Thailand possesses 3,219 kilometers of coastline divided between two bodies of water: the Gulf of Thailand to the east and south, and the Andaman Sea to the west. The Gulf of Thailand coastline extends approximately 1,840 kilometers from the Cambodian border near Trat Province south to the Malaysian border at Narathiwat Province. The Andaman Sea coastline runs approximately 740 kilometers along six provinces: Ranong, Phang Nga, Phuket, Krabi, Trang, and Satun. These two coasts differ fundamentally in their oceanography, with the Gulf of Thailand being a semi-enclosed tropical sea with average depths of 45 meters and maximum depths of 80 meters, while the Andaman Sea represents open ocean with depths exceeding 4,000 meters in the offshore trenches. The shallow continental shelf of the Gulf extends 160 kilometers offshore in some areas, creating conditions for extensive coral reef systems and seagrass beds. The Andaman coast features steeper underwater topography with dramatic limestone karst formations both above and below the waterline.
The Chao Phraya River extends 372 kilometers from the confluence of the Ping and Nan rivers at Nakhon Sawan Province to the Gulf of Thailand at Samut Prakan Province south of Bangkok. The river drains a basin of 160,000 square kilometers, approximately one-third of Thailand's land area. The Chao Phraya delta begins roughly 180 kilometers from the river mouth and encompasses approximately 13,000 square kilometers of the Central Plains. River discharge averages 21 cubic kilometers annually, with peak flows during September and October reaching 3,500 cubic meters per second at Bangkok. The river carries an estimated 13 million tons of sediment annually to the Gulf, building the delta seaward at rates measured between 1.5 and 2.5 meters per year historically, though modern damming has reduced this rate. The Chao Phraya serves as the primary transportation corridor for Bangkok, with approximately 50,000 people using the 72-kilometer Chao Phraya Express Boat service daily on 31 piers between Nonthaburi and Phra Pradaeng. Cargo barges transport an estimated 9 million tons of goods annually on the lower river, primarily rice, construction materials, and petroleum products.
Bangkok sits at 1.5 meters above mean sea level where the Chao Phraya River splits into multiple distributaries before reaching the Gulf of Thailand 25 kilometers downstream. The city was founded in 1782 by King Rama I specifically because of its riverine position, which provided both defense and trade advantages over the previous capital at Thonburi on the opposite bank. The Grand Palace occupies 218,400 square meters on the eastern bank of the Chao Phraya in the historic Rattanakosin Island district. Water transport shaped Bangkok's development, with canals called khlongs forming the primary transportation network until road construction accelerated in the 1960s. The original city layout followed Ayutthaya's design with canals providing both transport and defense. Today, approximately 1,682 canals remain in Bangkok, down from an estimated 2,600 in 1950. Khlong Saen Saep, running 18 kilometers east-west through central Bangkok, carries approximately 60,000 passengers daily on its boat service between Wat Si Bun Rueang and Wat Sriboonreung. The riverfront has become increasingly developed for commerce and tourism, with Asiatique The Riverfront occupying a former warehouse complex on 11 hectares along the eastern bank and reporting over 20 million visitors since opening in 2012.
The Mekong River forms 976 kilometers of Thailand's border, primarily with Laos along the northeastern provinces. The river enters Thailand at the tripoint with Myanmar and Laos in Chiang Rai Province and follows the border eastward to Loei, Nong Khai, Bueng Kan, Nakhon Phanom, Mukdahan, Amnat Charoen, and Ubon Ratchathani provinces before exiting into Cambodia. The Mekong at Nong Khai measures approximately 800 meters wide during the dry season from November to May and expands to 1,200 meters during the wet season from June to October. Average discharge at Chiang Saen in the far north measures 2,410 cubic meters per second annually, increasing to 5,740 cubic meters per second at Mukdahan downstream due to tributary inflows. The river drops approximately 100 meters in elevation across its Thai border section, creating numerous rapids. The First Thai-Lao Friendship Bridge opened in 1994 connecting Nong Khai Province to Vientiane, spanning 1,170 meters across the river at a cost of 638 million baht. Three additional Friendship Bridges have since opened: the second at Mukdahan in 2006, the third at Nakhon Phanom in 2011, and the fourth at Chiang Khong in 2013. The Mekong supports significant fisheries with estimates of 50,000 tons of fish harvested annually from the Thai border section, though catches have declined approximately 30 percent since 2000 according to surveys by the Mekong River Commission.
Phuket Island covers 543 square kilometers, making it Thailand's largest island. The island lies in the Andaman Sea connected to the mainland by the 660-meter Sarasin Bridge and the parallel 730-meter Thao Thepkrasattri Bridge. Phuket's coastline extends 140 kilometers with 30 named beaches. Patong Beach on the west coast runs 3.5 kilometers and represents the island's primary tourist center with an estimated 2.5 million visitors annually. Phuket Town serves as the provincial capital with a population of 79,308 as of 2019. The island's interior rises to 529 meters at Mai Thao Sip Song. Tin mining drove Phuket's economy from the 16th century through the 1980s when the industry collapsed due to falling tin prices and deposit depletion. Tourism grew rapidly beginning in the 1980s, with Phuket International Airport handling 10.9 million passengers in 2019. The December 26, 2004 Indian Ocean tsunami struck Phuket's west coast with waves reaching 5 to 10 meters, causing 259 confirmed deaths on the island and extensive damage to Patong, Kamala, and Khao Lak areas. Tsunami warning systems were subsequently installed, with 79 warning towers operating across Phuket as of 2020. The island receives an average of 2,200 millimeters of rainfall annually, concentrated between May and October during the southwest monsoon season.
The Phi Phi Islands consist of six islands in Krabi Province 40 kilometers from the mainland. Phi Phi Don, the largest at 9.73 square kilometers, supports a permanent population of 2,500 residents concentrated in the Tonsai Bay area. Phi Phi Leh, the second largest at 2 square kilometers, remains uninhabited. Maya Bay on Phi Phi Leh's west coast gained international attention following the 2000 film "The Beach" and subsequently received up to 5,000 visitors daily during peak season. Thai authorities closed Maya Bay in June 2018 for environmental recovery after surveys documented severe coral damage and a 90 percent decline in reef fish populations compared to 2000 levels. The bay reopened in January 2022 with a daily visitor limit of 300 people and a prohibition on boats entering the bay, requiring tourists to moor at a floating platform offshore. The islands consist of limestone reaching 190 meters elevation on Phi Phi Don and feature dramatic vertical cliffs dropping directly into water depths exceeding 20 meters. Phi Phi Don has no roads, with transportation via longtail boat or walking paths. The islands were severely impacted by the 2004 tsunami, with Tonsai Bay's twin bays connected by a low-lying isthmus particularly affected, resulting in 1,500 to 2,000 deaths including both residents and tourists. The area was rebuilt with improved building codes requiring structures to be elevated and strengthened.
Phang Nga Bay extends 400 square kilometers between the mainland Phang Nga and Krabi provinces and Phuket Island in the Andaman Sea. The bay contains approximately 100 limestone karst islands rising vertically from water depths of 6 to 27 meters. Khao Phing Kan, known internationally as James Bond Island after appearing in the 1974 film "The Man with the Golden Gun," consists of two leaning rocks and lies within Ao Phang Nga National Park, established in 1981 and covering 400 square kilometers including 42 islands. The limestone formations began forming approximately 260 million years ago during the Permian period and were subsequently uplifted and eroded by tropical weathering and sea level changes. The bay's unique morphology creates a shallow estuarine environment with significant mangrove forests covering approximately 100 square kilometers within the bay system. The mangroves include Rhizophora species along the water edge and Avicennia and Sonneratia in less saline areas. Traditional Muslim fishing villages built on stilts over the water exist at Koh Panyee, where approximately 1,500 residents live in structures elevated 2 to 3 meters above the water surface. The bay experiences a tidal range of 1.5 to 3 meters, creating strong currents through the narrow passages between islands.
Koh Samui in Surat Thani Province covers 228.7 square kilometers, making it Thailand's second-largest island after Phuket. The island lies 35 kilometers from the mainland in the Gulf of Thailand. Samui's population was recorded at 62,500 in 2018, concentrated primarily along the coastal areas. Coconut farming dominated the island economy until tourism expansion began in the 1980s. The island produces approximately 2 million coconuts monthly, still shipped to Bangkok and other mainland destinations. Samui Airport, opened in 1989 and privately owned by Bangkok Airways, handled 2.3 million passengers in 2019. The airport features an open-air design with thatched roofs and requires all aircraft movements to cease during heavy rain due to the open terminal structure. Chaweng Beach on the east coast extends 7 kilometers and contains the highest concentration of tourist development. The island receives approximately 1,900 millimeters of rainfall annually, but with a different monsoon pattern than the Andaman coast, experiencing highest rainfall from October to December when the northeast monsoon brings moisture from the Gulf of Thailand. This inverted pattern makes Samui a dry-season destination when the Andaman coast experiences heavy rain. The island's interior rises to 635 meters at Khao Pom, with the upland area still predominantly forested with coconut plantations on slopes.
Koh Phangan lies 15 kilometers north of Koh Samui and covers 125 square kilometers. The island's permanent population numbers approximately 12,000 residents. Koh Phangan gained international recognition for its Full Moon Party events on Haad Rin beach, which began informally in 1988 and now attracts an estimated 10,000 to 30,000 attendees monthly during peak tourist season. The parties occur on the night of the full moon each month, with authorities designating Haad Rin Nok beach as the official location. The island features more rugged topography than Koh Samui, with 65 percent of the island covered in mountainous forest. Khao Ra at 627 meters represents the highest point. The island has approximately 30 beaches, many accessible only by boat or hiking trail due to the interior mountains extending close to coastlines. Than Sadet Waterfall National Park covers 65.93 square kilometers of the northeast portion of the island, established in 1983. The Than Sadet waterfall system drops 120 meters in multiple cascades before reaching the sea. King Rama V visited the waterfall 14 times between 1888 and 1909, with his initials carved into rocks at the site. Ferry services connect Koh Phangan to Surat Thani mainland port at three primary piers, with crossing times ranging from 2.5 to 6 hours depending on vessel type and departure point.
The Andaman coast experiences a southwest monsoon season from May through October bringing an average of 2,400 millimeters of rainfall to coastal areas, with Ranong Province in the north recording up to 3,500 millimeters annually. The northeast monsoon from November through February brings dry conditions with minimal rainfall and calmer seas. Sea surface temperatures in the Andaman Sea range from 28°C to 31°C year-round. The Gulf of Thailand has a more complex pattern with the southwest monsoon bringing rainfall from May to October to the northern and central Gulf coast, while the northeast monsoon from October to January affects the southern Gulf coast from Surat Thani to the Malaysian border. This creates the distinct pattern where eastern Gulf islands like Koh Samui experience dry conditions from January to August while western Andaman coast destinations remain dry from November to April. Wave heights in the Andaman Sea during monsoon season regularly exceed 2.5 meters, with 3 to 4-meter waves common during peak monsoon months of August and September. The Gulf of Thailand remains generally calmer with wave heights typically below 2 meters even during monsoon periods due to the semi-enclosed nature of the gulf.
Railay Beach in Krabi Province occupies a limestone peninsula of approximately 1.5 square kilometers accessible only by boat due to high limestone cliffs blocking land access. The peninsula features four primary beaches: Railay West, Railay East, Phra Nang, and Tonsai. The limestone cliffs reach heights of 200 meters and contain over 700 established rock climbing routes graded from beginner level 5.5 to expert level 5.14c on the Yosemite Decimal System. Climbing development began in the late 1980s, with Railay now ranking among Southeast Asia's primary climbing destinations, attracting an estimated 50,000 climbers annually. The cliffs consist of Permian-age Ratburi limestone, approximately 250 million years old, characterized by pockets, stalactites, and overhanging formations. Phra Nang Cave at the southern end of Phra Nang Beach contains a shrine dedicated to a sea princess, with local fishermen leaving wooden phallus offerings for good fortune and fertility. The cave extends approximately 40 meters into the cliff. Longtail boats provide the primary access from Ao Nang Beach 15 minutes away or Krabi Town 40 minutes away. No roads or vehicles exist on the peninsula, with all transport via wheelbarrow or porter. The area experiences significant tidal variation of 2 to 3 meters, exposing extensive reef flats during low tide.
The Similan Islands consist of nine granite islands 70 kilometers northwest of Phuket in the Andaman Sea. The islands form part of Mu Ko Similan National Park, established in 1982 and expanded to include two additional northern islands in 1998, bringing the protected area to 140 square kilometers. The islands run on a north-south axis over 25 kilometers. Koh Similan, the ninth and largest island, covers 2.9 square kilometers. The granite boulder formations contrast sharply with the limestone karst topography dominant elsewhere on the Andaman coast. The islands formed approximately 280 million years ago from the same granite batholith that surfaces in Phuket and Khao Lak. The waters surrounding the islands contain 28 dive sites to depths of 40 meters with visibility typically ranging from 15 to 40 meters. The underwater environment includes large granite boulders, swim-throughs, and coral coverage estimated at 40 to 60 percent on shallow reefs. Notable dive sites include Elephant Head Rock off Island 8, featuring three granite pinnacles rising from 30 meters depth with channels and caves. The islands open to visitors from October 15 to May 15 annually, closing during monsoon season. Park regulations implemented in 2018 limit daily visitors to 3,325 people across all nine islands combined, with overnight stays restricted to specific camping areas. The islands have no permanent population or freshwater sources.