Bathymetry · plates 1 m to 25 m, widening with depth
How the lake is distributed by depth
What you are looking at
Lake Superior is the largest lake of fresh water on Earth by surface area, and the shape of its floor is why it dominates the system by volume as well. Multiplying this grid cell by cell gives a volume larger than Michigan, Huron, Ontario and Erie combined.
The floor falls away fast. Where Erie spends most of its area on a shelf, Superior spends most of its area already deep, and the plates crowd into narrow bands along the shore before dropping into the main trough.
The two features that break the trough are both easy to find from above. Isle Royale runs as a long ridge near the north-west shore, and the Keweenaw Peninsula reaches out from the south, splitting the western end into two arms.
All five lakes are drawn on one depth scale, and Superior is the only one that reaches the dark end of it. Everything in this system below 300 m is here.
Lake Michigan runs almost due north to south for five hundred kilometres, and the deep water stays in the northern half. The deepest sounding in the grid sits mid-lake, west of the Leelanau Peninsula.
The southern end is the opposite: a wide, gently shelving floor that comes up slowly toward the Indiana and Illinois shore. That shelf is the same system of sand that builds the dune country above the waterline.
Green Bay is almost a separate lake, held off behind the Door Peninsula and shallow enough that it shows in the bright end of the ramp while the main basin beside it has already gone dark.
Michigan and Huron sit at the same surface level and are joined at the Straits of Mackinac, so hydrologically they are one lake. They are drawn separately here because NOAA compiled them separately. Pan north-east and you can cross between them without the map changing.
Lake Huron has the most complicated outline of the five, and the floor explains why. The main basin is a deep, clean trough, while the north-east is a maze of channels and islands where the bedrock comes to the surface.
Manitoulin, the largest island in any lake on Earth, closes off the North Channel. Below it the plates drop away quickly; above it they stay in the bright shallow bands.
Saginaw Bay reaches south-west into Michigan and barely gets deep at all. It reads gold across almost its whole length, the same signature the western basin of Erie shows.
On the shared scale the main basin runs as dark as Michigan's while the North Channel and Saginaw Bay stay up in the bright bands. That contrast inside one lake is most of what makes Huron odd.
Lake Erie is the shallowest of the Great Lakes by a wide margin. Its average depth is a fifth of Superior's, and the whole lake holds less water than any of the other four.
The shape is three basins in a row. The western basin west of the islands averages roughly 7 m, shallow enough that a hard autumn blow can stack the water at one end and bare the bottom at the other. The central basin is a remarkably flat plain that barely passes 24 m anywhere. Only the eastern basin holds real depth, in a trough south-east of Long Point.
The ridge crossing between Point Pelee and Lorain is a drowned end moraine, a glacier's dumped rubble left standing when the meltwater rose around it. It still works as a sill, holding the western basin apart from the central one.
On the shared scale Erie is the outlier. It never leaves the bright half of the ramp. Put the five lakes side by side and this is the one that reads as a shelf rather than a basin.
Put Lake Ontario next to Lake Erie and they look like near neighbours: same corner of the continent, Erie draining into Ontario over Niagara. Their floors could hardly be less alike. Ontario averages nearly five times Erie's depth.
The main basin is a long, smooth trough with the deepest water toward the eastern end. The floor is strikingly featureless through the middle, which is what a deep basin filled with fine sediment looks like from above.
The east end is the exception. Where the lake narrows toward the St. Lawrence, the bedrock breaks the surface as the Thousand Islands, and the plates crowd into a tangle of channels.
Ontario reaches nearly as deep as Michigan in a fraction of the area, so on the shared scale it sits among the darker lakes in the system despite being the smallest of the five.
All five lakes share one scale and one tile set, so a colour means the same depth whether you are looking at Erie or Superior, and you can pan from one lake to the next without the map reloading. Plates are a metre apart down to 25 m and then widen with depth, to 2.5 m, 5 m, 15 m and 25 m, which keeps Erie's shelf finely contoured without burying Superior under four hundred lines. The grid is 3 arc-seconds, roughly 90 m per cell, compiled from soundings collected since about 1900, and it is the finest covering any whole Great Lake. None of them has been resurveyed with modern multibeam.
Print sheets
Six posters, one per lake and one of the whole system. Each sheet is the shape its lake is, which is why they are not all the same size: Michigan is taller than it is wide and will not go on a landscape sheet. A single-lake sheet spends the whole colour ramp on its own depth range; the system sheet keeps the shared scale this page uses, so a colour means the same thing on all five.
