When Mexico became independent in 1821 a chain of six towns in the El Paso area situated from three to five miles apart stretched along the southern bank of the Rio Grande. In the early 1830s the capricious river formed a new channel south of the old one, thus placing three of them-Ysleta, Socorro, and San Elizario-on an island some twenty miles in length and two to four miles in width. For the remainder of the Mexican period this area was called La Isla, "the Island." The Rio Grande continued to flow primarily through its new channel, and by 1848, when the river became the boundary between the United States and Mexico, water had ceased to flow in the Río Viejo, or old riverbed.
A flourishing agriculture existed on the Island that featured a fertile soil mixed with a sandy loam and irrigated by a network of acequias, or irrigation canals. The principal products were corn, wheat, fruits, and vegetables. The quality and flavor of the grapes, wine, and brandy produced by the vineyards ranked with the best to be found in the viceroyalty of Spain, according to almost every official who visited the area. Most of the land-haciendas, ranches, and farms-with the exception of the ejidos, the communal holdings of the mission Indians, was owned by the wealthy Paseños of El Paso del Norte (Ciudad Juárez) across the river, the largest town and political capital of the area. Supplementing the Island's agricultural base was the Chihuahua trade along the historic Old San Antonio Road, a natural extension of the Santa Fe trade with Missouri that began a decade earlier.
Ysleta and Socorro were established as missions by refugees of the Pueblo Revolt in New Mexico in 1680-Ysleta for the Tiguas and Socorro for the Piros. Both missions were swept away by the flooding river in the early 1830s, but they were at length replaced by the present structures-Socorro in 1843 and Ysleta in 1851. San Elizario, a presidio that was moved upriver to the El Paso area in 1879, became the nucleus of a town. According to a census of 1841 the total population of the three Island settlements was 2,850. Socorro was the largest with 1,101, San Elizario was second with 1,018, and Ysleta had 731 residents. Each town was governed by an alcalde appointed by the ayuntamiento of El Paso del Norte, which was controlled by the landowning and mercantile aristocracy. Most heads of families on La Isla were farm workers or servants. Indians were dependents, and poverty was widespread. The two-room adobe structure was the general pattern for all.
During the Mexican War Col. Alexander Doniphan's force brought the El Paso area under United States control. The Treaty of Guadalupe Hidalgo of February 2, 1848, which officially ended the war, provided that the new international boundary was to be "the Rio Grande . . . following the deepest channel . . . to the point where it strikes the southern boundary of New Mexico." American officials declared the Island to be United States territory, and in November 1848 Col. John M. Washington, military governor of New Mexico, appointed T. Frank White president and directed him to extend his jurisdiction over all of the territory east of the river that had formerly been a part of Chihuahua. In February 1849 Mexican officials reported that an armed force of the United States had occupied Ysleta, Socorro, and San Elizario and had taken possession of all the lands, including ejidos. Mexican protests proved futile.
Meanwhile, by mid-1849 the discovery of gold in California had brought into the El Paso area hordes of immigrants, many of whom decided to remain. By late 1849 five settlements had been established by Anglo-Americans-Frontera, Hart's Mill, Coon's Ranch, Magoffinsville, and Stephenson's Ranch. These five, together with the three Mexican settlements on the Island, indicated that a bilingual, bicultural complex was taking shape at the Pass of the North. That the Mexican town of San Elizario should become El Paso County's first county seat is significant.