Water is for the most part an incompressible fluid. Since the fluid cannot compress, force is propagated through the fluid medium in shear. This shear movement “ waves”, are manifested at the air water interface as a projection equal and opposite of force (gravity). These projections are transitory and unstable and as such, disintegrate by cascading under the force of gravity. The cascading water mixes with air breaking up the light bouncing off the mixture. The white is color (or lack thereof) since the light doesn’t transmit directly across the interface.
Whitewater is an Energy Dissipation Event
Entropy, Second Law of Thermodynamics. Water changes its energy state as acted upon by gravity. The loss of potential energy and conversion to kinetic energy yields the net difference friction and subsequent heat loss. Where does the heat go? The heat warms the water subject to the friction. This is why the lower stretches of the river are warmer than the upper.
Energy, Momentum, the integrated product of mass and velocity has two sides of the equation. These are sub and supercritical flow regimes. The astute rafter sees sparkling water in shallow depths as the water changes velocity in accordance with the continuum. This sparkling is water yielding its potential for kinetic energy resulting in tiny waves. The sparkling water is supercritical flow. Subcritical flow is for the most part, absence of waves.
On the flip side of the equation, Hydraulic Jumps are an exchange in energy from super to subcritical flows. Backwater head resistance forces the change in energy state to a quiescent flow. The bottoms of most rapids have a hydraulic jump.
Stay out of Holes. Holes are frequently expressed at the tongue of a hydraulic jump. Holes swallow you up and hopefully ………spit you out. Think of it as a ride in a washing machine on 4 inch boat. There is nothing good in there and other than experience. If you seek punishment, there is an impressive hole on the back side of Maytag at the higher flows. There are a few notable holes in Cataract Canyon. This is a good blog topic,”Notable Holes”
Rapids are frequently expressed by a numeric numbering system based on a statistically probabilities. Every river is different at different flows; know it for what it is. A low flow river may exhibit dangers that don’t exist at higher levels. Always scout and safety first.
Generally speaking, a class 6 is a 100% probability of an inversion. Similarly, a class 5 is a 50% probability of an inversion. A class 4 is a 50% of 50% probability of an inversion. A class 3 is a 50% of a 50% of a 50% probability of an inversion. Think of it as a declining logarithmic curve. Some folks rate low head irrigation dams as a 6. This is not statistically accurate. However the risk associated with entrapment in some low head dams is unacceptable. Dams designed to pass debris while maintaining headwater do not usually fall into this category. It’s all about the risk.