Experts can read a stretch of rapids, spotting the hazards and seeing the safest way through. Knowing the amount of water flowing in a river is important because the river's speed increases as more water flows through it. Double the water means double the speed, so a mild rapid becomes a dangerous one during the rainy season. Laminar describes the smooth-flowing currents in an unobstructed river. Even these currents can be complicated, for their speeds vary.
Surface water is slowed by wind, while deep currents are slowed by friction with the riverbed.
Water in the middle, a few feet below the surface, usually runs the fastest. Turbulence occurs when obstacles, such as rocks or a sudden narrowing of the river channel, obstruct the current's flow. Obstacles force too much water into too little space, so the water runs faster and laminar sheets break into individual ribbons of current. Then things get really complicated. If water runs into a boulder, a turbulent zone is created where the water and rock collide.