Zoom in on a lake in Québec 3D and, until now, you hit a lid. A smooth, perfectly flat plate, laid down where the water begins — as if the relief stopped dead at the shore. It wasn't a bug : it was the shadow of a physical limit of the laser. Now that lid lifts. Beneath 2,203 Québec lakes, the real bottom appears : basins, deep holes, shoals and shelves, exactly as they plunge below the surface.
This improvement doesn't come from LiDAR — it comes from another family of measurements, and from an open dataset Québec has been building for decades : the bathymetry of its lakes.
Why lakes were flat lids
Québec 3D's relief comes from airborne topographic LiDAR, whose near-infrared laser (wavelength ~1,064 nm) does not pass through water : over a lake, the laser reports only the elevation of the surface mirror, never the bottom. The processing chain therefore flattens each water body to the elevation of its surface → a lid.
Note that green-laser bathymetric LiDAR (532 nm) does exist and can sound shallow water — but it is not the source used here. Québec 3D's land relief rests on the topographic LiDAR coverage ; the bathymetry comes from elsewhere.
Sounding: measuring depth with sound
Where LiDAR times light in air, bathymetry times sound in water. The echo sounder, under a boat, measures how long the acoustic echo takes to return from the bottom. Depth follows from :
D = c × Δt / 2
where c is the speed of sound in water (~1,500 m/s) and Δt the round-trip travel time of the wave. The speed actually varies with temperature, pressure and water composition (about 1,450 m/s in cold fresh water, 1,500 m/s in salt water) ; 1,500 m/s is the common operational approximation. A boat follows transects, its position logged continuously by GNSS, and the points are then interpolated into a grid of depths : the bathymetric numerical model (MNB in French).
The laser draws the landscape seen from the sky ; the echo sounder draws it seen from the hull. Two rangefinders — one of light, one of sound — handing off exactly at the waterline.
The data: the Géobase des bathymétries de lac du Québec (GBLQ)
Compiled and released as open data by the MELCCFP (Ministère de l'Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs), on Données Québec. The MNB encodes a depth (zero at the shore), which is what lets the bottom join without a step. The MELCCFP is credited as the producer. Licence : CC-BY 4.0.
From measured depth to carved bottom
bottom elevation = surface elevation − depth. Since depth is nil at the shore, the bottom rejoins the ground exactly at the edge — an invisible join, and the surrounding land does not move by a single pixel.
Why 2,203 lakes, and not all
The index lists 3,280. Only 2,204 (67%) have a carvable numerical MNB ; the other 1,076 (33%) come only as a PDF chart (an image, not a grid of numbers — unusable in 3D). Of those 2,204, a single one falls outside the LiDAR footprint. That leaves 2,203 lakes carved, ~4,900 km² of lakebeds, some beyond 100 m deep.
The most emblematic case of this limit is Lac Saint-Jean itself : its vast basin is released only as a PDF chart, never as a numerical grid — so it stays smooth, while several smaller neighbouring lakes, genuinely surveyed, are carved. The rule is simple : we only carve what the source publishes as numbers.
What a portrait of the bottom is good for
Four worlds :
Civil safety — A lake is not a surface : it is a volume. Knowing the bathymetry means knowing a reservoir's holding capacity and its behaviour in a flood. During a drought, the available water volume is computed by subtracting the bottom model from the surface model. In a dam failure, the geometry of the bottom governs the drawdown rate and the flood wave downstream. For emergency navigation in murky water, mapped shoals become safety landmarks. Spill-response teams can model a contaminant's dispersion from density currents, themselves dictated by the shape of the basin and the stratification of the water masses.
Economy — Bathymetry underpins several sectors. In hydroelectricity, a reservoir's usable volume and its progressive siltation — the buildup of sediment that eats into storage capacity — are measured by comparing successive bathymetric surveys. For drinking water, the ideal intake sits at a known depth, below the thermocline but above the anoxic zone. Sub-lacustrine cables (telecommunications, power transmission) need a route that avoids holes and outcrops. Port infrastructure, docks and industrial water intakes rely on precise surveys for sizing and maintenance. Commercial fishing benefits from habitat mapping, which depends on depth, substrate and temperature.
Recreation and the outdoors — Sport fishing rests on knowing fish habitat : the holes where deep-dwelling species shelter in summer, the shoals that concentrate baitfish, the thermal slopes that guide their movements. Boating and recreational navigation gain in safety when reefs and shoals are located. Freshwater diving — wrecks, drowned structures, underwater geology — is planned with a map of the bottom. Kayaking and shoreline cottaging benefit from knowing the access points, near-shore depths and safe swimming zones. These uses, incidentally, inform the planning and management of water bodies at the municipal scale.
Environment — Bathymetry is the foundation of limnology — the study of lake ecosystems. Depth and basin morphometry determine summer thermal stratification : the epilimnion (warmed surface layer), the metalimnion or thermocline (zone of rapid transition), and the hypolimnion (cold deep water). This stratification governs oxygenation : in deep, eutrophic lakes the hypolimnion can turn anoxic, creating dead zones for wildlife. Spawning grounds sit at precise depths, on specific substrates that bathymetry helps to locate. Tracking siltation and sedimentation documents eutrophication and contaminant load. Finally, the basin's geometry feeds the hydrodynamic models that simulate water circulation, nutrient transport and a lake's response to climate change.
Dive below the surface
In 3D, in “Bare earth” mode, glide over a mapped lake and watch its basin sink below the water level — the same scene, now with its submerged relief.
Explore a lake in 3D →The takeaway
Topographic LiDAR stops at the water surface ; bathymetry takes over beneath it. A simple “surface minus depth” gives 2,203 lakes their volume back, where the two measurements meet.