Historically, about 4,000 kilometers of inland waterways consisting of the rivers and canals of the central plain and the Chao Phraya Delta formed the backbone of the transportation system. Although in the twentieth century railroads and roads assumed a dominant position in the central plain, waterways still carried a sizable portion of the total traffic. Waterborne freight, chiefly consisting of rice, accounted for about 17 percent of total freight transported countrywide in the 1980s. Large numbers of small craft also transported passengers. During the rainy season about 1,600 kilometers of waterways were navigable by barges of up to 80 tons and 1.8-meter draft, which could travel from the Gulf of Thailand to as far north as Uttaradit. Navigation was reduced to about 1,100 kilometers of waterways in the dry season, and traffic could navigate only to Nakhon Sawan, roughly halfway to Uttaradit. Shallow-draft vessels could navigate the interconnected network of canals throughout the year, and Bangkok, Ayutthaya, and other towns had floating markets where a great deal of trading activity took place. Some sections of the Mekong River were also navigable.