When the United States worried about migrants in small boats

Here's a short passage from Roger Greaves's book Reading Madeleine that describes one of the British writer Robert Henrey's adventures in Canada as a young journalist in the early 1920s:

He was sent to Niagara to investigate a rumour that an organisation had been formed for passing aliens into the United States. The passengers were collected in Toronto or Hamilton, taken by car to the Canadian bank of the Niagara river a couple of miles below the falls and a crossing was attempted by night. 

The immigrants were packed in a small boat and the other side would be signalled with a flash lamp. However, the tangle of branches and undergrowth proved a good hiding place for United States immigration officers, who were always on the watch.