After climbing up the steep fellside, avoiding a ravine and plodding across pathless, marshy ground, three patches of water, little bigger than glorified ponds, came into view. These tarns, or small mountain lakes, lying just below the Lakeland peak of Glaramara, might sound a feeble prize for so much effort, but they were now officially bagged, the first in my tarnbagging quest to visit every single upland sheet of water in the Lake District national park.

There are hundreds of these "eyes of the mountain", as the landscape painter William Heaton Cooper once described them, scattered across the fells. They range from Broad Crag Tarn, not far from the summit of Scafell Pike, England's highest mountain, the popular beauty spot of Tarn Hows, to the numerous small pools half-hidden among rocks. Some can be half a kilometre long while others almost small enough to jump across.

Tarnbagging is the sport of collecting these tarns and for a small, but growing, band of outdoor lovers reaching them is an end in itself, rather than something to glance at on the way up to a summit. The beauty of it is that it takes you off the beaten track and into quieter parts of the fells – no mean feat during weekends and bank holidays. Rather than traipsing up and down the same well-trodden paths, ingenious circuits can be devised that give the walker a completely different perspective of the fells.

At the moment, tarnbagging is the poor cousin of the much more popular pursuit of collecting peaks. But unlike Munro-bagging – climbing all the Scottish peaks over 3,000ft – there's no great publishing industry around it. Apart from a few blogs there is just one modern guide dedicated to the sport, The Tarns of Lakeland (1995), by John and Anne Nuttall. Over a four-year period, the couple visited almost every speck of blue, listing, sketching, and devising walking routes to link them up.

But what exactly distinguishes a tarn (the name being related to the Norse tjörn, meaning small lake or tear drop), from a pool of trapped water? Some say it must have a permanent outflow, while others that it should be a natural feature, rather than manmade. You could include only those that are named, but that would exclude some of the finest ones, and then of course there's the question as to whether it should be permanent or not. Ultimately it's down to the individual, but I was in need of a definitive list. Thankfully the Nuttalls have counted 335 within the national park boundaries, but it was hard deciding where to begin.

I am someone whose walking trips usually have to include at least one mountain top, which was going to be a hard habit to break. So, for my first foray in to the bagging business, I opted for the Glaramara route, as it combined both peaks and plenty of tarns. Running along from the summit to the top of Allen Crags is a ridge with a necklace of tarns stretched out along it.

Once the steep ascent of the route had been negotiated, the ridge path offers amazing views. In front of you are the central fells with the massive Great End in the foreground, Great Gable across the valley on the right, and the Langdales on the left. But forget about these famous names. As the path undulated the joys of tarnbagging began to reveal themselves. Small ones sitting on grassy platforms, the larger High House Tarn, possibly named after an old mountain hut as well as the unnamed, one of which Alfred Wainwright describes as a "gem".

In the past I would have steamed on by these en route to the next peak, but now I could actually stop to enjoy them. There was though the dilemma of how one actually "bags" a tarn. I started out by dipping my hand in each but then settled for just looking. For some, it's wading in the water, while there are stories of people who insist on having a drink from the pool. Good luck to them. But with the recent growth in the popularity of open water swimming, there are hardened souls who aim to swim in as many of them as possible.

However, this is hardly a new trend. In November 1959, Harry Griffin, the Guardian's legendary Lakeland Country diarist, reported in the paper that two Grasmere men, Colin Dodgson and Timothy Tyson, had "discovered the Lake District afresh" by bathing in approximately 463 tarns. This was a little task they set themselves as a rest from driving up to Scotland each weekend to complete the Munros. It was an amazing feat, especially as Tyson was aged 75 when he finished. It took them eight years to do and sometimes they had to break ice before entering the water. Some, though, must have been little deeper than a bath tub.

Following on in this tradition is Peter Hayes, an outdoor swimmer who devised the concept of swimhiking. In his 2008 book, Swimhiking in the Lake District and North East England, he lists at least 30 walks that include tarns. Hayes though, has specific rules about bagging, saying: "You cannot merely swim in the tarn, you have to cross it (stopping to appreciate the view in the centre)". Of course, with one of his own specially designed "swimsacs" you can get to the other side, put on your clothes and continue walking to the next tarn.

Back on the route, after leaving the summit of Allen Crags (but not before scurrying down the hillside to investigate a couple of tiny pools I'd spotted on the map), the path joined the main mountain pass track of Esk Hause. Before heading back to the valley though, there was one final tick I had to get on this tarnbagging expedition.

Sprinkling Tarn, near the base of Great End, is a much larger stretch of water than the pools on the ridge. It's a perfect upland tarn, sitting at the very hub of the mountains, reflecting the amazing scenery around it and containing trout. It's hard not to disagree with Heaton Cooper, who in his 1960 book, The Tarns of Lakeland (essential background reading for any aspiring tarnbagger), described it as "the most completely satisfying of all the tarns of the Lakeland".

After bagging at least seven tarns (a few together usually counts as one), I was certainly satisfied. As I turned to go though, I noticed a small patch of water just beyond Sprinkling Tarn – another one for the collection.

• Richard Nelsson is the editor of the Guardian Book of Mountains