Maintaining the level of the canal is vitally important. Obviously, if the water is too shallow, boats get stuck on the bottom. But if the water is too deep, it puts too much pressure on the banks and lock gates; and it can lift swing bridges off their mountings. So, along the canal there are slots which lead to culverts or rivers and allow water to run off. The water level cannot rise above the level of the slot .But that water isn’t always wasted. The slots under Coffee House Bridge used to fill the reservoir of Bootle watermill. Nowadays, they just run off into the Rimrose brook.
The slots at the top of of Stanley locks fill the pound between the top lock and the next. That pound has slots that fill the next pound down, and so on.
There is an added bonus. The canal between Appley Locks and Liverpool is 45Km long and over 10m wide. Storm drains along that route (over)flow into the canal, which then spreads the water to all of the runoffs along that stretch. Additionally, the very large surface area of the canal is able to absorb a great deal of flood water. It’s much prettier than an underground pipe, too. In fact, it’s a Sustainable (Urban) Drainage System.