A rower has an injured hand and a coxswain narrowly avoided being hit on the Waikato River, when a group of teenage boys pelted them with large rocks.

Hamilton police say the rowers had to seek shelter under the bridge around 7.30 last night, when the group of boys threw the rocks from the Claudelands Rail Bridge.

Five members of the group, aged from 13 and 15, have been charged with assault with a weapon and police are working to identify four others who were involved.

A tourist from Christchurch photographed the incident.