As an opening statement it should be said that a water crossing should not be attempted by the inexperienced, especially deep water crossings.  A water crossing can be fatal to an engine and even worse, to occupants of the vehicle in the worst case scenario.  Never underestimate the force of flowing water.  Water weighs about 62.4 pounds per cubic foot and typically flows downstream at approximately 6 to 12 miles an hour.  For each foot the water against a vehicle, 500 pounds of lateral force are applied to the vehicle.  As a vehicle enters water, 1500 pounds of buoyant force are produced for each foot that the vehicle is submerged.  This basically means that a vehicle weighs 1,500 pounds less for each foot of water.  When the buoyant force exceeds the weight of the vehicle, it begins to float and is now at the mercy of the water as  the flowing force of the water is exerted on the vehicle, pushing it downstream.  For typical cars, this translates into a floating car in just two feet of water.  To put it into a 4x4 perspective, a 4x4 vehicle is typically lifted to some degree.  This puts the buoyant part of the vehicle, mainly the body although tires are quite buoyant, higher than cars, but in many cases not by much.  The key point is that like a boat, the water displaced creates the buoyancy and vehicles actually float until water seeps into them.  But by the time water seeps in, it may be too late.

That being said, a water crossings in a 4x4 can sometimes be some of the most exciting and challenging situations you do when offroad.  Water crossings can also become some of the most expensive.

A 4x4 is not an amphibious vehicle, nor is it waterproof.  Even those 4x4 vehicles that have been modified to tread water have their limitations when crossing through deeper waters. The depth to which a vehicle can be submerged depends greatly on the vehicle itself.  Those factors not only apply to the vehicle but the age of the vehicle as seals deteriorate.  As a rule of thumb, the depth limit of a vehicle is about the top of the tires and even that is probably too deep.

Thinking Ahead

As a rule of thumb, it's a good idea to have your recover equipment easily accessible and ready for use.  When you need your recovery equipment it may be in an emergency situation where time is critical.  If your recovery equipment is buried beneath other gear, the last thing you want to do is have to hunt for it especially if you are stuck in a stalled vehicle in the middle of a river crossing. 

Unfamiliar Waters

Prior to a crossing unfamiliar water is to walk it.  Yes, get wet.  If you cannot walk it you cannot cross it with your vehicle. If the water is flowing too quickly or is too deep to safely walk the crossing, then it cannot be driven across without serious risk.

Breathers

Axles, manual transmissions and transfercases typically have breathers.  Breathers are designed to allow air pressure to equalize between the atmosphere and the inside of the mechanical component.  During use, axles, transmissions and transfercases heat up.  When a hot axle or gear box hits cold water it rapidly cools.  This causes the air pressure inside the axle tube, differential housing, and gearboxes to reduce as the air molecules to contract.  This in turn causes air to be drawn in via the diff breathers.  If your breathers are below the water level or getting splashed, water will also be sucked into the breathers.  Extending your low lying breathers, especially differential breathers, up higher into the chassis area using flexible tubing will allow a cooling component to draw dry air rather than water during a water crossing.  This is a necessary modification for 4x4's that frequently encounter water crossings.  Of course even with breather extensions, there is no guarantee that water will not enter your gearbox and differentials.  Rapid temperature reduction, worn seals, loose bolts, all can be the cause of water getting into places it shouldn't be.  As a preventative measure and the very least giving the axles, differentials, gearbox and transfercase time to cool down means there won't be as sudden a pressure drop in these mechanical components causing water to be sucked in.

Engines breath air.  As air is pulled into the engine, it is compressed by the pistons.  Water does not compress.  If water gets sucked into your air intake on the engine, it can result in serious damage.  The engine attempts to compress what is in the cylinder and if enough water enters a cylinder, even a smaller amount of water, the engine can instantly stop as it tries to compress what can not be compressed, which results in something called commonly called "hydro-locking".  This usually results in bent or broken pistons, valves and crankshaft.  An expensive end to your day.

So where is your air intake?  Is it behind the grill?  Behind the headlight?  On top of the engine or high atop the vehicle relocated by a snorkel?  The location of your air intake is critical to how well a vehicle can handle a crossing as far as the air intake goes.  Many vehicles have their air intakes in optimal locations to allow cold air into the engine such as directly behind the headlight, or through the fender wall.  Unfortunately these locations are great places for directing water into the engine.  Examine your air intake to see where it is pulling air from.  If it is right out there where a head of water will funnel right into the air intake during a water crossing, you may want to consider relocating it or at the very least consider a way to temporarily and quickly modify the intake to prevent damage during a water crossing. 

NEVER attempt a water crossing where the water depth is above the height of your air intake.  Vehicles that regularly encounter water crossings, should consider a more permanent relocation of the air intake such as a snorkel to raise the air intake.  As an obvious word of caution, do not consider the location of the air intake to be the depth that you can submerge your vehicle!

The Engine Bay

Aside from submerging your vehicle to the point where it floats, your engine bay is the primary concern when driving through water.  Almost as important as the air intake is your ignition system.  In many cases a wet ignition system will cause the engine to stall.  Not a good thing in the middle of a river.  Water on spark plug wires (especially old, worn out wires), an old seal on the distributor cap (or no seal at all), a wet coil or wet ignition module can all cause an engine to stall.  Typically though water in the ignition system usually results in just a stalled engine and not actual engine damage.  But when it does occur, it's usually in a place where you cannot easily dry off your ignition system components such as in the middle of a crossing. So protecting the important components from water can be the difference between getting across and going into recover mode.

When you move through water, the front of the vehicle creates a bow wave.  This raises the water level at the front of the vehicle, the primary point of water entry into the engine bay.  For that reason, go slow, don't plow through the water as fast you can to get to the other side.  As a preventative measure, some people  place a tarp across the front of the vehicle to minimize water entering the engine bay so long as you keep moving forward.  The result of the tarp method is less water for the radiator fan to spray over the ignition system and less of a chance for the fan to warp and slice into the radiator.  It also helps to reduce the chances of water entering the air intake.  To minimize stalling from a wet ignition, inspect seals, wires, and all other ignition system parts and replace worn out parts and seals. It’s also a good idea to spray all ignition system parts with water repellent before they get wet.  Diesel engines usually do better with water crossings since they do not have the ignition system to worry about.

Inspect your radiator fan.  Take note of type of radiator fan you have. Most 4x4's have a viscous coupling type fan, also called a clutch style fan.  These fans do not spin at full speed when the engine is cool and can be a benefit when encountering a water crossing.  You may also find that you have a fixed type fan that always spins at the speed of the engine.  To check your fan turn off the engine and try spinning the fan.  If the fan turn easily with the engine off and there appears to be a clutch mechanism at the center of the fan this is most likely a clutch type fan you will probably can get away without taking off the fan belt. If the fan doesn't turn easy or it is definitely the fixed type fan, then you should consider removing the fan belt before entering a water crossing.  The reason is when a fan encounters water, it acts much the same way as a propeller and a light-weight radiator fan will flex opposite the direction that it blows air, which is towards the radiator. With most engines having little clearance between the radiator and radiator fan, this means you may see contact as the fan bends, possibly slicing into and damaging your radiator.

When you encounter an unfamiliar water crossing and there is no way to easily examine the depth, sometimes the best way to scout out your crossing is to get wet and walk it.  Walking a water crossing is the best way to find a good track for your wheels to follow and locate any hidden potholes and large rocks as well as checking the depth of the water.  Since water has a way of distorting the view of the bottom and making it hard to estimate the depth it's a good idea to walk in the line that you intend to take, making note of the wheel tracks and objects between the wheel tracks.  Some people place markers in the water to mark hidden objects or drop-offs or deeper holes.  A marker could be a tall stick or even a tethered balloon.

Before beginning the water crossing mentally picture your route.   It's a good idea to take off your seat belt and wind down your window.  When entering the water typically using low range second gear at about 1500-2000 rpm creates just about the right bow wave.  This bow wave of water helps to maintain momentum and push water ahead of your engine bay.  Get the speed just right is tricky.  Do not attempt to drive too fast through the water.  The idea is not to get across as fast as you can.  Too fast will send water over your hood and possibly into your intake.  Too slow may flood the engine bay.  Hopefully you follow your line get to the other side with no issues.  If you have to stop, slow or reverse, avoid using the clutch as this may allow water and debris to get between the friction plate and the flywheel.  If you start to loose traction, its important not to over-rev the engine.  Back-off the accelerator and move the front wheels side to side a little in the hope that the wheels may regain traction.

If you stall the engine, put the vehicle in neutral without using the clutch and attempt to restart. If your lucky and you didn't flood the intake or drench your electrical components the engine will fire up.  Its normally best to use 1st gear low range and with a minimum of clutch usage, try to get moving again.  If you find yourself stalled in the middle of a crossing and you can not re-fire the engine, depending on the water depth, its advisable to climb out of your window rather than open the door and flood your interior including electrical components, seats, gear and carpets.

After encountering a water crossing you want to be sure to inspect specific things that could be affected by the water.  Any time you encounter a water crossing that is axle depth or deeper, check the differential oil for water contamination. Even if you have an extended differential breathers, this is not a guarantee that water has not entered the differential.  If you do not have differential breathers this should mean a mandatory inspection.  Even though the best time to check the differentials for water is right after the crossing, most of the time, people will not check it right there and will continue wheeling the rest of the day.  However checking the differential fluid is relatively easy to do.  Since water is heavier than your gear oil oil, it will collect at the lowest point in the differential, at the drain plug.  To properly inspect it after a crossing allowing time for your axles vehicle to cool a bit, then loosen the drain bolt and run a small amount of the gear oil into a glass or cup. You'll see water if it's present.  Also a milky or chocolate milk colored oil indicates water is present.  If you're unlucky enough to have water present in your gear oil, the differential oil should be drained and replaced. Driving your vehicle for any distance with water mixed with the gear oil can damage your ring and pinion.  If you had taken on water in the differential and have to drive out, allow the axle to cool for a while and attempt to drain as much water as possibly without draining any larger quantity of gear oil.  Collect this oil and water mixture in a container and save it until you get home, then dispose of it properly.  Never dispose of oil into the environment. 

Aside from the axle differentials your gearbox, transfer case and engine oil can all become contaminated by water. These however usually only take on water when a vehicle is stationary in deeper water.  Also check other electrical items such as your electric winch.  Winches typically are not used for extended periods of time but when they are needed, they may have been damaged or may have seized due to the water exposure.  Periodic maintenance and testing should be part of your routine anyway.  After a water crossing give the winch a quick test of it's operation.

Water crossing should be taken seriously.  Water has the potential to do expensive damage to your vehicle's engine and drive train. But with proper preparation and post inspection and maintenance, the challenge of a water crossing can be as enjoyable as any other offroad challenge.