Azusa police combed 1 1/2-miles along the San Gabriel River on Thursday and found 25 makeshift encampments where some people have been living for months.
Officials estimate 50 men, women and children were living in unsafe conditions on federal land downstream from dams in the Angeles National Forest, said Azusa police Lt. John Momot.
No arrests were made, Momot said, but people were given 30 days to vacate or face a citation for trespassing.
The riverbed property, owned by the Army Corps of Engineers and maintained by Los Angeles County Flood Control, is policed by Azusa officers.