Colorado River Delta
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The Colorado River Delta is the region where the
Colorado River The Colorado River () is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and in northern Mexico. The river, the List of longest rivers of the United States (by main stem), 5th longest in the United St ...
once flowed into the
Gulf of California The Gulf of California (), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Vermejo''), is a marginal sea of the Pacific Ocean that separates the Baja California peninsula from ...
(also known as the Sea of Cortez) in eastern
Mexicali Municipality Mexicali Municipality is a Municipalities of Mexico, municipality () in the Mexican States of Mexico, state of Baja California. Its municipal seat () is located in the city of Mexicali. As of 2020, the municipality had a total population of 1,04 ...
in the north of the state of
Baja California Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of B ...
, in northwestern
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
. The delta is part of a larger geologic region called the Salton Trough. Historically, the interaction of the river's flow and the ocean's tide created a dynamic environment, supporting freshwater,
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
, and saltwater species. Within the
delta Delta commonly refers to: * Delta (letter) (Δ or δ), the fourth letter of the Greek alphabet * D (NATO phonetic alphabet: "Delta"), the fourth letter in the Latin alphabet * River delta, at a river mouth * Delta Air Lines, a major US carrier ...
region, the river split into multiple braided channels and formed a complex
estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime enviro ...
and terrestrial
ecosystems An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
. The use of water upstream and the accompanying reduction of freshwater flow has resulted in the loss of most of the
wetlands A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...
of the area, as well as drastic changes to the aquatic ecosystems - an ecosystem collapse.


Natural history

Until the early 20th century, the Colorado River ran free from its
headwaters The headwater of a river or stream is the geographical point of its beginning, specifically where surface runoff water begins to accumulate into a flowing channel of water. A river or stream into which one or many tributary rivers or streams flo ...
in the
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
of
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
southwest into
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
, where it flowed into the
Gulf of California The Gulf of California (), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Vermejo''), is a marginal sea of the Pacific Ocean that separates the Baja California peninsula from ...
. Significant quantities of nourishing
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
from throughout the Colorado River Basin were carried downstream, creating the vast Colorado River Delta and preventing intrusion of seawater into the
Mexicali Mexicali (; ) is the capital city of the States of Mexico, Mexican state of Baja California. The city, which is the seat of the Mexicali Municipality, has a population of 689,775, according to the 2010 census, while the Calexico–Mexicali, Cale ...
and
Imperial Valley The Imperial Valley ( or ''Valle Imperial'') of Southern California lies in Imperial and Riverside counties, with an urban area centered on the city of El Centro. The Valley is bordered by the Colorado River to the east and, in part, the S ...
s. Prior to the construction of major dams along its route, the Colorado River fed one of the largest
desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
in the world. Spread across the northernmost end of the Gulf of California, the Colorado River delta's vast
riparian A riparian zone or riparian area is the interface between land and a river or stream. In some regions, the terms riparian woodland, riparian forest, riparian buffer zone, riparian corridor, and riparian strip are used to characterize a ripar ...
, freshwater, brackish, and tidal
wetlands A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...
once covered and supported a large population of plant, bird, aquatic, and terrestrial life. Because most of the river's flow reached the delta at that time, its freshwater, silt, and nutrients helped create and sustain a complex system of estuarial
wetlands A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...
that provided feeding and nesting grounds for birds, and spawning habitat for fish and marine mammals. In contrast to the surrounding Sonoran Desert, the Colorado River delta's abundance was striking.


Human history


Early history

Early explorers reported
jaguar The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...
s,
beaver Beavers (genus ''Castor'') are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are the second-large ...
s,
deer A deer (: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) ...
, and
coyote The coyote (''Canis latrans''), also known as the American jackal, prairie wolf, or brush wolf, is a species of canis, canine native to North America. It is smaller than its close relative, the Wolf, gray wolf, and slightly smaller than the c ...
s in the delta, in addition to the abundance of
waterfowl Anseriformes is an order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which i ...
,
fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
, and other marine and
estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime enviro ...
organisms (Spamer, 1990; ). Early explorers also encountered local people known as the Cucapá, or the people of the river. The Cucapá are descendants of the Yuman-speaking Native
Indigenous peoples of the Americas In the Americas, Indigenous peoples comprise the two continents' pre-Columbian inhabitants, as well as the ethnic groups that identify with them in the 15th century, as well as the ethnic groups that identify with the pre-Columbian population of ...
and have inhabited the delta for nearly a thousand years. Spanish explorer Hernando de Alarcón made the first recorded contact with the Cucapá in 1540 and reported seeing many thousands. The Cucapá used the delta
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...
extensively for harvesting Palmer's saltgrass ('' Distichlis palmeri''), a wild grain that grows in salty soil, and for cultivating
maize Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...
(corn),
beans A bean is the seed of some plants in the legume family (Fabaceae) used as a vegetable for human consumption or animal feed. The seeds are often preserved through drying (a ''pulse''), but fresh beans are also sold. Dried beans are tradition ...
, and squash.


After dam construction

Today, conditions in the delta have changed. Like other desert river deltas, such as the
Nile Delta The Nile Delta (, or simply , ) is the River delta, delta formed in Lower Egypt where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's larger deltas—from Alexandria in the west to Port Said in the eas ...
and the
Indus River Delta The Indus River Delta forms where the Indus River flows into the Arabian Sea, mostly in the southern Sindh province of Pakistan with a small portion in the Kutch Region of India. The delta covers an area of about , and is approximately across ...
, the Colorado River delta has been greatly altered by human activity. Decades of dam construction and water diversions in the United States and Mexico have reduced the delta to a remnant system of small wetlands and brackish mudflats. As reservoirs filled behind dams and captured floodwaters, freshwater could no longer reach the delta. The construction of
Hoover Dam The Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado, Black Canyon of the Colorado River (U.S.), Colorado River, on the border between the U.S. states of Nevada and Arizona. Constructed between 1931 and 1936, d ...
in the 1930s marked the beginning of the modern era for the Colorado River Delta. For six years, as Lake Mead filled behind the dam, virtually no freshwater reached the delta. Even spring flooding was captured. This ecologically devastating event was repeated from 1963 to 1981 as
Lake Powell Lake Powell is a reservoir on the Colorado River in Utah and Arizona, United States. It is a major vacation destination visited by approximately two million people every year. It holds of water when full, second in the United States to only the ...
filled behind the Glen Canyon Dam. With these reservoirs now filled, the dams are used to regulate flow so that water can be reliably apportioned among the users of the Colorado River Compact, and its use maximized. Most flood flows can be contained, regulated, and added to the river's capacity to sustain the
Western United States The Western United States (also called the American West, the Western States, the Far West, the Western territories, and the West) is List of regions of the United States, census regions United States Census Bureau. As American settlement i ...
' urban centers and
agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...
. Floodwaters are released only when the
Bureau of Reclamation The Bureau of Reclamation, formerly the United States Reclamation Service, is a List of United States federal agencies, federal agency under the U.S. Department of the Interior, which oversees water resource management, specifically as it ...
, the agency managing the dams, predicts flows that exceed the system's capacity for use and storage. The loss of freshwater flows to the delta over the twentieth century has reduced delta wetlands to about 5 percent of their original extent, and non-native species have compromised the ecological health of much of what remains. Stress on ecosystems has allowed
invasive plant An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native speci ...
s to out-compete native species along Colorado River riparian areas. Native forests of cottonwood and
willow Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...
have yielded to sand and mudflats dominated by the nonnative
tamarisk The genus ''Tamarix'' (tamarisk, salt cedar, taray) is composed of about 50–60 species of flowering plants in the family Tamaricaceae, native to drier areas of Eurasia and Africa. The generic name originated in Latin and may refer to the Tamb ...
(also known as salt cedar), arrow-weed, and iodine bush, a transformation that has decreased the habitat value of the riparian forest.


High flows in 1980s

Full reservoir conditions coupled with a series of flood events throughout the 1980s and early 1990s resulted in flood releases that reached the delta. These flows reestablished an active floodplain and revegetated many areas of the floodplain within irrigation and flood control levels, and helped to reestablish riparian forests. A scheme is currently in place which aims to rejuvenate the
wetlands A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...
by releasing a pulse of water down the river delta.


Ecology

The delta supports a variety of wildlife, including several threatened and endangered species. Mexico's Environmental Regulations on Endangered Species lists the following endangered species found in the terrestrial and aquatic regions of the delta (Diario Officiel, 1994): *the
desert pupfish The desert pupfish (''Cyprinodon macularius'') is a rare species of teleost, bony fish in the family Cyprinodontidae. It is a small fish, typically less than 7.62 cm (3 in) in length. Males are generally larger than females, and have br ...
, also listed as an endangered species in the U.S., the largest remaining population anywhere is in the Ciénega de Santa Clara *the Yuma rail, also listed as endangered in the U.S. *the
bobcat The bobcat (''Lynx rufus''), also known as the wildcat, bay lynx, or red lynx, is one of the four extant species within the medium-sized wild cat genus '' Lynx''. Native to North America, it ranges from southern Canada through most of the c ...
*the vaquita porpoise, the world's smallest marine cetacean, listed as a species of special concern by the U.S. Marine Mammal Commission. There are thought to be 12 vaquitas left in the world. *the totoaba, now virtually extinct, a steel-blue fish that grows up to 2 m (7 ft) and 136 kg (300 pounds), and once supported a commercial fishery that closed in 1975 (Postel et al., n.d.). * the Colorado delta clam, once an extremely abundant species and important in the trophic dynamics of the ecosystem. Although not extensively studied, the delta's significance for migratory birds is indisputable, as it is the principal freshwater marsh in the region. A total of 358 bird species have been documented in the Colorado River Delta and the upper Gulf of California region. From these, two are listed as endangered, six as threatened, and sixteen are under special protection in Mexico. Two wintering species and five breeding species have been locally extirpated, including the southwestern willow flycatcher, the fulvous whistling duck, and the
sandhill crane The sandhill crane (''Antigone canadensis'') is a species of large Crane (bird), cranes of North America and extreme northeastern Siberia. The common name of this bird refers to its habitat, such as the Platte River, on the edge of Nebraska's S ...
.


Biosphere reserve

The
Gulf of California The Gulf of California (), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Vermejo''), is a marginal sea of the Pacific Ocean that separates the Baja California peninsula from ...
lies within the jurisdictional boundaries of Mexico and its
states State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...
of
Baja California Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of B ...
and
Sonora Sonora (), officially Estado Libre y Soberano de Sonora (), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is divided into Municipalities of Sonora, 72 ...
. In 1974, the Mexican government designated portions of the upper Gulf and lower Colorado River Delta as a reserve zone. The
United Nations Educational, Scientific and Cultural Organization The United Nations Educational, Scientific and Cultural Organization (UNESCO ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) with the aim of promoting world peace and International secur ...
(UNESCO) designated over of Gulf of California and Colorado River Delta Biosphere Reserve as a
Biosphere The biosphere (), also called the ecosphere (), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to mat ...
Nature reserve A nature reserve (also known as a wildlife refuge, wildlife sanctuary, biosphere reserve or bioreserve, natural or nature preserve, or nature conservation area) is a protected area of importance for flora, fauna, funga, or features of geologic ...
in June 1993. Within this , over nearest the Colorado River Delta are designated as the Reserve "core area", with the remaining of open water and shoreline designated as a "buffer area". UNESCO considers areas for designation as
Biosphere Reserve A nature reserve (also known as a wildlife refuge, wildlife sanctuary, biosphere reserve or bioreserve, natural or nature preserve, or nature conservation area) is a protected area of importance for flora, fauna, funga, or features of geologic ...
s only after the nation in which the site is located submits a nomination. Once designated, Biosphere Reserves remain under the sovereign jurisdiction of the countries where they are located. Federal Mexican governmental agencies with administrative authority over the Biosphere Reserve include the National Commission of Protected Natural Areas ( CONANP) and the Secretary of the Environment, Natural Resources and Fisheries ( SEMARNAP). In addition to designation as a Biosphere Reserve, within Colorado River Delta (Humedales del Delta del Río Colorado) are designated as a
Ramsar Wetland The Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat is an international treaty for the conservation and sustainable use of Ramsar sites (wetlands). It is also known as the Convention on Wetlands. It i ...
under the U.N. Convention on Wetlands. Ramsar Wetlands are wetlands of international importance in terms of their ecology, botany, zoology, limnology or hydrology. The U.N. designation is considered following a nomination by the nation in which the site is located.


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See also

*
Aral Sea The Aral Sea () was an endorheic lake lying between Kazakhstan to its north and Uzbekistan to its south, which began shrinking in the 1960s and had largely dried up into desert by the 2010s. It was in the Aktobe and Kyzylorda regions of Kazakhst ...


References


External links

*Officia
Upper Gulf of California and Colorado River Delta Biosphere Reserve websiteMap of the Biosphere Reserve (in español)A Delta Once More: Restoring Riparian and Wetland Habitat in the Colorado River Delta
*[https://web.archive.org/web/20050621210518/http://www.ibwc.state.gov/Files/Minutes/Min306.pdf IBWC Minute 306 (Conceptual Framework for U.S. – Mx Studies for Future Recommendations Concerning the Riparian and Estuarine Ecology of the Limitrophe Section of the Colorado River and its Associated Delta.)]
Proceedings of IBWC Colorado River Delta SymposiumColorado River Delta Research Coordination Network
{{Authority control Landforms of Baja California Landforms of Sonora River deltas of the United States Gulf of California Wetlands of Mexico Flooded grasslands and savannas Ecoregions of Mexico Ramsar sites in Mexico Landforms of Mexico Natural history of the Lower Colorado River Valley Salton Trough Sonoran Desert World Heritage Sites in Mexico Biosphere reserves of Mexico Important Bird Areas of Mexico Protected areas of Baja California