The Gold Rush and the annexation of California by the United States brought drastic changes to San Pablo. A stage route was established from San Francisco and Oakland to Sacramento, and ran right along the road we call San Pablo Avenue today. A small stage stop village began to appear around the Castro/Alvarado Adobe home at the corner of San Pablo Avenue and Church Lane. Saloons lined the main street, which was wide enough (60~100 feet) to accommodate the movement of cattle herds. The Castro descendants began to sell off portions of their Rancho to American ranchers and farmers. A Post Office was established, a school was built, and churches founded.

Many of the new ranchers were new immigrants to the United States, and most of them were Azore Island Portuguese. There were also immigrants from Chile, Germany, France, and China. A large camp of Chinese shrimp fishers lived along the edge of San Pablo Bay from 1870 to 1912. (San Pablo went all the way to the bay in those days.) A few African-Americans lived in the area as well. We can assume that a number of Mexicans, such as the Castros and Alvarados, continued to live here also.

In 1872, the people of Rancho San Pablo collected money to buy the largest U.S. flag in the state of California - 14 by 22 feet. This flag has been preserved in the collection of the San Pablo Historical Society.

By the 1900s, a small town of San Pablo was firmly established, and a big school was built there to accommodate children from nearby ranches. The prinicpal of this school was Walter Helms, for whom Helms Junior High is named. In 1914 San Pablo's first library opened with about 100 books. A number of property owners near downtown San Pablo subdivided their farm land for housing tracts during the first decade of the 20^th century. Most of the streets laid out at that time are still in use today.

In the 1920s a number of "speak easys" sold bootleg liquor and provided nightclub entertainment. Francisco Castro's original home, on the site of today's El Cerrito Center, was turned into a nightclub and speak easy. The Bay Cities Coursing Park, a dog racing track on Church Lane, was a popular local attraction.

The last remnant of this bucolic agricultural era is the Blume House museum. This home was built by German immigrants in 1905. It is fully furnished as most farm houses of the era. One of the outbuildings - the Bunk House - was also preserved and showcases farm equipment. The buildings are open to the public every Sunday and admission is free.