Dry Creek, is it public waters?

My family has a 90% of the time dry creek on the back of our property. The neighbors have horses and ride the creek from top to bottom crossing multiple farms. The other day there were a few ATV's who are now starting to make the same trek. We never thought twice about it until the neighbors asked for permission yesterday because they said someone down stream warned them they were tresspassing.

Is this tresspass or or they legal as long as they stay in the limits of the high water mark? (when there is water)

We have a few young family members, nieces and nephews who were invited to ride and we don't want to send them into an area looking for trouble.

Thanks,

Rod