Saimaa

Saimaa is the largest lake in Finland, and it isn't really a lake: it's a labyrinth. More than thirteen thousand islands split by narrows, bays that look like rivers, and basins joined by nineteenth-century canals.

The crossing cuts through its south-western corner by packraft, an inflatable boat that folds into a backpack. From Savitaipale to Mikkeli: 80.53 kilometres in 7 stages.

It is also the last refuge of the Saimaa ringed seal, which lives nowhere else on Earth. About five hundred are left. The rule is simple: two hundred metres away, steady course, never follow one.

It all started with a GPX file downloaded from the internet: a kayaker, June 2018. A track is what somebody felt like paddling on one particular day, and between our two ends it measured 104.5 kilometres.

It went round the Toijola peninsula on the outside: twenty-nine kilometres to gain twelve. Through the middle run two canals blasted out of the rock between 1874 and 1877, and they are still open to boats.

And it left Lake Kuolimo down the four drops of Partakoski. Kuolimo has a second outlet, Kärnäkoski, where the water falls all at once and can't be paddled, but the carry is a hundred and six metres past an old mill.

Drop the out-and-back detours, cut through the middle, and the crossing comes down to 80.53 kilometres. That's 24 kilometres less, more than a whole stage, and not a single rapid.

The 2018 track ran north to south. Thirty-two Septembers of weather reanalysis said otherwise: heading south, the wind is in your face 36 % of the daylight hours; heading north, only 16 %. The whole trip was reversed.

That puts the finish in Mikkeli, with the railway station two hundred metres from the slipway. The fixed day becomes the first one, set by a timetable, instead of the last, set by the weather.

Savitaipale means "the clay portage": boats were dragged from one lake to the next here long before us. It's the last shop for six days, so everything that will be eaten leaves from this beach.

The first day is short on purpose: you arrive by bus and still have to build the boat. The line hugs the eastern shore and avoids the open crossing of Isoselkä. It ends at the mouth of a seven-hundred-metre channel nobody had documented.

The carry: a hundred and six metres from water to water, between the mill and the fortress Suvorov built in 1793, when this was the border between Sweden and Russia. A few kilometres on, Russian trenches from 1914, blasted into bare rock.

Then the labyrinth begins: sheltered channels between islands, not a single service for fifteen kilometres. At the end of the day comes the most open stretch of water on the whole route, and that's where you decide to cross or to wait.

The longest stage, 16.56 kilometres. This is core seal habitat, and twice the route runs inside the Ristiina commercial fairway: four metres of draught, log rafts up to a kilometre long. They move at under two kilometres an hour. You don't wait for them; you overtake.

In 1789, during the war between Sweden and Russia, this narrows was sealed with a barrier. Then you come out onto Yövesi, open but less than it looks, and sleep on its shore.

Astuvansalmi: rock paintings some five thousand years old, painted from the water when the lake stood several metres higher. The old track left them nine kilometres away; now the route passes eleven metres from their jetty.

And the inside passage of Toijola: Varkaantaipale and Kirkkotaipale, dug in 1874–77. Speed limit, no wash allowed. The stretch that saves a whole stage turned out to be the best protected of the entire crossing.

The short stage, 9.22 kilometres, with the day's open water in the middle of Louhivesi. This is the true northern edge of the seal's range. And somewhere on this lake falls the most important decision of the trip: carry on or turn back.

Six days in, the first shop: Ristiina. From Löydö beach it's four kilometres on foot, or a bus. Everything you don't eat on the crossing is weight you've paddled.

Siikasalmi is a channel dredged in 1833, used by the tour boats out of Mikkeli. The last night is spent on Kyyhkylä beach, with dinner, a roof and a bus a few hundred metres away.

The last open water, Annilanselkä, and then the sheltering islands. 7.17 kilometres to go. You enter the town through the narrows under the Pursialankatu bridge, keeping to starboard.

The slipway at Mikkeli harbour. Two hundred and five metres to the railway station.

The dashed lines on the map are official fairways, and they can be measured. The route runs 11.27 kilometres inside one, 8.39 of them in shipping lanes. Of the 41 times it meets a fairway axis, only 5 are crossings; the rest you don't cross, you travel along them, looking back.

The point of no return isn't a point: it's a band. Before kilometre 27, turning back is cheaper in any paddleable wind; past 54, carrying on is. In between, that morning's forecast decides.

13 exits by road, measured one by one. In violet, the ones that leave you a walk from a bus; in grey, the ones where you have to call someone. The violet doesn't start until Ristiina.