When I was a child growing up in Putney, this was the time of year to decide for Oxford or Cambridge (in terms of who you supported in the Boat Race) - on no particular basis, for no particular reason, just because it was the big local event, with all the national media and thousands of spectators in attendance. But quite why it was so important was - and is- never quite clear. You would only ever catch a glimpse of the race: with only two crews in it, once they were gone, they were gone; and even once you could watch the whole thing on television, it was clear that all too often it would turn into a simple procession.
The only excitement was to watch the enthusiastic spectators who had gone down to the water's edge without realising that the flotilla of umpires' and spectators' launches would unleash a substantial wash all over their feet. Later, those of us who lived nearby would make an event of watching the start in person and rushing home to see the finish on TV (we could still make our own entertainment, you see).
Eventually I learnt about the Head of the River Race: this year's event was held yesterday. Over 400 crews race against the clock over the same four and a bit miles of the Oxford and Cambridge Boat Race, but in the opposite direction. Competitors range from Olympians to schoolboys and "Sunday morning gentlemen" crews, from all across the country, and indeed from other countries. I've rowed it myself several times (but decades ago, moving straight from the schoolboy to "Sunday morning gentleman" class, as it were), and it's by far the better spectacle, especially once the colourful traffic jams build up of crews waiting to come ashore and load up boats on to trailers, or to start the long row home to their own boathouses.
The difference - apart from the sheer numbers of people and the length of the thing (you could see the first hundred or so crews come home, go and do some shopping or have lunch and come back to find plenty more still gasping their way over the line) - is in the spectators. This is a sort of community event: each crew has its own supporters here (I heard plenty of French, German, Spanish and Italian as well as a wide range of British accents), the boathouse and pub balconies are packed with partisans who mostly know something about what they're watching, and there are food-stalls and people selling things that rowers find useful.
Incidentally, I used to live in a flat behind the boathouses in this clip - I walked past it, as well as my childhood home, and felt not a pang of nostalgia.