Introduction

River current in rapids produce an amazing amount of force.  A common boating term for this is "Push".  Many intermediate boaters find the Cheat Canyon pretty straightforward at 1' on the bridge gauge.  Increase the bridge gauge to 3' and the run becomes a nightmare for them.  River currents on large bodies of water also need to be taken into consideration when planning how long it will take to paddle a certain distance.  If you can paddle 3 mph on a flat lake and you are paddling against a 2 mph current, your net speed is only 1 mph.  Fortunately in whitewater, we are typically paddling with the current (at least most of the time).

Water is Heavy!

Many that are new to our sport grossly underestimate the power of water.  It isn't unusual for a small stream to flow at 300 CFS.  One CFS of water weighs 62.5 pounds.  Since pins often occur where the river necks down, a very large portion of the stream's flow is pressing down on the trapped object.  Over time, most boaters discover they need to work with the river instead of fighting a losing battle.  The AW site's national river database: AW River DB is a great safety tool.  It is color coordinated to let you know when your favorite streams are reasonably safe to paddle.  River levels may also rise unexpectently from isolated storms upstream.  When paddling isolated runs, we need to pay attention to the overall river level and potentially take off the river when the level is too high for safe boating.

Water is Powerful

Water exerts a great deal of force on objects.  The following chart is from River Rescue, 4th Edition - Les Bechdel & Slim Ray:

The force increases in proportion to the square of the water velocity.  Water is heavy as well, it weighs 62.4 lbs per cubic foot (or 8.35 lbs/gal).  Figure an average kayak holds 75 gallons without air bags, the total weight is: (75 * 8.35) + 40 (average kayak weight) or 666 Pounds.  Now you can understand why we insist on paddlers installing air bags in their boats.

Constrictions

Rapids are formed in a number of ways which makes them so interesting.  Here is a trick question.  If a run is flowing at 1,000 CFS in the pool above a rapid which is 500' wide, what is the CFS in the rapid which is only 100' wide?  Believe it of not, it is still 1,000 CFS.  That same 1,000 CFS is going to be a great deal more powerful though and you better be prepared for this change.  Chances are pretty decent that you can paddle back against the current in the pool above.  Good luck trying the same in the rapid.  Since the flow (amount) of water is the same, the current is going to be much faster in the rapid.

Conclusions

Since water currents are both heavy and forceful, how does that impact the typical whitewater boater?

• Install Floatation - If you and your boat get separated on a fast moving stream, the water you displace will make recovering your boat your boat faster and safer.
• Work with the current, try not to fight it.  A great way to unsterstand this principle is doing attainment drills.
• Obey the "Safe Eddy Rule".  Never stand in moving water deeper than your knees.  As you can see from the table above, a foot entrapment is going to be challenging to escape.
• Stay off rivers in flood stage.  The AW site color codes these as purple in their river database.  Rivers in flood escape their river banks and weave through trees and brush - always a deadly combination for whitewater boaters.
• Monitor river levels.  If your run is rising quickly, make a determination for what level is no longer safe and pull off immediately when the river reaches that level.
• Fast current requires setting a steeper boat when performing ferry maneuvers.  Using waves to assist in ferrying is really helpful.  You will get to practice this technique on the second day in the Feeder Canal.

Here is a short article from the ACA on the effects of wind, waves, and water by Mike Aronoff: Wind, Waves, Current.