The Trinity River rises in three principal branches: the East Fork, the Elm Fork, and the West Fork. A fourth headstream, shorter and smaller, is known as the Clear Fork. The East Fork of the Trinity River rises in central Grayson County and flows south seventy-eight miles, through central Collin, western Rockwall, eastern Dallas, and western Kaufman counties, to the southwestern part of Kaufman County, where it joins the West Fork. The Elm Fork of the Trinity rises in eastern Montague County and flows southeast eighty-five miles, through Cooke and Denton counties, to a confluence with the West Fork, which forms the Trinity River proper a mile west of downtown Dallas in central Dallas County (at 32°48' N, 96°52' W). The West Fork of the Trinity rises in southern Archer County and flows southeast 180 miles through Jack, Wise, Tarrant, and Dallas counties and along the county line between Ellis and Kaufman counties, to its junction with the East Fork of the Trinity. The Clear Fork of the Trinity rises in northwestern Parker County and flows first southeast and then northeast forty-five miles to join the West Fork of the Trinity at Fort Worth in central Tarrant County. From the junction of the East and West Forks the Trinity River continues southeast, forming all or part of the county lines between Kaufman and Ellis, Ellis and Henderson, Henderson and Navarro, Freestone and Anderson, Anderson and Leon, Leon and Houston, and Houston and Madison counties. It then cuts across northern Walker County to form a portion of the county line between Walker and Trinity counties and continues as the county line between Trinity and San Jacinto and San Jacinto and Polk counties. At the northern line of Liberty County the Trinity turns almost directly south, cutting across Liberty and Chambers counties, to drain into Trinity Bay just west of Anahuac (at 29°45' N, 94°42' W).
The Trinity flows 423 miles from the confluence of the Elm and West forks to the coast, making it the longest river having its entire course in Texas. The river rises on the North Central Plains, but most of its course is in the Coastal Plains area. The total drainage basin area is 17,969 square miles and includes all or part of thirty-seven counties. The population of the Trinity River Basin in 1980 was 3.2 million. Of these, 75 percent live in Dallas and Tarrant counties. The largest cities in the basin include Dallas, with a 1980 population of 904,100, and Fort Worth, with a population of 385,100. Other cities in the basin with a population of 50,000 or more are Arlington, Garland, Irving, Richardson, Plano, Grand Prairie, and Mesquite. The upper Trinity Basin has rolling topography and narrow stream channels. Soils in the region are deep to shallow clay, clay loam, and sandy loam that support elms, sycamores, willows, oaks, junipers, mesquites, and grasses. The middle and lower Trinity Basin is gently rolling to flat terrain with wide, shallow stream channels. Clay and sandy loams predominate and support water-tolerant hardwoods, conifers, and grasses. In addition to several dams on the river's tributaries, the Trinity is dammed just above Camilla in San Jacinto County to form Livingston Reservoir.
Annual rainfall in the watershed varies from thirty to forty inches in the upper basin to forty to fifty inches in the lower. Rapid surface runoff during intense thunderstorm activity frequently produces flash floods on the smaller tributaries and upper reaches of the river. Slow-moving floods, sometimes of long duration, are common in the middle and lower basin area. The extreme lower reaches of the river are also subject to hurricane-induced surge tides and strong winds (see HURRICANES). The annual flow of the stream averages five million acre-feet but is highly irregular because the rainfall is often concentrated-so much that it has caused several destructive floods. The most disastrous flood on record was that of 1908. Reservoirs on the upper branches control the floods to a certain extent and provide municipal water supplies.
The Trinity has been identified as the stream that the Caddo Indians called Arkikosa in Central Texas and Daycoa nearer the coast, as well as the one that Réne Robert Cavelier, Sieur de La Salle, in 1687 called River of the Canoes. The name Trinity (La Santisima Trinidad) is supposed to have first been applied to the present stream by Alonso De León in 1690. Domingo Terán de los Rios in 1691 called the same stream Encarnación de Verbo. Domingo Ramón in 1716 probably applied the name Trinity to the present Brazos, for, when he later reached the Trinity, he was told by the Indians that other Spaniards called the stream the Trinity. The Marqués de Aguayo and other later explorers used the name Trinity consistently.
During the colonial period of Texas history, the land along the lower course of the Trinity was settled as far up as Anderson County. The Anahuac disturbances were among the most historically significant events of the era. Settlement up the Trinity valley continued to advance rapidly in the period of the republic. Beginning about 1836 numerous packet boats steamed up the Trinity River, bringing groceries and dry goods and carrying down cotton, sugar, cowhides, and deer skins. One of the largest of these early steamers was the Scioto Belle, put in service in 1844. Some of the packets penetrated as far as Magnolia, ten miles west of Palestine, and in 1854 one reached Porter's Bluff, fifty miles below Dallas. Often their movements were impeded by snags or sand bars or halted by low water. Following a convention on Trinity improvement in 1849 at Huntsville, Congress in 1852 authorized a survey of the river. In the next year an army engineer's report mentioned the Trinity as the deepest and least obstructed river in Texas, said that seven steamboats were in operation in its lower channel, and estimated that navigation was practicable. Under a Texas act of 1858 a bar was removed from the river's mouth. Navigation fell off during the Civil War, but in 1868 Job Boat No. 1 reached Dallas with a cargo, after a voyage of a year and four days from Galveston. In the years before 1874 nearly fifty boats continuously navigated the river as far north as Trinidad in Kaufman County and Porter's Bluff in northern Navarro County. In the peak season of 1868–69 boats carried 15,425 bales of cotton down the Trinity. With the construction of railroads to Dallas in the early 1870s the river traffic began to die. But high railroad rates and the prospect of Dallas as a major port kept the dream of a navigable Trinity River alive. Since that time numerous schemes to make the Trinity navigable have been proposed. Several proposals received considerable attention, and some construction was undertaken, but the dream of a port of Dallas has never been realized (see TRINITY RIVER NAVIGATION PROJECTS).
Over the past century the waters of the Trinity have become increasingly polluted. Runoff containing pesticides and herbicides and dumping of industrial and human waste-particularly in the Dallas-Fort Worth metroplex-have combined to cause serious deterioration of water quality. The most severely affected area is the 250-mile-long stretch that extends from Dallas-Fort Worth to the headwaters of Lake Livingston. By the early 1960s the river below Dallas for 100 miles was so polluted that the United States Public Health Service described it as "septic." Since that time efforts have been made to clean up the river. A water quality management plan was adopted in the 1970s, however pollution problems have continued. In its master plan published in 2010, the Trinity River Authority of Texas characterized what it referred to as “Legacy Pollutants”—agents that have been “banned for decades, yet are still found in the environment in concentrations deemed to be detrimental for humans. The sources of these are typically unknown or contaminated sediment that, were it to be removed, could cause greater harm.” Water samples continued to show high levels of bacteria in highly urbanized areas of the Trinity River basin.