Chao Phraya River in the context of "History of Thailand"

Bangkok, officially known in Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand. The city occupies 1,568.7 square kilometres (605.7 sq mi) in the Chao Phraya River delta in central Thailand and has an estimated population of 11.4 million people as of 2024, 15.9% of the country's population. Over 17.4 million people (25% of Thailand's population) live within the surrounding Bangkok Metropolitan Region as of the 2021 estimate, making Bangkok a megacity and an extreme primate city, dwarfing Thailand's other urban centres in both size and importance to the national economy.

Bangkok traces its roots to a small trading post during the Ayutthaya era in the 15th century, which eventually grew and became the site of two capital cities, Thonburi in 1767 and Rattanakosin in 1782. Bangkok was at the heart of the modernization of Siam during the late 19th century, as the country faced pressures from the West. The city was at the centre of the country's political struggles throughout the 20th century, as Siam—later renamed Thailand—abolished absolute monarchy, adopted constitutional rule, and underwent numerous coups and several uprisings. The city, incorporated as a special administrative area under the Bangkok Metropolitan Administration in 1972, grew rapidly during the 1960s through the 1980s and now exerts a significant impact on Thailand's politics, economy, education, media, and modern society.

Mass migration of Tai peoples from China (Guangxi) to Mainland Southeast Asia and Northern Thailand occurred between the 8th-10th century. The Mainland region was ruled by the Khmer Empire since 800 CE. The Thai established their own kingdoms: the semi-legendary Singhanavati Kingdom (691 BCE–638 CE) evolved into the Ngoenyang Kingdom (638–1292). In 1220, the Khmer controlled Sukhothai was conquered by the Thais and made the capital of the Sukhothai Kingdom. By 1220, the long declining Khmer Empire was mostly overrun by Thais. By the 13th century, the Sukhothai Kingdom (1238–1438) had replaced the Mon kingdoms in Central Thailand. During the reign of King Ramkhamhaeng the Thai script was created in 1283, the arts flourished, Thai institutions were developed, and people called themselves "Thai" as freed people from foreign rule.

In 1351, the Suphannabhum city-state (modern-day Suphan Buri), and Lavo city-state (modern-day Lop Buri) merged into the Ayutthaya Kingdom which was founded near the Chao Phraya River. Duarte Fernandes was the first European to arrive in Ayutthaya in 1511. Siam became the popular international name for Thailand.

Phra Nakhon Si Ayutthaya (Thai: พระนครศรีอยุธยา, pronounced [pʰráʔ ná(ʔ).kʰɔ̄ːn sǐː ʔā.jút.tʰā.jāː]), or locally and simply Ayutthaya, is the capital of the Phra Nakhon Si Ayutthaya province of Thailand. Ayutthaya was once the capital of the Ayutthaya Kingdom. Located on an island at the confluence of the Chao Phraya and Pa Sak rivers, Ayutthaya is the birthplace of the founder of Bangkok, King Rama I. The ruins of the old city are preserved in the Ayutthaya Historical Park.

Central Thailand (Central Plain) (historically also known as Siam or Dvaravati) is one of the regions of Thailand, covering the broad alluvial plain of the Chao Phraya River. It is separated from northeast Thailand (Isan) by the Phetchabun mountain range. The Tenasserim Hills separate it from Myanmar to the west. In the north it is bounded by the Phi Pan Nam Range, one of the hilly systems of northern Thailand. The area was the heartland of the Ayutthaya Kingdom (at times referred to as Siam) and is still the dominant area of Thailand since it contains the world's most primate city, Bangkok.

Thonburi (Thai: ธนบุรี) is an area of modern Bangkok. During the era of the Ayutthaya Kingdom, its location on the right (west) bank at the mouth of the Chao Phraya River had made it an important garrison town, which is reflected in its name: thon (ธน) a loanword from Pali dhána 'wealth', and buri (บุรี), from púra, 'fortress'. The full formal name was Thon Buri Si Mahasamut (กรุงธนบุรีศรีมหาสมุทร 'City of Treasures Gracing the Ocean'). For the informal name, see the history of Bangkok under Ayutthaya.

In 1767, after the sack of Ayutthaya by the Burmese armies, General Taksin took back Thonburi and, by right of conquest, made it the capital of the Thonburi Kingdom, crowning himself king until 6 April 1782, when he was deposed. Rama I, the newly enthroned king, moved the capital across the river, where stakes driven into the soil of Bangkok for the City Pillar at 06:45 on 21 April 1782 marked the official founding of the new capital. Thonburi remained an independent town and province, until it was merged with Bangkok in 1971. Thonburi stayed less developed than the other side of the river. Many of the traditional small waterways, khlongs, still exist there, while they are nearly gone from the other side of the river.

The Lavo Kingdom (Thai: อาณาจักรละโว้) was a political entity (mandala) on the left bank of the Chao Phraya River in the Upper Chao Phraya valley from the end of Dvaravati civilization, in the 7th century, until 1388. The original center of Lavo was Lavapura and was shifted to Ayodhya (Xiān) in the 1080s. However, since both Ayodhya or Xiān and Lavo separately sent embassies to the Chinese court in the late 1200s, these two polities were potentially individual states.

Before the 9th century, Lavo, together with other supra-regional settlements, such as Si Thep, Sema (th), Phimai, Nakhon Pathom, and others were the centers of the mandala-style polities of Dvaravati. Due to several circumstances, including climate changes and the invasions of the surrounding polities, several Dvaravati centers lost their prosperity, and the mandalas in the Menam Valley was then split into three groups: Lavo (modern Lopburi) to the east, which was more often in touch with the Angkorean and pre-Angkorean worlds, Suphannaphum (modern Suphanburi) to the west, which had more contact with the Mon and Malay worlds and the northern polities, which had more complexity in culture, ethnic, and linguistic than the aforementioned two polities. Meanwhile, the Mun–Chi mandalas allied with Kambudesha in the Tonlé Sap basin.

Wat Arun Ratchawararam Ratchawaramahawihan (Thai: วัดอรุณราชวราราม ราชวรมหาวิหาร pronunciation ) or Wat Arun (Thai pronunciation: [wát ʔarun], "Temple of Dawn") is a Buddhist temple (wat) in the Bangkok Yai district of Bangkok, Thailand. It is situated on Thonburi on the west bank of the Chao Phraya River. The temple derives its name from the Hindu god Aruṇa, often personified as the radiations of the rising sun. Built with a fusion of Indian, Chinese, and Khmer influences, incorporating elements of both Hindu and Buddhist symbolism, as well as reflecting Siamese tradition and identity. Wat Arun is among the best known of Thailand's landmarks. Although the temple has existed since at least the 17th century, its distinctive prang (spire) was built in the early 19th century during the reigns of Rama II and Rama III.

The Pa Sak River (Thai: แม่น้ำป่าสัก, RTGS: Maenam Pa Sak, IPA: [mɛ̂ːnáːm pàː sàk], pronunciation) is a river in central Thailand. The river originates in the Phetchabun Mountains, Dan Sai District, Loei Province, and passes through Phetchabun Province as the backbone of the province. It then passes through the eastern part of Lopburi Province and Saraburi Province, until it joins the Lopburi River northeast of Ayutthaya Island, before it runs into the Chao Phraya River southeast of Ayutthaya near Phet Fortress. It has a length of 513 km (319 mi) and drains a watershed of 16,291 km (6,290 sq mi). The annual discharge is 2.4 km (0.58 cu mi).

The valley of the Pa Sak through the Phetchabun Mountains is a dominant feature of Phetchabun Province. Water levels vary seasonally. To address drought problems in the lower Pa Sak valley, the Pa Sak Cholasit Dam (เขื่อนป่าสักชลสิทธิ์) in Lopburi Province was built in the 1990s. The 4,860-meter (15,940 ft) wide and 36.5-meter (120 ft) high dam retains 0.785 km (0.188 cu mi) of water. The dam also supplies about 6.7 MW of electricity.