There is one clearance that stops you, and several that only change your route. Most people quote them as if they were the same thing.
There is no way around it. Every other clearance figure below costs you a route option; this one costs you the Loop.
All of these have a way through. Only the first one doesn't.
| Clearance | Where | If you're taller |
|---|---|---|
| 19'6" | Illinois River, mile 300.6 | Nothing. This is the one. |
| 17'0" | Chicago River | Take the Cal-Sag Canal instead |
| 20–21' | Erie Canal to the Oswego Canal — the usual New York route | Not the binding number — anything that clears the Illinois bridge clears this too |
| 17'0" | The "Triangle Loop" — Lake Champlain, the Chambly Canal, the St. Lawrence | Take Oswego. You lose Montreal, Quebec and the Rideau |
| 15'0" | The Erie west to Lake Erie, then the Welland | Take Oswego. You lose the western Erie |
| 22'0" | Trent-Severn Waterway | Not usually the binding number — but know it if you're tall |
Sources give the Oswego route as 20 feet in some places and 21 in others. Plan for 20 and you're covered either way. The two 17-foot figures are unrelated to each other — one is a river in Illinois, one is a canal route in New York.
This is the part almost nobody says out loud.
Every number above is a planning figure, and they move with the water. A charted clearance assumes a pool level. Pools rise with rain and with how the system is being managed, and the number you actually pass under on the day may be lower than the one you planned around.
The Erie is the clearest example. The bridges themselves — the pair past Three Rivers Junction — measure 15'6", and AGLCA advises planning for 15'0". We use theirs. But the canal's own lowest measured clearances are 15'7.44" at bridge E-123, normal pool and 14'8.4" at Guard Lock West, MAXIMUM pool — and that last figure is nearly a foot below the number most people are working from, and below both of the ones above.
So "this boat can't do the Erie" is never a verdict. Height puts a route in question. The figure to check is today's pool, not the chart — and the people to ask are the lockmasters, who in some places can lower a pool to get a boat through.
The 17-foot Triangle figure works partly for that reason: it depends on the pool being dropped at a railroad bridge north of Champlain Lock 3 that sits at roughly 15'6" otherwise.
For at least one bridge on the route, the clearance is measured electronically and published every hour. One Looper found the feed was down — so he read the physical board at the bridge instead, which showed a little over fourteen feet. He went through slowly, with about six inches to spare, because he had re-measured his own air draft and knew it to the inch.
In the same conversation he mentioned having seen published figures for that clearance ranging from fourteen feet to seventeen. Three feet of spread in the public record, an official live gauge that happened to be down, a board at the bridge, and a number he had measured himself.
Check the published figure. Read the board. And know your own number. He needed all three, and the one that saved him was the one he controlled.
Where most people get this wrong.
Listings usually do give an air draft. What they rarely give is a figure anyone has confirmed on that particular boat — and there are three reasons it drifts:
The only figure worth relying on is one someone measured, on that hull, with everything lowered that can be lowered. A tape measure doesn't care how much you want the boat.
The 19'6" figure and the route-option clearances are AGLCA's, published on greatloop.org in their FAQ and route-segment pages. The measured Erie clearances are the canal's own. Both are public, and we've linked them so you can check us. What we added is putting them side by side and saying which is a wall and which is a route choice — we haven't measured a bridge.
If we've got something wrong here, tell us and we'll fix it — support@caniloop.net. A correction from someone who has run it is worth more than anything we can write.