Why is this canal so shallow?

In this video I moor up on an aqueduct, visit two pubs to see which one is the best, beach-comb by a river, explain how my water tank works, and why the Macclesfield canal is so shallow (with my artist's interpretation of a cross section of the canal).

Why is the Macclesfield so shallow? More info from the Macclesfield Canal Society:
“...the water level in the top length from Marple to Bosley (and including the top length of the Peak Forest Canal) may be as much as six inches lower than normal during the summer months! This leads to difficulty in mooring away from wharves.
The Macclesfield Canal was built as a 'state of the art' waterway. This meant that Thomas Telford, who laid out the route and designed the canal, employed all of the lessons he had learnt on earlier canals. As a result the Macclesfield 'suffers' from the fact that it was designed very specifically for commercial carrying - how was Telford to know that leisure would soon become its principal purpose? 
The Macc has a very shallow and tapered bottom. The deep channel is just wide enough for two boats to pass in the centre, indeed, deeper draughted boats would question use of the word deep... The tapers from the deep channel to the bank ensure that the depth at the sides is insufficient to moor at many places along the canal. The canal was designed for boats to moor at the wharves and nowhere else. Although modern steel narrow boats are shallower draughted than a loaded working boat they still find it difficult to moor close in along much of the length. Fibreglass cruisers don't usually experience these difficulties.” - go to https://macclesfieldcanal.org.uk/olds... for more history on the canal.

Filmed on iPhone 11 Pro, edited on Lumafusion for iPad

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