Hello!
When the settlers came for the gold rush, they sometimes forced the Mexican Californians out of their lands in order to be able to stay there. When the gold rush ended, some settlers stayed there because they liked the location, which resulted in the Mexican Californians losing their lands. It certainly had a negative effect on the people already living there.
You can feel free to let me know if you have any questions regarding this!
Thanks!
- TetraFish