Immigration records are a treasure for family historians, providing not only evidence of our ethnicity but also personal information we probably couldn't find anywhere else. If you can find the ship manifests or passenger lists, they could yield genealogical treasures.
The ship's place of origin could provide evidence of one's ethnicity, a documented connection to ancestral homelands - unless they transferred ports, of course. Some passenger lists included the age and place of birth, too, which is most helpful in distinguishing between people sharing the same name. The timing of their voyage could indicate why they emigrated. For example, my maternal 3rd great-grandfather, John Dickinson, came from Sheffield, England, in the mid to late 1800s to Pittsburgh, Pennsylvania. What brought him here? Well, he was a "steel forgeman" back in Sheffield and in the 1870s, Pittsburgh boomed with new steel mills, making it one of the foremost steel producing cities in America at the time. (To this day, their NFL football team is called the Pittsburgh Steelers - my favorite). So, it appears the Dickinsons came to America for opportunity. On the other hand, my paternal 3rd great-grandfather, William McGinnis, came from Northern Ireland to America in 1849 or 1850. What was happening in Northern Ireland in 1850? It was about five years into the Great Irish Famine, which killed over a million people and caused over 2 million to flee the country - not just for a chance at a better life but to save their lives. My "Timeline of Events" was created to help see what major events were taking place at any given time. It is a work in process but many major events are listed and may provide some clues as to what was happening during their lives.
The other passengers on ship manifests may also provide clues. For example, half of the Mayflower passengers were members of the Leiden Congregation who came from the Netherlands. You may also be able to determine who they traveled with. Perhaps their future husband or wife was on the ship with his or her parents, which would provide more clues for you to follow. Some manifests indicate how much money they were carrying, their occupation, any distinguishing marks on their bodies, their height, weight, and/or eye color.
Being from the northeast, when I think of 19th and 20th century immigration to America, I think of Ellis Island, Castle Garden, and the Ports of Philadelphia and Boston. My immigrant ancestors all arrived in the northeast, and most stayed in that area, with the exception of my great-grandfather, Percy Daniels. He was born in Olympia, Washington, but his parents had gone west by land, following the Oregon Trail. As far as I know, none of my ancestors arrived on the west coast, so I had never given much thought to immigration stations in the west. It was a book called "Angel Island, the Ellis Island of the West", by Mary Bamford, that brought the Angel Island Immigration Station to my attention. As you can see on the map, the analogy is fitting. Like Ellis Island sits in the New York Harbor, Angel Island sits in the San Francisco Bay.
After Mexico ceded California to the United States in 1848, Angel Island was used as a military base, just as Ellis Island was formerly used by the military for decades, first as a fort (Fort Gibson) and later as a battery and naval magazine where ammunitions were stored.
Castle Garden, now called Castle Clinton, located in Battery Park, New York City, was America's first immigration station. More than 8 million immigrants reported their arrival at Castle Garden between 1855 and 1890. Ellis Island didn't open until about 1892 and from then until 1954, about 12 million immigrants were processed there.
Although immigrant lists were kept, there were no restrictions on immigration to America until 1882. It was then that Congress passed The Chinese Exclusion Act, restricting the immigration of Chinese laborers and they soon realized the need for a designated place on the west coast for immigrants to be processed and inspected upon arrival and Angel Island was chosen as the location.
The administration building for processing immigrants at Angel Island was opened in 1910. The immigrants arriving there came primarily from Germany, Russia, India, Africa, Japan, Korea, and China. Because only skilled Chinese immigrants were allowed entry, some who didn't qualify claimed they were sons or daughters of Chinese Americans already in the country, thereby claiming they were automatically citizens. Those immigrants had to be thoroughly interrogated and detained while their claims were investigated. Witnesses had to be called and relationships had to be verified, a process that took anywhere from 3 days to 22 months. Those who were detained wrote poems in Chinese on the wooden walls at Angel Island, expressing their disdain for the process, their treatment, and unsanitary conditions in the detention center.
Over a million immigrants were processed at Angel Island between 1910 and 1940 and about 80% were granted entry. Of those, it is estimated that 175,000 were Chinese and 117,000 were Japanese.
In August of 1940 the main building and part of the women's quarters were destroyed by fire and the station and detainees were relocated to a facility in San Francisco. Three years later, in 1943, the Chinese Exclusion Act was repealed. The buildings on Angel Island sat in disrepair for decades until the 1970s, when demolition was considered. A group of advocates, however, fought to preserve the site and it was made into a public park. The poems of the detained Chinese immigrants on display as a reminder of the suffering they endured in hopes of becoming American citizens.
For more information about Angel Island, see the Wikipedia article found here. You might also enjoy this short video:
Finally, here is the book that first peaked my interest in Angel Island. Read it free, courtesy of Archive.org: