نَهْرٌ جَارٍ
A flowing river.
In this analysis, flowing properly belongs to the water; a river designates its location or channel. Calling the river “flowing” transfers the property to a connected place.
An advanced point is that expressions can become fully conventional in ordinary usage. Rhetorical analysis here clarifies the original predicational association; it need not deny that native speakers normally understand such a phrase effortlessly.