These floods soon spread through the provinces of northern, northeastern, and central Thailand along the Mekong and Chao Phraya river basins.

Sixty-five of Thailand's 76 provinces were declared flood disaster zones, and over 20,000 square kilometres (7,700 sq mi) of farmland was damaged.

[1] The disaster was described by an unnamed Thai government official as "the worst flooding yet in terms of the amount of water and [number of] people affected".

[3] The World Bank has estimated 1,425 trillion baht (US$46.5 billion) in economic damages and losses due to flooding, as of 1 December 2011.

[6] Disruptions to manufacturing supply chains affected regional automobile production and caused a global shortage of hard disk drives which lasted throughout 2012.

Remnants of tropical storms that strike Vietnam or the peninsular south commonly increase precipitation, resulting in further risk of flooding.

Much effort, including a system of drainage tunnels begun in 2001,[10] has been put into preventing the inundation of the capital city, which lies near the mouth of the Chao Phraya and is prone to flooding, with considerable success.

To combat rising sea levels a barrier dam stretching from Chonburi to Hua Hin has been proposed, at a cost of 500 billion baht.

[14][a] Within one week thirteen persons had been reported dead, with ongoing flooding in the provinces of Chiang Mai, Lampang, Lamphun, Mae Hong Son, Nan, Phrae, and Uttaradit in the north, and Bung Kan, Nakhon Phanom, Nong Khai, Sakon Nakhon, and Udon Thani in the upper northeast.

[15] Flooding was still ongoing by late-August, as heavy rains were expected to continue for longer than usual due to the effect of a multi-year La Niña event.

[21] By the beginning of October, most dams were already near or over capacity and were being forced to increase their rate of discharge, potentially worsening downstream flooding.

Barriers protecting industrial estates failed, resulting in the flooding of dozens of major factories and a country-wide disruption of the manufacturing supply chains.

In Khon Kaen Province alone, floods destroyed close to 350,000 350,000 rai (56,000 ha; 220 sq mi) of land, stranding 315 families of Mai Si Wilai village in the middle of the swollen Nong Kong Kaew Lake in Chonnabot District, while in Phra Lap municipality on the outskirts of Khon Kaen city, over 700 displaced residents of Phra Kheu village called the shoulder of a provincial highway home.

Saba Yoi, Khuan Niang, Rattaphum, and Singha Nakhon Districts were declared disaster zones after flood waters as deep as five meters covered nearly all areas.

[28] Parts of the Phahonyothin Highway leading out of Bangkok became inaccessible, causing severe traffic jams on alternative routes.

[30] The entire Rangsit campus of Thammasat University in Pathum Thani, north of Bangkok, serving as the largest evacuation centre, was flooded with two meters of water.

[31] As the situation continued, several floodgates controlling the Rangsit/Raphiphat canal system became overwhelmed and were forced to release water towards more residential areas.

The Rangsit gymnasium served as the main means of exit from the town for those who did not have ability to leave on boat or four-wheel drive.

The Chao Phraya River itself, and pumping stations around Bangkok drain approximately 420,000,000 square metres (4.5×109 sq ft) per day.

However, the releases from the dams upstream of Bangkok coupled with additional rainfall, led to estimates that 16,000,000,000 cubic metres (5.7×1011 cu ft) of flood waters must be drained.

Thailand's Royal Irrigation Department predicted, provided there is no new rain, that it would take 30–45 days for this quantity of water to reach the sea.

[47] The 24/7 Emergency Operation Center for Flood, Storm and Landslide was set up on 20 August under the Disaster Prevention and Mitigation Department of the Ministry of Interior to coordinate warning and relief efforts.

[citation needed] On 16 October 2011, the USS George Washington (CVN-73) aircraft carrier, as well as several other United States Navy ships were deployed to Thailand to assist in relief work.

[58] Smith Dharmasarojana, former Director-General of the Meteorological Department and Chairman of the National Disaster Warning Council Foundation, also criticized the operation, claiming that "accelerating boats in the middle of the wide Chao Phraya River is a waste because they propel only the water on the surface.

The persons concerned must think twice [about them]..."[60] Sukhumbhand Paribatra, a member of the opposition party and Democrat Governor of Bangkok, apparently seized on the flood as an opportunity to grandstand and made comments such as "please believe me and only me", and charged that 800,000 sandbags provided by the federal government were of questionable construction.

In response, the government released photos of the bags, construction details, and suggested that the governor was perhaps ill-advised by a third party in an attempt to allow him to save some face.

[citation needed] Once the flood arrived at Pathum Thani Province, the media presented flood-related news more frequently and made car owners in Bangkok and nearby precincts panic.

On 8 October 2011 the 10-metre high water barrier in Nikom Rojna Industrial Estate, which housed many manufacturing plants, collapsed.

[85] The TAT wanted tourists to know tourist destinations such as Chiang Rai, Chiang Mai, Mae Hong Son, Pai, Kanchanaburi, Ratchaburi, Pattaya, Sattahip, Chonburi, Chantaburi, Phetchaburi, Hua Hin, Cha-am, Phuket, Krabi, Phang Nga, Surat Thani, Trang, Satun, Hat Yai, Rayong, Trat, Chumphon, Ranong, Ko Pha Ngan, and Ko Samui had not been affected by flooding at all.

[86] An added concern became even more pressing when it became clear on 7 November 2011 that Rama II Road, the last open link to the south, was expected to flood.