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107-699: The Sacramento–San Joaquin River Delta , or California Delta , is an expansive inland river delta and estuary in Northern California . The delta is formed at the western edge of the Central Valley by the confluence of the Sacramento and San Joaquin rivers and lies just east of where the rivers enter Suisun Bay , which flows into San Francisco Bay , then the Pacific Ocean via San Pablo Bay . The Delta

214-432: A density current that deposits its sediments as turbidites . When the river water is less dense than the basin water, as is typical of river deltas on an ocean coastline, the delta is characterized by hypopycnal flow in which the river water is slow to mix with the denser basin water and spreads out as a surface fan. This allows fine sediments to be carried a considerable distance before settling out of suspension. Beds in

321-479: A chance to learn about the disaster that represented the largest domestic loss of life in the U.S. in World War II, and the impact it had on race relations in the U.S. military. Brannan Island State Recreation Area offers boat launches and facilities that support fishing, windsurfing, waterskiing, and sailing, in addition to campsites, picnic areas and trails to allow visitors to explore the marshes and islands of

428-485: A common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation , and sea access for trade. Deltas often host extensive industrial and commercial activities, and agricultural land is frequently in conflict. Some of the world's largest regional economies are located on deltas such as the Pearl River Delta , Yangtze River Delta , European Low Countries and

535-537: A delta but enter into the sea in the form of an estuary . Notable examples include the Gulf of Saint Lawrence and the Tagus estuary. In rare cases, the river delta is located inside a large valley and is called an inverted river delta . Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an inland delta , and often occurs on former lake beds. The term

642-533: A fan. The more often the flow changes course, the shape develops closer to an ideal fan because more rapid changes in channel position result in a more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape. Most large river deltas discharge to intra-cratonic basins on

749-399: A few main distributaries. Once a wave-dominated or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there is a lot of water around – such as floods or storm surges . These distributaries slowly silt up at a more or less constant rate until they fizzle out. A tidal freshwater delta is

856-535: A flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example. See endorheic basin . The generic term mega delta can be used to describe very large Asian river deltas, such as the Yangtze , Pearl , Red , Mekong , Irrawaddy , Ganges-Brahmaputra , and Indus . The formation of a delta is complicated, multiple, and cross-cutting over time, but in

963-573: A hypocynal delta dip at a very shallow angle, around 1 degree. Fluvial-dominated deltas are further distinguished by the relative importance of the inertia of rapidly flowing water, the importance of turbulent bed friction beyond the river mouth, and buoyancy . Outflow dominated by inertia tends to form Gilbert-type deltas. Outflow dominated by turbulent friction is prone to channel bifurcation, while buoyancy-dominated outflow produces long distributaries with narrow subaqueous natural levees and few channel bifurcations. The modern Mississippi River delta

1070-463: A marshy interior "bowl" that flooded intermittently with the seasons and tides. The height of these natural levees, formed from overbank deposits of sediments deposited by annual river floods, ranged from one foot (0.30 m) above mean high tide at Sherman Island on the Sacramento River, to seven to eight feet (2.1 to 2.4 m) at Andrus , Staten and Tyler Islands, located further east near

1177-481: A mat up to 6 feet (1.8 m) thick, shutting off light and becoming acidic when they decompose. The Delta was formerly populated by large herds of deer and tule elk ; their trails were so vast that early Spanish explorers supposed the area was inhabited by cattle. Significant numbers of California grizzly bears could also be found in the Delta. The extensive wetlands of the Delta supported massive bird populations, half

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1284-439: A mature delta with a distributary network. Another way these distributary networks form is from the deposition of mouth bars (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth bar, which splits the river into two distributary channels. A good example of

1391-420: A north–south route for migratory birds, crosses the Delta. The migratory birds provide both birdwatching and hunting opportunities. Including the northern Suisun Marsh , there are over 150 private duck clubs in the area. The levee system allowed farmers to drain and reclaim almost 500,000 acres (780 sq mi; 2,000 km) of the Delta, then a tidal marsh. Once the rivers were confined to their riverbeds,

1498-502: A risk for human consumption as well. After numerous rounds of mitigation following the 2005 Smelt Biological Opinion (BiOp) lead to pumping restrictions from the State and Federal projects from the Delta, a number of potential solutions to the Delta's problems have been proposed. These followed the CALFED process which had offered several staged solutions. A first set of alternatives would maintain

1605-862: A river valley through which the continuation of the Sacramento and San Joaquin Rivers flowed to the Pacific Ocean. When sea levels rose again, ocean water backed up through the Carquinez Strait into the Central Valley; the combination of the narrow strait and tidal action pushing inland dramatically slowed the current of these rivers and forced them to drop sediment. The early delta was composed of shifting channels, sand dunes, alluvial fans and floodplains that underwent constant fluctuation because of rapidly rising seas – one inch (25 mm) per year. About 8,000 years ago,

1712-423: A sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary". Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries. Each tributary mimics this salinity gradient from its brackish junction with the mainstem estuary up to

1819-447: A simple delta three main types of bedding may be distinguished: the bottomset beds, foreset/frontset beds, and topset beds. This three-part structure may be seen on small scale by crossbedding . Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments. Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in

1926-419: A threat that has the potential to impact human life. Several types of toxic cyanobacteria have appeared in the Delta, with Microcystis aeruginosa being one of the most common types. Microcystis aeruginosa produce microcystins which are hepatotoxins that can cause liver cancer. Frequent Microcystis blooms have impacted the food web of the Delta at multiple trophic levels . Microcystis spp. blooms in

2033-542: Is a good example of a fluvial-dominated delta whose outflow is buoyancy-dominated. Channel abandonment has been frequent, with seven distinct channels active over the last 5000 years. Other fluvial-dominated deltas include the Mackenzie delta and the Alta delta. A Gilbert delta (named after Grove Karl Gilbert ) is a type of fluvial-dominated delta formed from coarse sediments, as opposed to gently sloping muddy deltas such as that of

2140-651: Is a major sign that Mars once had large amounts of water. Deltas have been found over a wide geographical range. Below are pictures of a few. Georgiana Slough Georgiana Slough , is a slough within Sacramento County, California . It is located in the Sacramento–San Joaquin River Delta , and links both the Sacramento River and the San Joaquin River above their confluence in

2247-432: Is also provided for wildlife refuges and habitat conservation works. The Contra Costa Canal and North Bay Aqueduct also take Delta water, supplying the nearby San Francisco Bay Area. Locally, the Delta provides water for cities and towns in five counties and for over 1,800 agricultural users. The Delta is also a popular site for recreation and is used for sailing, waterskiing, houseboating, fishing, and hunting. The Delta

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2354-534: Is also transected by the Middle River and Old River, former channels of the San Joaquin. These rivers transport more than 30 million acre-feet (8.9 cu mi; 37 km) of water through the Delta each year – about 50 percent of all California's runoff. Nearby cities include Lodi and Stockton to the east, Tracy and Manteca to the south, Brentwood to the southwest, and Pittsburg and Antioch to

2461-576: Is called by the Ionians ", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. Despite making comparisons to other river systems deltas, Herodotus did not describe them as "deltas". The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, referring to both as islands, but did not apply the word delta. According to

2568-469: Is essential to provoke the slowing of river current and the resulting sediment deposits that now make up the Delta. The Delta in its contemporary (pre-1850s) state began to form about 10,000 years ago at the end of the last Ice Age. During the Ice Age global sea levels were about 300 ft (91 m) lower than today, and the Delta region, as well as Suisun Bay, the Carquinez Strait and San Francisco Bay, were

2675-564: Is home to over 100 marinas and 25 yacht clubs. In 2010, over seven million 'annual visitor boating days' occurred on the Delta, according to a local government survey. The Delta is home to several state and regional parks, reflecting a variety of recreational activities. These include historical interpretative centers, including Delta Meadows and Locke Boarding House and the Old Sacramento State Historic Park . The Port Chicago Naval Magazine National Memorial offers visitors

2782-522: Is not considered a true river delta, but rather an inverted river delta , as it formed inward (sediments accumulating progressively inland) rather than outward. The only other major river delta in the world located this far inland is the Pearl River Delta in China. The main source rivers include the Sacramento River from the north, the San Joaquin from the southeast, and the Calaveras and Mokelumne Rivers from

2889-526: Is recognized for protection by the California Bays and Estuaries Policy . Sacramento–San Joaquin Delta was designated a National Heritage Area on March 12, 2019. The city of Stockton is located on the San Joaquin River at the eastern edge of the delta. The total area of the Delta, including both land and water, is about 1,100 square miles (2,800 km). Its population is around 500,000. The Delta

2996-615: Is required again. Delta waterways are also major transportation corridors for agricultural products; the ports of Sacramento and Stockton , located on the Sacramento and San Joaquin rivers, respectively, are the most important inland ports in California. The San Joaquin River throughout most of the Delta and the lower Sacramento River below its connection to the Sacramento Deep Water Ship Channel are routinely dredged to allow

3103-689: Is shipped to both the Benicia Refinery and the Martinez Refinery by water. Nearby is the Port Chicago Marine Ocean Terminal , a facility that supports the U.S. Navy. The Delta is often considered the nexus of California's statewide water system . About half the total river flow in the state passes through this region, from which water is exported to other areas of the San Joaquin Valley, Southern California and portions of

3210-490: Is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta . The triangular shape of the Nile Delta was known to audiences of classical Athenian drama ; the tragedy Prometheus Bound by Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta". Herodotus 's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it

3317-452: Is used as a sedimentation enhancing strategy to reduce subsidence and restore the wetlands in the Delta. Land subsidence has endangered the Delta's system of protective levees, occasionally triggering levee failure and subsequent flooding. Using the Delta as the primary valve in a linear water system may have been convenient for a period, but that convenience has come at a high price for the environment. —Paul Shigley, The Devil Is in

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3424-501: The Central Valley , which includes both the irrigation water for its $ 17 billion agricultural economy and the drinking water for about 25 million people. About 500 plant and animal species inhabit the Delta, which is one of the largest estuaries in western North America. Before agricultural development of the Delta region, the Delta's many islands were forested by Tule , bulrush , and various reeds that flourished in intermittently flooded, low-lying marshy areas. Over thousands of years,

3531-641: The Greater Tokyo Area . The Ganges–Brahmaputra Delta , which spans most of Bangladesh and West Bengal and empties into the Bay of Bengal , is the world's largest delta. The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water, in its case Lake Baikal . Researchers have found a number of examples of deltas that formed in Martian lakes . Finding deltas

3638-526: The Nile Delta and Colorado River Delta are some of the most extreme examples of the devastation caused to deltas by damming and diversion of water. Historical data documents show that during the Roman Empire and Little Ice Age (times when there was considerable anthropogenic pressure), there was significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat. Ancient deltas benefit

3745-510: The Port of Stockton to the San Joaquin's confluence with the Sacramento near Antioch. "The virgin California Delta was so vast, wild and confusing – its sloughs meandered everywhere and led nowhere – that John C. Frémont lost a whole regiment in there for several days, and some who ventured in just disappeared." Marc Reisner , A Dangerous Place (2003) The Delta was formerly located at

3852-584: The Arkansas or Swampland Act , which allowed for the transfer of title for wetlands to private owners on the conditions that the land would be reclaimed. In California, more than two million acres (3,100 sq mi; 8,100 km) of wetlands were partitioned under the Swampland Act, of which 500,000 acres (780 sq mi; 2,000 km) were in the Sacramento–San Joaquin Delta, roughly

3959-662: The Bay area to supply some 1,130,000 acres (1,770 sq mi; 4,600 km) of farmland and 23 million people in central and Southern California. The Delta provides an estimated seven million acre-feet (2.1 cu mi; 8.6 km) of water per year, of which about 100,000 acre-feet (0.030 cu mi; 0.12 km) are exported to the San Francisco Bay Area, 1.7 million acre-feet (0.50 cu mi; 2.1 km) are used locally, and over five million acre-feet (1.5 cu mi; 6.2 km) are exported to

4066-548: The Delta (2012) Land subsidence also allows brackish water intrusion into the Delta, an issue compounded by the diversion of up to 25% of the freshwater flowing into the Delta. The decreased volume of freshwater in the Delta has had a profound effect on its ecology. In most years, large dams in the Delta watershed fully hold back spring runoff; as a result the Delta is most susceptible to salinity intrusion between February and June. However, regulation provided by dams helps boost freshwater flows during dry summers and autumns, reducing

4173-448: The Delta can be attributed to a variety of factors with the most important being lowered streamflow . From 2004 through 2008 researchers collected different water quality parameters during the cyanobacteria blooms and determined that the blooms appeared after reaching a threshold of 19 °C (66 °F) which was exacerbated by reduced precipitation , reduced streamflow, and increased nutrient concentrations. They also determined that

4280-401: The Delta cannot be irrigated using conventional methods due to the highly absorbent quality of the peat soil and irregularities caused by land subsidence. Irrigation is typically carried out periodically by piping water into small "spud ditches", which spread water over large areas and raise the local water table . The heightened groundwater is then gradually depleted by the crop until irrigation

4387-532: The Delta from the summit of nearby Mount Diablo . For decades, the Delta was little utilized by the Spanish colonists. Expeditions from 1806 to 1812 failed to locate suitable mission sites in the Delta area. However, frequent military expeditions were made into the Delta from 1813 to 1845 in response to animosities between the Native Americans and the Spanish and later Mexicans; also several land grants were made in

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4494-682: The Delta in its current condition and configuration. The second would restore parts of the Delta more closely to its natural state but include the construction of an additional Peripheral Canal to maintain the water supply currently provided by the Delta. River delta River deltas are important in human civilization , as they are major agricultural production centers and population centers. They can provide coastline defence and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales , they are also important carbon sinks . A river delta

4601-580: The Delta levee system can be found in the Economic Sustainability Plan of the Delta Protection Commission. This study concluded that the Delta levee system is now relatively robust but should be improved to effectively eliminate the risk of failure in extreme floods and earthquakes. It emphasizes the significant value of the infrastructure that passes through the Delta, including water conveyance, in addition to life and property, and

4708-768: The Delta near Pittsburg , at the head of Suisun Bay , through its connection with the Mokelumne River . The entrance to the slough on the Sacramento River is just below Walnut Grove , at 38°14′21″N 121°30′59″W  /  38.23917°N 121.51639°W  / 38.23917; -121.51639 and runs between Tyler Island and Andrus Island to where it has its confluence with the Mokelumne River at 38°07′49″N 121°34′40″W  /  38.13028°N 121.57778°W  / 38.13028; -121.57778 northwest of Bouldin Island just above that rivers confluence with

4815-507: The Delta was 515,264 as of 2000. Altogether, the Delta covers 1,153 square miles (2,990 km), with 841 sq mi (2,180 km), or nearly 73 percent, devoted to agriculture. About 100 sq mi (260 km) of the Delta area is urban and 117 sq mi (300 km) are undeveloped land. The rivers, streams, sloughs and waterways of the Delta total about 95 sq mi (250 km) of surface, although this fluctuates greatly with seasons and tides. Geologically, it

4922-510: The Delta were found to decrease the diversity of the aquatic microbial community. Additionally, the cyanobacteria blooms in the Delta have led to decreased zooplankton mass and density . There is also concern for further spread throughout the food web via bioaccumulation . Microcystins were detected in the tissue of clams at levels much higher than the ambient water around them because of microcystin's ability to covalently bind to tissue. The increased occurrences of cyanobacteria blooms in

5029-477: The Delta's ecosystem. Delta fish populations have been significantly reduced due to the reclamation of marshland and diversions of fresh water. In 2004, the Delta smelt was found to be on the edge of extinction. The survival of the Delta smelt has been one of the largest environmental issues in California, as environmental measures enacted to protect its population have often reduced the amount of water available for federal water projects that depend on water pumped from

5136-568: The Delta. A 2010 study shows that the anglers in the Delta who ate these fish had higher levels of mercury in their system. Delta fish have also been found to have high levels of a toxin called methylmercury which originates from old gold rush era mines in the Sierras. In that same study, it was found that women along the delta were consuming high numbers of fish, making them more susceptible to poisoning. Nutria were found in Merced County in 2017 on

5243-678: The Greek geographer Strabo , the Cynic philosopher Onesicritus of Astypalaea , who accompanied Alexander the Great 's conquests in India , reported that Patalene (the delta of the Indus River ) was "a delta" ( Koinē Greek : καλεῖ δὲ τὴν νῆσον δέλτα , romanized:  kalei de tēn nēson délta , lit.   'he calls the island a delta'). The Roman author Arrian 's Indica states that "the delta of

5350-497: The Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. It is commonly a result of homopycnal flow. Such deltas are characterized by a tripartite structure of topset, foreset, and bottomset beds. River water entering the lake rapidly deposits its coarser sediments on the submerged face of the delta, forming steeping dipping foreset beds. The finer sediments are deposited on

5457-525: The Mokelumne River. An estimated 60 percent of the Delta flooded for up to two hours each day at high tide; during spring tides or river floods, it was not uncommon for the entire Delta to be under water. Humans have inhabited the Delta for up to 4,300 years. The estimated indigenous population of the Delta at the time of first contact with Europeans was about 3,000–15,000, predominantly Miwok and Maidu , with some estimates ranging up to 20,000. The Native Americans lived in villages of 200–1,000 people on

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5564-508: The Sacramento–San Joaquin Delta. The California Office of Environmental Health Hazard Assessment has issued a safe eating advisory for any fish caught in Central and South parts of the Delta due to elevated levels of mercury and PCBs. In addition, there is a notice of "DO NOT EAT" for any fish or shellfish from the Port of Stockton . There is a separate safe eating advisory for the Northern part of

5671-466: The San Joaquin Valley, coastal Central and Southern California. Intrusion of brackish water into the Delta is a recurring natural phenomenon; however, it became a serious issue after the development of agriculture in the upper Sacramento and San Joaquin valley reduced inflows. Multiple droughts between 1910 and 1940 caused significant salinity intrusion in the Delta because of the reduction of freshwater inflows. The growing Delta water quality issue provided

5778-472: The ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf . There are many other lesser factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to

5885-447: The area. Originally a Delta farming island, Franks Tract flooded in 1936 and again in 1938. The levees were never repaired and the island remained submerged, becoming an expanse of open water dotted with remnants of the original levees now the only landforms. In 1959, the area became a state park. Franks Tract State Recreation Area is accessible only by water and is used for fishing, waterfowl hunting, and boating. The Pacific Flyway ,

5992-1256: The basins feeding deltas have reduced river sediment delivery to many deltas in recent decades. This change means that there is less sediment available to maintain delta landforms, and compensate for erosion and sea level rise , causing some deltas to start losing land. Declines in river sediment delivery are projected to continue in the coming decades. The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes. People living on deltas often construct flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can not compensate for subsidence and erosion . In addition to interference with delta aggradation , pumping of groundwater , oil , and gas , and constructing infrastructure all accelerate subsidence , increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining , and cause saltwater intrusion . There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies . While nearly all deltas have been impacted to some degree by humans,

6099-422: The bird's-foot of the Mississippi or Ural river deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope ). As the gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within

6206-599: The bottom of a large inland sea in the Central Valley, which formed as the uplift of the California Coast Ranges blocked off drainage from the Sierra Nevada to the Pacific. About 560,000 years ago, water breached the mountains, carving out the present-day Carquinez Strait and San Francisco Bay . The drainage of all the water through this narrow gap formed a bottleneck in the Central Valley's outflow; this constriction

6313-450: The channel and a rise in the channel bed relative to the floodplain . This destabilizes the river channel. If the river breaches its natural levees (such as during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient. Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up

6420-501: The conditions are met Microcystis have the ability to dominate the lower trophic levels hence why they are able to bloom. Additionally, when microcystins are present in the system then the consumers in the food web are at risk due to the effects of bioaccumulation. Fish that are present and active during cyanobacteria blooms can often have microcystin levels high enough to produce sublethal effects. Because microcystins can concentrate inside fish at multiple trophic levels it also represents

6527-492: The delta that are named or not named. These are all surrounded by 1,100 miles (1,800 km) of levees that border 700 miles (1,100 km) of waterways. The southwestern side of the Delta lies at the foothills of the California Coast Ranges , while to the northwest sit the lower Montezuma Hills . Most of the Delta lies within Contra Costa , Sacramento , San Joaquin , Solano and Yolo Counties. The total human population of

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6634-496: The disruption of fish migration and salinity buildup in the eastern Delta, where salts can no longer be flushed to the sea by natural river flows. Since 1900, there have been over 160 levee failures in the Sacramento–San Joaquin River Delta. These numbers include multiple failures of a single levee structure. Levee failures, also known as breaches, can be caused by overtopping or structural failure . One of

6741-465: The east. The Calaveras and Mokelumne are both tributaries of the San Joaquin River. The Sacramento and San Joaquin Rivers join at the western end of the Delta near Pittsburg , at the head of Suisun Bay, although they are linked upstream by the Georgiana Slough , which was first used by steamboats in the 19th century as a shortcut between Sacramento and Stockton . The southwestern part of the Delta

6848-452: The eastern edge of the Delta, where the land was higher and less susceptible to flooding. Their lives centered around the abundant reeds or tules that grew on the Delta islands, which they used to make houses, boats, and garments. Staple foods included tule roots and pollen, acorns, wild fruits and seeds, fish and game. Europeans first entered the Delta region in 1772, when Spanish explorer Don Pedro Fages and missionary Juan Crespí observed

6955-612: The economy due to their well-sorted sand and gravel . Sand and gravel are often quarried from these old deltas and used in concrete for highways , buildings, sidewalks, and landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone. Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water. Urban areas and human habitation tend to be located in lowlands near water access for transportation and sanitation . This makes deltas

7062-473: The edge of the Delta. State officials are concerned that they will harm the infrastructure that sends water to San Joaquin Valley farms and urban areas. The Delta has seen numerous cyanobacteria blooms with increasing frequency over the past two decades. Cyanobacteria have the potential to produce cyanotoxins which can pose a risk to humans and animals upon contact. Because of this cyanobacteria blooms are seen as

7169-574: The end of the Last Glacial Period . In its natural state, the Delta was a large freshwater marsh , consisting of many shallow channels and sloughs surrounding low islands of peat and tule . Since the mid-19th century, most of the region has been gradually claimed for agriculture. Wind erosion and oxidation have led to widespread subsidence on the Central Delta islands; much of the Delta region today sits below sea level, behind levees earning it

7276-431: The entirety of Sherman Island, some 14,000 acres (22 sq mi; 57 km), was diked and drained by a system of levees, flumes and floodgates. By the 1890s, the original levee system had been largely replaced with stronger embankments consisting of clay dredged from nearby river bottoms. By 1900, about 235,000 acres (367 sq mi; 950 km), or nearly half of the Delta's land area, had been reclaimed. Most of

7383-437: The existing Delta infrastructure, forcing landowners to rebuild their levees higher and stronger; more flooding in 1878 and 1881 reinforced these notions. Although land holdings in the Delta were initially limited to 320 acres (0.50 sq mi; 1.3 km) per buyer, this limit was repealed in 1868, allowing large agricultural conglomerates to take entire islands and carry out massive reclamation projects. From 1868 to 1869,

7490-537: The farmable land in the Delta had been reclaimed by the 1920s. The Delta produces crops valued at about $ 650 million annually (for the 1998–2004 period), making it one of the most productive farming regions of the United States in terms of crop value per unit area. Agriculture provides secondary benefits of over $ 2 billion to the local economy. Major crops grown in the Delta include corn, grain, hay, sugar beets, alfalfa, tomatoes, asparagus, and safflower; various fruits are also raised here, as well as some livestock. Crops in

7597-399: The flood of 1878 wiped out the last of the elk herds in the Delta. The Delta is home to approximately 22 species of fish, including several Pacific salmon species, striped bass , steelhead trout , American shad and sturgeon . About two-thirds of California's salmon pass through the Delta on their way upstream to spawn. The small Delta smelt is a key indicator species for the health of

7704-414: The flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity , which diminishes the ability of the flow to transport sediment . As a result, sediment drops out of the flow and is deposited as alluvium , which builds up to form the river delta. Over time, this single channel builds a deltaic lobe (such as

7811-506: The fresh stream feeding the head of tidal propagation. As a result, the tributaries are considered to be "subestuaries". The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas as well as processes that are unique to the tidal freshwater setting. The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics. Many tidal freshwater deltas that exist today are directly caused by

7918-466: The geomorphology and ecosystem. Deltas are typically classified according to the main control on deposition, which is a combination of river, wave , and tidal processes, depending on the strength of each. The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river. Fluvial-dominated deltas are found in areas of low tidal range and low wave energy. Where

8025-400: The growth and decay of the various swampland plants formed a layer of peat 50 feet (15 m) deep in places, hence the extremely high fertility of Delta soils. Sediment deposits formed natural levees around the borders of islands, where larger trees, mainly willows, were able to take root and form large riparian forests . The riparian forests were most pronounced along the Sacramento River and

8132-538: The initial impetus for building dams on Central Valley rivers to boost dry-season freshwater flows. This eventually became the federal Central Valley Project (CVP), California's first major statewide water system, most of which was built between the 1930s and the 1960s. Today, the California Aqueduct and the Delta-Mendota Canal , two of the state's largest water conveyance facilities, both draw water from

8239-406: The lake bottom beyond this steep slope as more gently dipping bottomset beds. Behind the delta front, braided channels deposit the gently dipping beds of the topset on the delta plain. While some authors describe both lacustrine and marine locations of Gilbert deltas, others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with

8346-586: The lakewater faster (as opposed to the case of a river falling into the sea or a salt lake, where less dense fresh water brought by the river stays on top longer). Gilbert himself first described this type of delta on Lake Bonneville in 1885. Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and form prominent peninsulas at Naramata , Summerland , and Peachland . In wave-dominated deltas, wave-driven sediment transport controls

8453-566: The land of the Indians is made by the Indus river no less than is the case with that of Egypt". As a generic term for the landform at the mouth of the river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon . River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir . When

8560-486: The lower Mokelumne River below its confluence with the Cosumnes River . More extensive woodlands were prominent on the fringes of the Delta bordering the riparian zones. Composed primarily of valley oak , box elder and Oregon ash , these oak woodlands grew in bands that stretched up to 3 miles (4.8 km) inland. Farther away from water sources, vegetation gave way to grassland . The water hyacinth has become one of

8667-400: The lower trophic levels. There have also been large amounts of nutrients monitored in the Delta as a result of various human activities. The increased presence of Microcystis aeruginosa blooms in the Delta represents a continual threat for species at multiple trophic levels. The lower trophic levels are affected from both reduced diversity as well as reduced numbers through competition. When

8774-414: The migrating waterfowl in California continue to pass through the Delta. A survey in 2012 recorded a spring duck population of 48.6 million, the highest since counts began in 1955. Large mammals in the Delta have not fared so well; with most of their habitat converted to agriculture, remaining populations were highly susceptible to human impacts and natural disasters. Grizzlies were hunted to extinction, while

8881-417: The most destructive plants to the Delta water way. Within one year it will spread and cover as much as 6,500 square feet (600 m) of water space in one growing season. It has become a challenge to the agricultural community to eliminate. The plants spread by budding and spreading its seeds, and those seeds fall to the bottom of the water, where they can stay viable in the muck for years. The Hyacinth can form

8988-489: The most recent examples of levee failure in the Sacramento–San Joaquin River Delta occurred in June 2004 when a levee breach caused more than 150,000 acre-feet (190,000,000 m) of water to flood the entire island of Jones Tract . But, the significant improvements made to the Delta levee system since 1982 have reduced the incidence of failures to this one major failure in 30 years. The most up-to-date description and discussion of

9095-483: The negative attributes associated with climate change like reduced precipitation and increased temperatures could further increase the possibility of cyanobacteria blooms in the Delta. The high prevalence of nutrient concentrations in the Delta also plays a significant part in the increased frequency of Microcystis aeruginosa blooms. Microcystis benefits greatly from anthropogenic inputs of nitrogen which allows it to out-compete other primary producers and dominate

9202-483: The nickname "California's Holland ". Much of the water supply for Central California and Southern California is also derived from the Delta, via pumps located at the southern end of the Delta. The pumps deliver water for irrigation in the San Joaquin Valley and municipal water supply for Southern California. The Delta consists of approximately 57 reclaimed islands and tracts but there are nearly 200 islands in

9309-524: The onset of or changes in historical land use, especially deforestation , intensive agriculture , and urbanization . These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States. Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development. Other rivers, particularly those on coasts with significant tidal range , do not form

9416-429: The passage of large cargo ships. The Sacramento River corridor has been maintained to a depth of 7 ft (2.1 m) as early as 1899, and was deepened to 30 ft (9.1 m) in 1955. The Stockton Ship Channel has been dredged since 1913; however, its present depth of 37 ft (11 m) was only achieved in 1987. The Delta is also home to the port of Benicia , an automobile and bulk shipping facility. Petroleum

9523-612: The peat soil of the former tidal marsh was exposed to oxygen. As the oxygen-rich peat soil decomposed and then released carbon dioxide, profound subsidence of the land resulted, of up to 25 feet since the late 1800s. Currently, most of the Delta is below sea level, with a great deal of the western and central Delta at least 15 feet (4.6 m) below sea level. The California Department of Water Resources has experimented with re-flooding areas for wetland restoration , in order to sequester carbon and rebuild soil levels . In addition, shallow flooding of land to restore anaerobic conditions

9630-452: The rate of sea-level rise slackened, allowing wetland plants to take hold in the Delta, trapping sediment; the growth and decay of these plants began to form the vast peat deposits that make up the Delta islands. The Delta reached a stabilized form similar to its mid-1800s state about 2,000–3,000 years ago. Immediately before large-scale human development, most of the Delta islands had saucer-like cross sections, with low natural levees flanking

9737-480: The result of this process is the Wax Lake Delta . In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates

9844-478: The risk of salinity intrusion in these months. Diversions located at the southern end of the Delta, however, have negated some of the benefits of upstream dams. The powerful pumps that supply water for the Central Valley Project and State Water Project cause water in the Delta to flow from north to south instead of the natural direction of east to west. This has caused multiple environmental issues, such as

9951-573: The river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta , which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Tidal deltas behave differently from river-dominated and wave-dominated deltas, which tend to have

10058-414: The river water is nearly equal in density to the basin water, the delta is characterized by homopycnal flow , in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load. Where the river water has a higher density than basin water, typically from a heavy load of sediment, the delta is characterized by hyperpycnal flow in which the river water hugs the basin bottom as

10165-511: The same amount of land that has been developed to date. Agricultural interests in the Delta were protected by the building of levees, a colossal effort first undertaken by Chinese laborers from the 1850s to the 1870s. The Board of Reclamation, formed in 1861, collectivized levee construction in the Delta by grouping islands into areas known as reclamation districts . The early levees were built of peat, and were highly susceptible to wind and water erosion. The Great Flood of 1862 obliterated much of

10272-399: The sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches. At the mouth of a river, the change in flow conditions can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter

10379-514: The sediment source. When sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion , delta lobe switching, and auto cyclicity. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment. However,

10486-424: The shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coastline. The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin. With a high wave energy near shore and a steeper slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from

10593-660: The southern end of the Delta at the Clifton Court Forebay . Built in the mid-twentieth century, the former supplies water to the Los Angeles Basin and coastal central California via the State Water Project ; the latter, a part of the CVP, provides supplies of irrigation water in the fertile San Joaquin Valley. Although the vast majority of water supplied by these projects is used for agriculture and urban areas, some water

10700-650: The trailing edges of passive margins due to the majority of large rivers such as the Mississippi , Nile , Amazon , Ganges , Indus , Yangtze , and Yellow River discharging along passive continental margins. This phenomenon is due mainly to three factors: topography , basin area, and basin elevation. Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas. Topography along active margins tends to be steeper and less widespread, which results in sediments not having

10807-412: The value of the Delta as a Place. A 2019 article states that "a catastrophic levee failure, defined as 20 islands flooding at once, has a 62 percent chance of occurring in the next two decades if subsidence isn’t addressed." Simultaneous levee failures on the Delta's 57 islands in the wake of an earthquake which allow the inflow of brackish San Francisco Bay waters could threaten the water supply for

10914-489: The vicinity of the Delta, including one to John Augustus Sutter , who started the first significant European settlement in the Central Valley just north of the Delta near present-day Sacramento. The Spanish conscripted large numbers of Native Americans for labor on missions; many Native Americans fled deep into the Delta in order to escape their European masters. However, this did not protect them from diseases. A malaria epidemic in 1833 decimated local native populations; this

11021-408: The west. The state capital, Sacramento , is located just to the north of the Delta. The Sacramento River Deep Water Ship Channel connects the Delta to the Port of Sacramento , with its terminus located near Rio Vista , on the northwestern side of the Delta. The Stockton Ship Channel is a dredged and partially straightened section of the San Joaquin River cutting directly through the Delta from

11128-411: Was cheaper and simpler than other fertile regions of California. As a result, the Delta remained California's richest farming region (alongside Los Angeles County ) until the 1940s, when massive federal water projects finally made possible the full-scale irrigation of the main body of the Central Valley. Today, the Delta is still among the state's most productive farming regions. In 1850, Congress passed

11235-637: Was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River , which he visited in 1800. Other prominent examples include the Inner Niger Delta , Peace–Athabasca Delta , the Sacramento–San Joaquin River Delta , and the Sistan delta of Iran. The Danube has one in the valley on the Slovak–Hungarian border between Bratislava and Iža . In some cases, a river flowing into

11342-469: Was formed by rising sea level following glaciation, leading to the accumulation of Sacramento and San Joaquin River sediments behind the Carquinez Strait , the sole outlet from the Central Valley to San Pablo Bay. The narrowness of the Carquinez Strait coupled with tidal action has caused the sediment to pile up, forming expansive islands. Geologically, the Delta has existed for about 10,000 years, since

11449-465: Was probably exacerbated by the marshy geography of the area, which bred large amounts of mosquitoes. The agricultural value of the Delta was first recognized during the California Gold Rush , when farmers planted orchards on Delta islands to provide fresh fruit for mining camps in the Sierra Nevada . Because of the flat terrain coupled with year-round availability of fresh water, irrigation here

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