The plaque is missing; it read
This river town was established in 1849. Pioneers and freight wagons crossed the river nearby at Durham's Ferry following post roads to the southern mines. As a terminal for riverboats, the town played an important part in development of west side grain farming and cattle raising.
Recordnet.com: Plaque missing from city's monument
Between 1836, when the Spanish secularized the missions, and 1846, when the Americans took control of the state, the Mexican Government issued some 30 land grants in California, specifically for agricultural purposes, primarily the raising of cattle.
Three grants were issued along the San Joaquin River. Velentin Higuerra and Rafael Feliz were granted 35,000 acres for the El Pescadero Ranch, which subsequently was the site of San Joaquin City. This grant extended from about Banta to some distance below Grayson.
San Joaquin City was established in 1849, the year before nearby Durham's Ferry began shuttling pioneers, gold miners, farmers and their wagons across the river.
Riverboats filled with grain left the city for Stockton. The city's popularity gave rise to talk that it would rival Stockton as the main conduit between San Francisco and the Mother Lode, and the city reached a population of 1,400 in its heyday, according to estimates. The town slowly subsided into obscurity in the early 1900s as trains edged out riverboats. It was replaced by Vernalis with the coming of the railroad to the Valley’s West Side.