1998gsxr

i have read the press today and there was an article on page A22 reads as follows:
this guy and his son did a bit of motor x ,a police officer find his son for riding his motor x bike on the river bed for no licence reg etc ...... can they do this as i thought the river bed was not classed as a road way??
crazy i know but i do the same thing,take my boys out on there motor x bikes myself....