Reservoirs, dams and the intakes of hydroelectric plants sit at elevations where the starting atmospheric pressure is lower. That means decompression has to be corrected, the drive back down the mountain road afterwards has to be watched, and an evacuation that's almost never quick has to be planned for.
A good part of the professional diving done inland doesn't happen at sea level. Supply reservoirs, headwater dams, hydroelectric plant intakes and outfalls, and mountain lakes all sit at elevations that require the dive to be handled differently. This isn't an administrative nuance or a formality on the job sheet: it changes decompression and it changes emergency logistics.
The most common confusion among people arriving from a marine background is thinking that shallow fresh water is always a benign setting. It's usually an uncomfortable one: cold, poor visibility, fewer resources around, and a decompression physics that doesn't forgive mechanically carrying over the table you've used your whole career.
What counts as altitude diving
Altitude diving is diving carried out in a body of water high enough above sea level that local atmospheric pressure is appreciably lower. Exactly where that threshold is set and what correction is applied isn't something the diver decides: it's set by the table, the algorithm, or the software the company has authorised, and it's applied by the supervisor.
What the whole team should understand is why, because a procedure that isn't understood gets applied badly the moment something unexpected comes up: a stop that runs long, a diver who surfaces early, a second dive nobody had planned for that day.
Why altitude changes decompression
Every decompression table solves the same problem: during the dive, the body absorbs inert gas at the ambient pressure of the bottom, and during the ascent that gas has to be released in an orderly way so it doesn't form bubbles. What determines how demanding the ascent is isn't just the starting pressure, but the ratio between that pressure and the pressure you end up at.
The starting pressure is lower
On a sea-level job, the diver finishes the ascent at the reference atmospheric pressure. In a mountain reservoir, they finish at a lower atmospheric pressure. The result is that the same water column covered during the ascent represents a proportionally larger drop in pressure, and the margin against bubble formation narrows.
Two practical consequences follow from that. The first is that a sea-level table applied without correction tells the diver they have a margin they don't actually have. The second is that the stops, where there are any, sit at different depths from the usual ones, because they too are defined in terms of pressure, not absolute metres.
Actual depth and equivalent depth
To solve that problem, altitude procedures work with two separate figures. Actual depth is what the depth gauge reads, and it's what governs consumption, equipment and the logistics of the dive. Equivalent depth is what's fed into the table so the decompression calculation is valid at that elevation.
Mixing them up is one of the classic mistakes. A diver reasoning from actual depth thinks they're doing a comfortable profile; the calculation says otherwise. That's why the dive log should record both clearly, just as gas mixes and times are logged on any submerged civil-works inspection campaign.
There's a third factor that's easily forgotten: the depth gauge itself. Instruments calculate depth from pressure and need to be set up for the environment they're working in, both for the elevation and for the density of fresh water. Checking that is part of preparing the equipment, not something left to individual common sense.
Acclimatisation and the diver's starting condition
A team that drives up from the valley in the morning, works, and comes back down in the afternoon isn't in the same situation as one that's been staying at altitude for several days. The body takes time to equilibrate to a lower ambient pressure, and that difference translates into different criteria within the procedure.
The conservative option, and the usual one when in doubt, is to treat the diver as unacclimatised. That penalises the calculation and lengthens the day, but it's consistent with the logic of the trade: decompression doesn't allow for improvisation, and the consequences of getting it wrong can't be fixed on the fly.
On top of that there's general condition. Mountain work often comes after a long approach, carrying equipment, prior physical effort and sometimes dehydration from dry air. None of those factors improve decompression tolerance, and all of them can be assessed at the pre-dive briefing.
Travelling to altitude after diving
This is the point that has caused the most avoidable accidents in inland work, and also the easiest to overlook because it doesn't look like a diving operation. At the end of the day, the team packs up, loads the van, and leaves. If the way back means crossing a mountain pass, ambient pressure drops again while the body is still off-gassing inert gas.
The same thing, worse, happens with flying afterwards. The rule is simple to state and hard to follow when people are in a hurry: the surface interval before moving to a higher elevation or flying is part of the operation's plan and is decided before starting, not once everyone's already standing by the vehicle with their boots on.
The practical way to lock it in is to document it in the dive plan and the permit to work, with the expected time out of the water and the time after which travel is allowed. Once that time is written down, it stops being an argument between tired colleagues.
Risks that have nothing to do with decompression
Altitude comes with a package of conditions that, together, explain why these jobs are planned with more margin than an equivalent port job.
Cold water and stratification
The water in a mountain reservoir is cold at the surface and usually colder still with depth, with sharp temperature swings crossing the thermocline. Cold degrades manual dexterity, speeds up gas consumption and affects the off-gassing of inert gas itself, as well as cutting the useful working time. Thermal protection, managing relief divers and checking the diver on exit are handled with the same care as on any operation exposed to hipotermia.
Visibility and sediment
The bottom of a reservoir builds up fine silt that stirs up at the slightest disturbance and takes a long time to settle again. It's common to work by feel, with the guide line and voice communication as the only reliable orientation. That shapes the technique, requires a well-managed umbilical, and makes the diver's real-time description worth as much as any video.
Suction currents
This is the most serious risk and the one that gives the least warning. Any facility that draws in or discharges water generates a suction current capable of pinning a diver against a screen with nothing visible to signal the danger beforehand. The only real defence is effective, verified isolation of the facility before anyone gets in the water, exactly the standard used in hydroelectric and nuclear plants. An unverified machine shutdown is not isolation.
Structures and enclosed geometry
Galleries, intakes, gates, screens and chambers turn many of these jobs into operations with a physical ceiling over the diver's head, with no direct ascent to the surface. That condition governs everything else: it changes the rig, it changes umbilical management, and it changes the rescue plan.
Remote access, evacuation and the recompression chamber
The biggest operational difference from a coastal job isn't underwater, it's on the map. Many reservoirs are reached by narrow tracks, with patchy communications coverage and a considerable distance from the referral hospital. The nearest usable recompression chamber can be hours away.
That forces you to reverse the reasoning. Instead of planning the dive and then figuring out how to evacuate, you plan the evacuation first, and that constraint dictates the profile you're prepared to accept. A profile that would call for long decompression at a site with no resources nearby isn't an ambitious profile: it's a badly chosen one.
The emergency plan has to be written and verified before starting, with the helicopter pickup point if there is one, the alternative land route, the phone numbers that actually work from that spot, and the treatment centre to be notified. Checking that information, rather than trusting it just because it's in an old template, is a responsibility of the supervisor and the company signing off on the job.
Who sets the procedure
It's worth being explicit here. None of the above authorises anyone to work out an altitude correction on their own from what they've read. The applicable tables and algorithms, the acclimatisation criteria, the intervals before travelling, and the operational limits are all set by the company's authorised procedure and applied by the supervisor with the documentation in hand.
The diver's role is different, and no smaller: understanding the logic, spotting when something doesn't add up, declaring their real condition and recent dive history, and refusing a profile that isn't backed by the plan. That's also the attitude that separates a trained professional from someone with a certification who's recently qualified and short on operational judgement.
What kind of diver fits this work
Altitude work rewards versatility and patience over technical flair. What's wanted is someone used to working with zero visibility, with the judgement to describe what they're feeling, able to manage their own thermal comfort, and disciplined about logging. Inspecting concrete structures, cleaning screens, and handling mechanical components are the dominant tasks.
It's also a stable inland market niche, tied to infrastructure maintenance that can't be put off indefinitely. Anyone wanting to head that way will find the general route in the guide to how to train as a commercial diver, with the particular point that here the differentiating value lies in experience built up at hydraulic facilities and in the ability to work far from everything.
In closing
Altitude diving isn't an exotic variant of the trade: it's an everyday part of inland work that demands a change of mindset before getting in the water. The starting atmospheric pressure is lower, decompression has to be corrected with the authorised tool, acclimatisation matters, and the way back is part of the dive.
On top of that, the environment brings cold, poor visibility, suction currents, and distance from medical assistance that can't be solved by improvising. A team that plans the evacuation before the profile, that verifies the facility's isolation, and that writes down the time after which it's safe to come down the mountain has solved most of the problem before getting wet.
Frequently asked questions
Why don't sea-level tables work at altitude?
Because they're calculated on the assumption that the diver ascends towards a given atmospheric pressure, that of sea level. At altitude, the destination pressure is lower, so the same ascent represents a proportionally larger drop in pressure. Applying them without correction leaves the diver with less margin than the profile appears to give.
What does equivalent depth mean on an altitude job?
It's the depth entered into the table or the software so that the decompression calculation is valid at that elevation, and it doesn't match what the depth gauge reads. Actual depth is still what matters for logistics, gas mix and equipment; equivalent depth is only for the calculation. Both are set by the authorised procedure, not by the diver's own judgement.
Do you need to acclimatise before diving at altitude?
Going up to altitude and diving straight away isn't the same as having lived there for days, because the body takes time to equilibrate to the lower ambient pressure. Companies usually set a minimum time on site beforehand, or treat the case as unacclimatised, which is the conservative option. The specific decision falls to the company's procedure and the supervisor.
Is it dangerous to come down the mountain right after diving?
Yes, and it's one of the most common mistakes. Driving down seems harmless, but leaving a high elevation for the valley, or especially flying after diving, means continuing to reduce the ambient pressure on a body that's still off-gassing inert gas. The surface interval before moving is part of the plan and isn't improvised at the end of the day.
What's different compared with an equivalent port job?
Technically, the task can be identical: inspecting a structure, cleaning a screen, fitting a part. What changes is the environment: colder water, worse visibility, suction currents when the facility is in service, and a much greater distance to a hospital or a recompression chamber. Planning matters more than technique.
Published 4 September 2026 · By Arthur More, professional saturation diver
Photo: Ghost Fishing UK · dominio público via Openverse