The Water Line

Water crossings look like the most dramatic thing a travel vehicle does. They are actually the most calculable. Depth, current, bottom and exit: four questions with four answers, all of which you can get before a wheel touches the water, and all of which people skip because there is an audience.

Know your wading depth as a number

Every manufacturer publishes one. It is typically between 500 and 800 millimetres for a modern four wheel drive, and it assumes a still surface, an intact vehicle and a slow entry. The number is set by the lowest point through which water can reach something that matters: the air intake first, then the electrics, then the breathers on the axles, gearbox and transfer case.

Find out where your intake actually sits, in centimetres above the ground, and write it on a piece of tape inside the vehicle. Almost nobody knows this figure and everybody guesses high. A snorkel raises the intake, which is exactly one of the four problems solved: it does nothing for electrics, differential breathers or buoyancy.

Walk it, always

Take a stick, walk the line you intend to drive, and probe. You are checking for depth, for a soft silt bottom, for rocks and holes, and above all for the exit, because a crossing you can drive into and not out of is the classic way to fill a vehicle. If the water is opaque and fast, or if you would not be willing to stand in it, you have your answer.

Current matters more than people expect. Thirty centimetres of water moving at walking pace pushes a vehicle sideways; sixty centimetres moving fast can float the back of a lightly loaded one and take it downstream. The test is simple. If you struggle to keep your footing, the vehicle will struggle too.

An engine drowns in one second and takes three months to replace. The walk takes three minutes.

Driving the crossing

Enter slowly, at an angle that keeps the front wheels loaded, then hold a steady, slow speed in a low gear, typically second in low range. The goal is a small standing bow wave in front of the bumper. That wave creates a trough around the engine bay and is genuinely why steady is better than fast: too slow and water rises over the bonnet, too fast and the wave collapses and the engine bay fills.

Do not change gear in the water. Do not stop unless you have to. If the engine dies mid-crossing, do not attempt a restart, because a starter turning an engine full of water bends connecting rods in the first revolution. Recover the vehicle out, then investigate on dry ground.

After the water

Brakes are wet and will not work properly for the first few hundred metres. Drag them gently while moving to dry them before you need them, especially on a descent. Then, at the next stop, check the axle, gearbox and transfer breathers, and look at the oils: a milky dipstick means water has entered and the vehicle should not continue far. On a longer trip, plan to change the differential oils after a serious crossing, because water in a diff destroys it quietly over the following thousand kilometres.

Salt water is a separate case. Rinse everything with fresh water as soon as you can, including the underside, the brakes and the inside of the wheels. A tidal crossing in the morning becomes a corrosion problem for the rest of the vehicle's life.

The crossings not to make

Flash flood conditions in a wadi or a desert riverbed: no. A rising river you have not seen at its low point: no. Anything where a failure would put people in moving water: no, whatever the vehicle is. In the mountains, the same crossing that was ankle deep at eight in the morning can be impassable at four in the afternoon after a day of snowmelt, which is why an early start is a safety measure and not just a habit.

There is no shame in turning around. Every experienced traveller has a river they did not cross, and most of them can point to somebody who did.

Questions we get asked

Do I need a snorkel?

Only if you regularly cross water near your wading depth, or drive in very dusty conditions where a raised intake takes cleaner air. It is not a licence to go deeper: electrics, breathers and buoyancy set the real limit.

Should I use a water blind or radiator cover?

On older vehicles with mechanical fans it prevents the fan from throwing water over the engine bay and can stop a bent fan blade. Most modern vehicles have viscous or electric fans that make it unnecessary.

What if the engine stops in the water?

Do not restart. Get the vehicle out with a strap or winch, then check the air filter and the oil. A dry filter and clean oil usually mean you were lucky. Water in either means the engine needs work before it runs again.