[pear]] and [[plum]] orchards around the lake as well as [[vegetable]]s, [[hay]], [[honey]], [[nursery tree]], [[Christmas tree]], [[sod]]/turf, and [[wheat]] production bordering or near the lake.
==
Flathead Lake lies at the southern end of a geological feature called the [[Rocky Mountain Trench]]. The [[trench]], which formed with the [[Rocky Mountains]], extends north into the southern [[Yukon]] as a straight, steep [[valley]], which also holds the headwaters of the [[Columbia River]]. During the last [[ice age]] this trench was filled by an enormous [[glacier]]. As the glacier moved southward it carved out the trench. The Polson Moraine, near present-day [[Polson, Montana]], marks the southernmost extent of the glacier during the last ice age and thus is the site of the glacier's [[terminal moraine]].<ref name="moraine">{{cite book | url=https://books.google.com/books?id=s4y3c8fxeEwC&q=%22polson+moraine%22&pg=PA49 | title=Glacial Lake Missoula: And Its Humongous Floods | publisher=Mountain Press Publishing | author=Alt, David D. | year=2001 | pages=49 | isbn=9780878424153}}</ref>▼
Once known as "Salish Lake", this body of water was named for the [[Flathead Indians|Salish Indians]]. Early European Americans called them the Flathead Indians because of a misinterpretation of early [[Plains Indian Sign Language|Native American sign language]].<ref>{{cite journal|last1=Partoll|first1=Albert|title=The Flathead-Salish Indian Name in Montana Nomenclature|journal=[[Montana: The Magazine of Western History]]|date=1951|volume=1|issue=1|pages=37–47}}</ref> A common misconception is that the name is derived from a practice of [[Artificial cranial deformation|head flattening]] more common among tribes such as the [[Chinookan peoples|Chinook]]. There is no evidence to show that the [[Flathead Indians|Salish]] ever had this custom.<ref>{{Cite journal |jstor = 4515711|title = The Flathead-Salish Indian Name in Montana Nomenclature|journal = The Montana Magazine of History|volume = 1|issue = 1|pages = 37–47|last1 = Partoll|first1 = Albert J.|year = 1951}}</ref> Since the late 19th century, the Salish were mostly removed to the [[Flathead Indian Reservation]], located at the southern end of the lake.▼
[[File:CloudsAndLakeInMontana.jpg|thumb|left|
The large size of the Polson Moraine indicates that the glacier stalled here for many years before retreating. As the climate warmed, a portion of the glacier in the Mission Valley receded more slowly than the main body, which kept the lake basin from being filled with sediment. Eventually this ice also melted, forming a lake behind the moraine. Once the water reached the top of this [[moraine dammed lake|moraine dam]], it began to cut a channel through it. Most [[moraine dammed lake]]s drain quickly because water cuts entirely through the moraine.
At one time, probably when the valley was partially filled by a [[glacier]], the level of Flathead Lake was about {{convert|500|ft|m}} higher and drained through the valley west of [[Elmo, Montana]], which is at the end of Big Arm Bay, bottom center in the aerial photo above. Water carved out a wide, flat-bottomed pass with a deeper, narrow channel at the south edge of the pass. The deeper channel and traces of the dry riverbed are still visible from Route 28.{{citation needed|date=March 2012}}▼
The [[Flathead River]] and the [[Swan River (Montana)|Swan River]] (known also as the Bigfork River where it enters the lake) are the lake's major [[tributary|tributaries]]. The lake is inhabited by the native [[bull trout]] and [[cutthroat trout]], as well as the non-native [[lake trout]], [[yellow perch]], and [[Coregonus|lake whitefish]]. Local residents have reported sighting other aquatic fauna in the lake as well, such as [[sturgeon]] and the [[Flathead Lake Monster]].<ref name="missoulian">{{cite web | url=http://missoulian.com/lifestyles/hometowns/paul-fugleberg-flathead-lake-sturgeon-catch-still-controversial/article_19ad1b87-4266-5596-a863-f0490910447f.html | title=Paul Fugleberg: Flathead Lake sturgeon catch still controversial | work=Missoulian | date=5 April 2015 | access-date=10 April 2015 | author=Fugleberg, Paul}}</ref>▼
==Management==
The non-native [[Mysida|opossum shrimp]], (''Mysis diluviana''), were introduced by Montana Department of Fish, Wildlife and Parks in the Flathead drainage basin to encourage production of larger kokanee salmon; they migrated into Flathead Lake and have altered the ecosystem.<ref name="Bosworth">{{cite web | url=http://newwest.net/topic/article/how_non_native_shrimp_transformed_the_ecosystem_at_montanas_flathead_lake/C41/L41/ | last=Bosworth | first=Brendan | title=How Non-Native Shrimp Transformed The Ecosystem at Montana's Flathead Lake | access-date=15 December 2016}}</ref>▼
The Flathead River and the [[Swan River (Montana)|Swan River]] (known also as the Bigfork River where it enters the lake) are the major [[tributary|tributaries]] of the five tributaries that are within the Flathead Watershed. Numerous small streams flow into the lake, particularly on the wetter east shore.{{r|flbs}} The [[SKQ Dam|Seli’š Ksanka Qlispe’ Dam]], formerly known as Kerr Dam and built near [[Polson, Montana|Polson]], controls the top {{convert|10| feet||adj=mid| of the lake}}, generates [[hydroelectric power]], and provides water for [[irrigation]] to support a federal irrigation project in the area.<ref>{{Cite news |last=Drew |first=Micah |date=2024-04-03 |title=Energy Keepers Receives Approval for Flathead Lake Level Variation |url=https://flatheadbeacon.com/2024/04/03/energy-keepers-receives-approval-for-flathead-lake-level-variation/ |access-date=2024-04-05 |work=Flathead Beacon |language=en}}</ref> Minimum outflow levels from Flathead Lake are designated by the [[Federal Energy Regulatory Commission]] (FERC) and are based on flood risk management, power generation requirements, and biological needs to support aquatic life in river systems.<ref>{{Cite news |last=Mepham |first=Andy |date=2023-07-13 |title=Dam operators: Beginning of pattern of ever-decreasing levels in Flathead River Basin |url=https://www.kpax.com/news/western-montana-news/dam-operators-beginning-of-pattern-of-ever-decreasing-levels-in-flathead-river-basin |access-date=2023-07-15 |work=KPAX News |language=en}}</ref>
The lake is downstream of [[Hungry Horse Dam]] on the [[South Fork Flathead River]].<ref>{{Cite press release |date=July 7, 2023 |title=Reclamation's operation of Hungry Horse Dam |url=https://www.usbr.gov/newsroom/news-release/4576 |access-date=2023-07-19 |publisher=Bureau of Reclamation |language=en}}</ref> The Hungry Horse Dam is managed by the [[Bureau of Reclamation]] to provide beneficial flow conditions and to provide safe passage for migrating juvenile fish to reach the [[Columbia River Estuary]] and the Pacific Ocean.<ref>{{Cite web |date=2022-05-10 |title=Federal Columbia River Power System Water Management |url=https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/federal-columbia-river-power-system-water-management |access-date=2023-07-19 |website=NOAA Fisheries |language=en}}</ref> The Columbia River Technical Management Team makes operational recommendations to the agencies that control federal dams. Made up of representatives from four states, five federal agencies and six tribal nations, it prioritizes fish and wildlife above other system benefits.<ref>{{Cite news |last=Drew |first=Micah |date=2023-07-15 |title=Technical Team Rejects Request to Raise Flathead Lake Levels with Hungry Horse Release |url=https://flatheadbeacon.com/2023/07/15/no-action-taken-on-low-flathead-lake-levels-hungry-horse-reservoir/ |access-date=2023-07-19 |work=Flathead Beacon |language=en}}</ref> The Columbia River system has 14 projects that must be operated to meet congressionally authorized purposes.<ref>{{Cite web |last=Baldwin |first=Matt |date=2024-03-16 |title=Flathead Basin dam operators defend management decisions |url=https://dailyinterlake.com/news/2024/mar/16/flathead-basin-dam-operators-defend-management-decisions/ |access-date=2024-03-17 |website=Daily Inter Lake |language=en}}</ref>
Fishermen had introduced lake trout 80 years prior but remained at low densities until the non-native Mysis became established. The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout by providing a deep water source of food where little was available previously. Lake trout subsequently flourished on mysids; this voracious piscivore now dominates the lake fishery. The formerly abundant [[Kokanee salmon|kokanee]] were extirpated, and native bull and westslope cutthroat trout are imperiled. Predation by Mysis has shifted [[zooplankton]] and [[phytoplankton]] community size structure. Bayesian change point analysis of primary productivity (27-y time series) showed a significant step increase of 55 mg C m−2 d−1 (i.e., 21% rise) concurrent with the mysid invasion, but little trend before or after despite increasing nutrient loading. Mysis facilitated predation by lake trout and indirectly caused the collapse of kokanee, redirecting energy flow through the ecosystem that would otherwise have been available to other top predators (bald eagles).<ref>{{cite journal|last1=Ellis|first1=B. K.|last2=Stanford|first2=J. A.|last3=Goodman|first3=D.|last4=Stafford|first4=C. P.|last5=Gustafson|first5=D. L.|last6=Beauchamp|first6=D. A.|last7=Chess|first7=D. W.|last8=Craft|first8=J. A.|last9=Deleray|first9=M. A.|last10=Hansen|first10=B. S.|title=Long-term effects of a trophic cascade in a large lake ecosystem|journal=[[Proceedings of the National Academy of Sciences]]|date=3 January 2011|volume=108|issue=3|pages=1070–1075|doi=10.1073/pnas.1013006108|pmid=21199944|pmc=3024674|bibcode=2011PNAS..108.1070E|doi-access=free}}</ref>▼
==
▲Once known as "Salish Lake", this body of water was named for the [[Flathead Indians|Salish Indians]]. Early European
▲Flathead Lake lies at the southern end of a geological feature called the [[Rocky Mountain Trench]]. The [[trench]], which formed with the [[Rocky Mountains]], extends north into the southern [[Yukon]] as a straight, steep [[valley]], which also holds the headwaters of the [[Columbia River]]. During the last [[ice age]] this trench was filled by an enormous [[glacier]]. As the glacier moved southward it carved out the trench. The Polson Moraine, near present-day [[Polson, Montana]], marks the southernmost extent of the glacier during the last ice age and thus is the site of the glacier's [[terminal moraine]].<ref name="moraine">{{cite book | url=https://books.google.com/books?id=s4y3c8fxeEwC&q=%22polson+moraine%22&pg=PA49 | title=Glacial Lake Missoula: And Its Humongous Floods | publisher=Mountain Press Publishing | author=Alt, David D. | year=2001 | pages=49 | isbn=9780878424153}}</ref>
The summer of 2023 saw abnormally low water levels.<ref>{{Cite news |last=Heston |first=Kate |date=2023-11-15 |title=Zinke introduces legislation to regulate Flathead Lake water levels |url=https://dailyinterlake.com/news/2023/nov/15/federal-legislation-introduced-to-regulate-flathead-lake-levels/ |access-date=2023-11-15 |work=Daily Inter Lake |language=en}}</ref> Variations to the Flood Risk Management Plan were approved by the U.S. Army Corp of Engineers in 2024 in response to the continuing dry conditions.<ref>{{Cite web |last= |first= |date=2024-05-24 |title=Corps removes Flathead Lake level restrictions |url=https://nbcmontana.com/news/local/corps-removes-flathead-lake-level-restrictions |access-date=2024-05-25 |website=KECI NBC Montana |language=en}}</ref>
▲[[File:CloudsAndLakeInMontana.jpg|thumb|left|Some clouds over Flathead Lake in [[Polson, Montana]], as the sun is setting]]
▲The large size of the Polson Moraine indicates that the glacier stalled here for many years before retreating. As the climate warmed, a portion of the glacier in the Mission Valley receded more slowly than the main body, which kept the lake basin from being filled with sediment. Eventually this ice also melted, forming a lake behind the moraine. Once the water reached the top of this [[moraine dammed lake|moraine dam]], it began to cut a channel through it. Most [[moraine dammed lake]]s drain quickly because water cuts entirely through the moraine. However, Flathead Lake remains because a [[bedrock]] hill buried underneath the Polson Moraine prevented the moraine from being completely cut through so the meltwater never completely drained.<ref name="moraine"/>
▲At one time, probably when the valley was partially filled by a [[glacier]], the level of Flathead Lake was about {{convert|500|ft|m}} higher and drained through the valley west of [[Elmo, Montana]], which is at the end of Big Arm Bay, bottom center in the aerial photo above. Water carved out a wide, flat-bottomed pass with a deeper, narrow channel at the south edge of the pass. The deeper channel and traces of the dry riverbed are still visible from Route 28.{{citation needed|date=March 2012}}
▲
▲The non-native [[Mysida|opossum shrimp]], (''Mysis diluviana''), were introduced by Montana Department of Fish, Wildlife and Parks in the Flathead [[drainage basin]] to encourage production of larger kokanee salmon; they migrated into Flathead Lake and have altered the ecosystem.<ref name="Bosworth">{{cite web | url=http://newwest.net/topic/article/how_non_native_shrimp_transformed_the_ecosystem_at_montanas_flathead_lake/C41/L41/ | last=Bosworth | first=Brendan | title=How Non-Native Shrimp Transformed The Ecosystem at Montana's Flathead Lake | access-date=15 December 2016}}</ref>
▲Fishermen had introduced lake trout 80 years prior{{when|date=July 2023}} to the introduction of opossum shrimp, but they remained at low densities until the non-native Mysis became established. The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout by providing a deep water source of food where little was available previously. Lake trout subsequently flourished on mysids; this voracious piscivore now dominates the lake fishery. The formerly abundant [[Kokanee salmon|kokanee]] were extirpated, and native bull and westslope cutthroat trout are imperiled. Predation by Mysis has shifted [[zooplankton]] and [[phytoplankton]] community size structure. Bayesian change point analysis of primary productivity (27-y time series) showed a significant step increase of 55 mg C m−2 d−1 (i.e., 21% rise) concurrent with the mysid invasion, but little trend before or after despite increasing nutrient loading. Mysis facilitated predation by lake trout and indirectly caused the collapse of kokanee, redirecting energy flow through the ecosystem that would otherwise have been available to other top predators (bald eagles).<ref>{{cite journal|last1=Ellis|first1=B. K.|last2=Stanford|first2=J. A.|last3=Goodman|first3=D.|last4=Stafford|first4=C. P.|last5=Gustafson|first5=D. L.|last6=Beauchamp|first6=D. A.|last7=Chess|first7=D. W.|last8=Craft|first8=J. A.|last9=Deleray|first9=M. A.|last10=Hansen|first10=B. S.|title=Long-term effects of a trophic cascade in a large lake ecosystem|journal=[[Proceedings of the National Academy of Sciences]]|date=3 January 2011|volume=108|issue=3|pages=1070–1075|doi=10.1073/pnas.1013006108|pmid=21199944|pmc=3024674|bibcode=2011PNAS..108.1070E|doi-access=free}}</ref>
▲==Fish==
*Native
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==External links==
{{Commons category}}
*[http://www.flatheadbasincommission.org/ Flathead Basin Commission]
{{authority control}}
[[Category:Bodies of water of Lake County, Montana]]▼
[[Category:Geology of Montana]]▼
[[Category:Lakes of Flathead County, Montana]]
[[Category:Lakes of Montana]]
[[Category:Lakes of the Rocky Mountains]]
▲[[Category:Geology of Montana]]
▲[[Category:Bodies of water of Lake County, Montana]]
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