The gravel bars are often covered with willow thickets. It sounds like the shooting occurred on the gravel bar.
“On Monday afternoon, empty medical supply packets lay next to the bloody spot on the gravel bank where first responders treated Dart. Police tape fluttered from the “Keep Out” sign that Crocker had planted in the gravel.”
Then I would say if the center of the river was his propety line and there was substanial vegetation on the sand bar it was his property.