You need to begin by defining "watershed year" then look at what was happening in the United States of America prior to 1890 compared with what was happening during 1890 and subsequent years.
According to dictionary.com, the fourth definition of watershed is defined as "an important point of division or transition between two phases, conditions, etc."
1890 is the the point of division between two phases in this country's history. Prior to 1890, the country had been through the Civil War and Reconstruction periods. The country was expanding within and was trying hard to stay out of international events. It was in the midst of an industrial revolution and growing economically.
1890 began a period of international growth. We gained new colonies and territories. Our navigational abilities improved. People like Theodore Roosevelt, Henry Cabot Lodge, and Alfred Thayer Mahan wanted to show other countries that we were a world power.
Although I have never heard 1890 referred to as a watershed year in the US, there are three events from that year that you could point to if you wanted to make this claim.
- Passage of the Sherman Antitrust Act. You can argue that this marked a very important step in government intervention in the economy. We would see much more intervention in the soon-to-come Progressive Era, but you could say this is the start.
- Wounded Knee massacre. This can be seen as the last act in the Indian wars that the US waged. After this, you could say that the frontier was closed, the Indian wars over.
- Publishing of Alfred Thayer Mahan's book. His arguments about the importance of sea power helped to bring about a period of American imperialism in this decade and early in the 1900s.