| Lake Alamosa | |
|---|---|
 Simulation of Lake Alamosa's surface, looking towardsBlanca Peak | |
  Lake Alamosa | |
| Location | San Luis Valley,Colorado |
| Coordinates | 37°12′N105°25′W / 37.2°N 105.42°W /37.2; -105.42 |
| Type | former lake |
| Basin countries | United States of America |
Lake Alamosa is aformer lake inColorado. It existed from thePliocene to the middlePleistocene in theSan Luis Valley, fed byglacial meltwater from surrounding mountain ranges. Water levels waxed and waned with theglacial stages until at highstand the lake (high water level in the lake) reached an elevation of 2,335 meters (7,661 ft) and probably a surface of over 4,000 square kilometers (1,500 sq mi), but only sparse remains of the former waterbody are visible today. The existence of the lake was postulated in the early 19th century and eventually proven in the early 20th century.
The lake eventually overflowed into theRio Grande river system during the middle Pleistocene. The overflow cut down a valley that eventually drained the lake, leaving only theSan Luis Closed Basin as a remnant. TheAlamosa Formation is a rock formation left by the lake.Groundwater resources are contained trapped between sediments left by the former lake.
The lake was in southernColorado, at high altitude, covering most of theSan Luis Valley[1]/San Luis Basin north of theSan Luis Hills.[2] It reached an elevation of 2,335 meters (7,661 ft) at highstand.[3] Westwards, Lake Alamosa spread to theSan Juan Mountains close toMonte Vista to the west. Northwards, it almost reached the present-day location ofSaguache.[2] It was about 105 kilometers (65 mi) long in north-south direction and reached a maximum width of 48 kilometers (30 mi),[4] making it one of North America's largest high elevation lakes, comparable only to the historicLake Texcoco in Mexico.[1] The surface area may have exceeded 4,000 square kilometers (1,500 sq mi) at highstand.[5] It is likely that some water seeped out of the lake before its final overflow[6] asgroundwater.[7]
Saddleback Mountain was an island in Lake Alamosa, and its northern side was swept by waves during storms owing to the lake's long fetch.[2] Additional islands were located in its southern basin; today they are hills.[4] The city ofAlamosa[8] and possibly also the town ofCenter, Colorado are located within the former lake's floor.[9]San Luis Lake presently occupies a small basin in the former lake floor;[10] the basin was previously occupied by a leftover of Lake Alamosa, Lake Sipapu.[11]
Because of the old age of Lake Alamosa, shorelines and shoreline features are often buried or eroded; in many places former shorelines are only highlighted by changes in vegetation patterns or slope angle.[12]Barrier bars,lagoons andspits formed in Lake Alamosa when it reached its highest stand.[13] In particular, two spits at Saddleback Mountain are testimony to the existence of the former lake,[14] and additional large spits formed close toSierra del Ojito and at theBrownie Hills.[4] Barrier bars dammed creeks which in turn formed lagoons;[12] such lagoon-bar systems are found for example aroundTrinchera Creek.[15]Wind-eroded rock structures occur along its former southeastern shore.[13]
During the glacial times of Lake Alamosa's existence, the basin was colder than today and possibly also drier. The lake was nourished byglacialmeltwater coming from theSan Juan,Sawatch, andSangre de Cristo Mountains. Presently, the climate atAlamosa is cold and dry, with mean annual temperatures of 5 °C (41 °F) and mean annual precipitation of 180 millimeters per year (7.1 in/year).[16]
The San Luis Basin is the largest[17] basin of theRio Grande Rift and is bordered to the east by afault. It constitutes an east-tiltedhalf-graben[1] that is subdivided by additional fault systems. Its southern reaches are covered by the volcanicServilleta Formation.[8] The valley forms the headwaters of theRio Grande.[17]
Lake Alamosa existed for about 3 million years,[17] from thePliocene to the middlePleistocene.[13] Rocks in the San Luishorst stretch across the entire Alamosa Basin and together with 4.8-3.7 million years oldbasalticlava flows of theTaos Plateau blocked the basin from draining to the south.[18]Tectonic uplift of theJemez lineament,[19]tectonic subsidence of the Lake Alamosa area[20] and the emplacement of these lava flows may have obstructed former drainages,[16] but evidence of prior southwards drainage has been found only recently.[21][5] South of Lake Alamosa[22] an even older lake, Lake Sunshine, occupied theSunshine Valley during the Pliocene.[23]
Many lakes in western North America are cyclical, becoming deep and large duringstadials and shallow and small duringinterstadials. Lake Alamosa briefly reached elevations of 2,292–2,304 meters (7,520–7,559 ft) at Hansen Bluff four times during its history.[24] According to modelling, ongoing sedimentation in the lake basin would have led to gradually increasing water levels during each stadial over time.[16] Aside from climatic influences, theBishop Tuff eruption of theLong Valley caldera in California and theHuckleberry Ridge eruption of theYellowstone Caldera depositedtephra in Lake Alamosa.[25] Sedimentation occurred in the basin floor, although slip along theSangre de Cristo fault to the east created accumulation space.[18]
It overflowed about 440,000 years ago,[3] when it reached a highstand duringoxygen isotope stage 12, one of the majorglaciations of theNorthern Hemisphere.[1][18] This date is not firmly established, however,[26] and the overflow might have occurred between 690,000 and 440,000 years ago[6] or after 376,000 years ago.[27] The glaciation was ending at that time, and meltwater from declining glaciers may have helped raise the water levels until the lake overflowed.[16] Continuing uplift in theSouthern Rocky Mountains may also have played a role.[28]
Lake Alamosa overflowed through theCostilla Plain intoNew Mexico,[13] in a gap between theFairy Hills and theBrownie Hills. From there water would have flowed across the Costilla Plain and theTaos Plateau to join theRio Grande andRed River west ofQuesta, New Mexico.[29]Faulting had weakened rocks at the overflow threshold, thus facilitating erosion of the threshold.[30] More than one channel may have been active as overflow path before the flow became focused on the present-day path of the Rio Grande.[31] The water would have reached theCulebra Creek and eventually theRed River at La Junta Point;[32] the Red River constituted the headwater of the Rio Grande during that time. The overflow of Lake Alamosa into the Rio Grande expanded its catchment by about 22,000 square kilometers (8,500 sq mi)[16]-18,000 square kilometers (6,900 sq mi), adding the high, glaciatedSan Juan Mountains, the upperSangre de Cristo Mountains andSawatch Range to its watershed.[1]
Water levels dropped quickly after the overflow began, preventing the formation of the repeated shorelines that are common at shrinkingpluvial lakes[31] where the decline in water levels is paced byevaporation.[33] The decline probably was not as quick as during theBonneville flood ofLake Bonneville, another example of a lake overflow event in North America, as the rocks at Lake Alamosa were more solid and there is no clear indication of a catastrophic overflow flood.[32] Later research has indicated that thedowncutting of the outlet may have continued for several hundred thousand years after the initial breach,[34] and was accompanied by an integration of the Rio Grande all the way to theGulf of Mexico.[34] That the lake drained aftergroundwater sapping has also been postulated but is less likely.[35]
After the drainage of the lake the lakefloor was eroded by streams, which redeposited its sediments, and soils formed.[3] The lake would have been replaced bydry lakes and anastomosing stream networks.[36] Creeks cut into the lake floor, forming valleys that later filled withalluvium.[33]
TheAlamosa Formation, ageological formation, fills the basin of Lake Alamosa;[13] it represents the deep water deposits of Lake Alamosa.[37] Impermeable deposits of the lake such as the "blue clay"[17] generategroundwater accumulations that are exploited forirrigation purposes.[38] The VerdosAlluvium of theDenver Basin may correlate to certain deposits on the shores of Lake Alamosa.[39]
While it was formerly thought that theGreat Sand Dunes were formed from sediments on the floor of Lake Alamosa but later research indicated that these sediments probably played an only minor role. The development of the dunes however certainly post-dated the disappearance of Lake Alamosa[40] as the lake would have impeded the transport of sand across the San Luis Valley.[41]
Decades before the region was settled and earlier than other geologic expeditions such as those ofJohn Wesley Powell,[30] in 1811-1812Jacob Fowler recorded the following:[29]
I Have no doubt but the River from the Head of those Rocks up for about one Hundred miles Has once been a lake of about from forty to fifty miles Wide and about two Hundred feet deep—and that the running and dashing of the Watter Has Woren a Way the Rocks So as to form the present Chanel.
The lake is named after the Alamosa Formation, which in turn received its name from Siebenthal 1910[1] who in that year postulated the existence of a former lake in the Alamosa Basin.[8] In the same year proof of the existence of the lake was found in well logs.[1]
Later, the geologistFerdinand Vandeveer Hayden in 1875 wrote a more detailed account of the former lake, although he got some details wrong, and proposed the name "Coronado's lakes" for Lake Alamosa and another lake he believed to have existed in theCostilla Plain.[30] It took almost a century after 1910 until a more detailed analysis of the deposits of the former lake - including identifying its former maximum elevation - was possible, however.[42] Most information on the history of Lake Alamosa's water levels has been obtained at the "Bachus pit"gravel pit inAlamosa County, as elsewhere lake deposits have been altered. Both beach and deep-water deposits (from the highstand before overflow) are encountered there.[43]
{{citation}}: CS1 maint: work parameter with ISBN (link){{cite book}}: CS1 maint: location missing publisher (link)Pleistoceneproglacial lakes and related seas | |||||||||||||||||||||||||||||
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