The Summit

As most of our readers will know, canals (generally) have locks interspersed along their lengths, the purpose being to help a boat go up hills (or down).

Now, bearing in mind that almost all canals round the country are linked, once you get to the top of a hill, then what ?  Well, you go down again, the other side. The top level is called the canal summit. Filling a lock requires 50,000 gallons of water to be run into it, and the same amount run out to go down the other side – total of 100,000 gallons of water each time a boat goes over the summit; that’s one million gallons of water for just ten boats, but where does it all come from ?

A river would seem an obvious source, but remember we’re at the top of a hill. Pumped maybe ? – most canals were built before steam became widely used, so no. Waterwheel driven pumps again need a river, so a bit restricted there too.  The solution was to build reservoirs at the summit level, holding sufficient water to allow for even the busiest boating activity when the canals were the the main freight routes up and down the country.

We’re at the ‘Tring Summit’, the highest point on the Grand Union Canal (opened 1805) as it passes over the Chilterns, and up here there are four large reservoirs, dug out and with dams built using nothing more than shovels !

Here’s one of them –

The drawing below, shows the four reservoirs. The main line of the Grand Union runs from the right of the map, to the top. The arm going to the bottom is the Wendover Arm, and is the main ‘feeder’ receiving water from two of the reservoirs, the other two feed the main line directly.  The arm going off to the left is the Aylesbury Arm – more on that in the next post.

The four summit reservoirs

We see a few herons on our travels, but they usually fly away as soon as you get anywhere near them, this one at a lock didn’t though, presenting an opportunity for a photo for you.

A very tame heron