You will also meet a ton of people that are more into the sport than you ever expected. Because of my hobby, I've had the privilege to meet some of the best river guides in the United States, and pick their brains about how they identify good rapids. Quite surprisingly, the identification of awesome rapids to take trips on is not done with observation or intuition in the classical sense. It comes from a serious grasp of fluid dynamics.
Fluid dynamics is the study of the physical forces involved when interacting with water. I was introduced to the concept by one of the expert guides I had the privilege to go on a private trip with. I asked how I could quickly learn how to read the river like they do, and they rattled off these things.
1) Flow Compression
The first thing I had to learn is that water, being a compound of hydrogen and oxygen molecules, has a lot of space hidden within its structure. What this means is that of a complete atom (Proton, Neutron, and Electron) there is a lot of space between the physical particles.
What this means is that the fluid itself can be compressed to change is behavior. The easiest way to imagine this is with a hose of flowing water. When you use your thumb to narrow the nozzle at the end of the hose, the pressure goes up. While your thumb stays there, you can actually lower the amount of water going into the hose and maintain the same pressure jet coming out around your thumb. This is a great example of water compression.
In the river, compression can come in many forms. Two rocks near each other, a rock just below the surface, or even a deeper trench can change the amount that a particular flow is compressed. As the flow is compressed, there is more pressure. According to the guide, the best place to look for new rapids is on narrow stretches of the river, where the flow is compressed.
2) Turbulent Flow
There are two different ways to categorize the entirety of a flow of water, those are laminar and turbulent flow. If you imagine your garden hose again, if you turn your water on high, the water holds its cylindrical shape for the first couple of feet of its flow, and then breaks up into smaller droplets as it reaches the ground.
That first couple of feet is a great example of a laminar flow. That is where the flow pressure and surface tension overcome gravity, wind, and other forces acting upon it to keep its shape. After a couple of feet, the cumulative forces acting upon it make it a turbulent flow, which means that the water is aerated.
In terms of white water rafting, you are always riding on a turbulent flow, which brings into question some really interesting physics questions; specifically, how to account for gravity. When you are riding a very light turbulent flow, or on the laminar flow of a slow river, the force of the water pushing up on you counteracts gravity; this is called buoyancy.
However, when you start to aerate the water, there is less force holding you up, and you start to feel the effects of gravity more. This is why in heavy rapids you feel like the raft "drops". You literally felt the effect of overcoming your displacement and fell under the force of gravity toward the riverbed. This is also the reason that some guides will avoid the heavy white-water areas with a full boat. Its not that they doubt your skill or bravery, but imagine if over 1000lbs just entered a couple of feet of free fall, then was overcome with dozens of pounds per square inch of water pressure. Instead of complaining, thank them; they just out-physic'd disaster.
3) Steady Flow
When talking about white water rafting, you need to understand the difference between laminar and turbulent flow before understanding steady vs. unsteady flow. Laminar describes how the flow of water can be measured, in terms of buoyancy and aeration.
Steady flow described a laminar or turbulent flow over time. Again, lets return to the metaphor of the hose in your backyard. Lets assume that instead of your thumb you placed a nozzle on the end of the hose to increase the pressure. This would be a steady flow, as that nozzle wouldn't change the diameter, flow, or location of compression over time. However, if you are using your finger, then you would classify the flow as unsteady, as it would be impossible for you to keep your finger in the exact same place and allow the exact same amount of water to pass by at all times.
In white water rafting, this is most easily seen with tributaries. These are other smaller rivers and streams that meet up with the river that you are moving on. These create eddies of unsteady flow; which means that after a rainfall, that area is going to exhibit more turbulence, and a higher water pressure than usual.
For advanced rafters, this means go harder and bigger; but for beginner groups, this requires the boatman to slow down and carefully go through it. As we've learned, combining gravity falls with higher water pressure is extremely dangerous for those who have never experienced it.
But for those that want a real adventure, get on that raft right after a rainfall, and see the perfect storm of fluid dynamics in action.