Water makes small choices. A trickle of liquid always moves downward, below, however imperceptible the angle of decline. When water encounters an obstacle, it seeps under and around, or crowded by a jostling flood, flows over. A single pebble might turn a deluge. There is no effort.
Study the surface of the river to learn what lies underneath it. This is the way of the spring river: to scout from the banks as the flow subsides to find what changes the winter floods brought to the channel. The water, turned by a pebble, can move boulders in the winter as it courses down from the mountains.
Bright lines shining across the current are stones covered only by shallow water marking a riffle. The water turns and dances, laughing over the lines of resistance.