If you take the subway or S-Bahn to Ohlsdorf, you can start the tour north of the Stadtpark and Alsterdorf. Here the Alster is still a stately river. Bridges span the water, parks and nature reserves border the course. Even here, the cyclist must be alert and always change the bank at the right time in order not to lose sight of the course of the river. But the route is always a feast for the eyes. Yellow arrows on trees point the way to the source (with a "K" - presumably for "Kayhude" - provided). But the markings are often well hidden, and a well-signposted cycle path would possibly miss the adventure. In the district of Poppenbüttel there is a weir. A short break in the rippling water is just as good here as it is in the numerous small cafés that line the path. Soon after, the hobby cyclist left the city of Hamburg. The Alster is getting narrower, the rural towns are becoming more tranquil. Here, at the latest, good maps are essential, even if the path initially follows the course of the Alster more or less closely. Thatched houses, lush green meadows to the left and right of the route leave the hustle and bustle of the city far behind the cyclist. The idyllic town of Wakendorf II is passed through - the destination is getting closer. The Alster is now only two meters wide at times, other smaller towns are crossed and it is not always easy to keep your bearings: where has it gone, the Alster? Passers-by give various useful tips ("It's that way to the beer", "You could fight your way through the fields", "There was an old railway line that leads right to the source"). The best thing to do as an "explorer" is to rely on your maps again and finally head straight for the village of Rhen - and there the street "An der Alsterquelle". And so the cyclist reaches the small town of Henstedt-Rhen. Signs point the way to the source of the Alster. In a wooded area, a wooden plaque describes the beginning and origin of the river as "collected source in a spring bog". The spring itself: quite unspectacular, but nicely surrounded by masonry and provided with an iron plate. If you don't want to swim back to Hamburg, you can cycle to Norderstedt, about ten kilometers away.