A little digging can help you understand the shape of things when we talk about digging the Ohio Canal (later called the Ohio & Erie Canal). We often speak about the canal’s minimum dimensions being 40’ wide at the top, 26’ wide at the bottom, and 4’ deep. It’s a trapezoidal shape, referred to as a canal's prism (imagine an upside down triangle with the lower third lopped off). It could be wider and deeper at any section, but it had to be at least those dimensions in every section (except for the locks). I’ll try to give you a down-to-earth explanation as to why.
Most construction projects must address this groundbreaking primary concern: How much is it going to cost? You want high quality, but at a reasonable price. One way to cut costs is to copy someone else’s high quality work. The Erie Canal (NY) had canal locks 90’ long by 15’ wide, and that canal worked great and had started to prosper. Ohio Canal engineers decided to design their locks to be 90’ long by 15’ wide. Brilliant! (There’s a lot of engineering that went into why those dimensions were chosen for the locks, but we’ll skip that in the name of blogging economy.) Canal boats grew and designs changed somewhat over the years, but they always had to be no larger than about 85’ long and 14.5’ wide in order to fit through the locks.
Take into consideration that boats needed to pass each other on the canal. The canal had to be wide enough and deep enough to safely complete those maneuvers. The expectation was that a fully laden canal boat would have no more than about a 3’ draft (3’ of boat below the water line). So, the design of the canal called for a minimum water depth of 4’, allowing for 1’ of water between the bottom of the boat and the bottom of the canal, and wide enough at the bottom to allow two boats to pass each other safely from the top of the water down to at least 3’ below the water line.
So why not just make the canal a squared-off ditch about 35’-40’ across? First, when digging the canal you want to move the least amount of material possible because everything you move costs you money. Next, you want to keep the canal as small as practicable because you’ll need enough water to keep the canal filled. And, because you’re filling your ditch with moving water and moving boats creating wakes (waves created by the boat moving through the water), erosion of the canal banks is a serious concern. Throw in the need to line areas of the canal bed and banks with "puddle" (a mixture of clay and gravel) that will stop the water from seeping through porous ground, and you’ve got yourself an engineering dilemma. Vertical walls would be highly susceptible to erosion, and material could fall into the canal, impeding the flow of water and traffic. It would also be difficult to adhere lining material to a vertical surface.
The solution to the considerations above is to gently slope the banks of the canal to reduce erosion from the flow of canal water and the wakes off the boats. The lining material rests atop the gentle slope of the canal wall.
The Erie Canal (NY) had minimum dimensions of 40’ across at the top and 28’ across at the bottom, creating a prism that widened 2’ on each side for every 1’ of elevation. During construction of the Ohio Canal, they decided to go with a narrower 26’ minimum bottom width, which created an even more gently sloped bank (widening about 2.5’ on each side for every 1’ of elevation), further reducing erosion while still allowing enough room for two canal boats to pass. Sometimes, you can improve upon the original while saving a little money.
If you want to learn more about the Ohio & Erie Canal, an excellent resource is “Ohio’s Grand Canal: A Brief History of the Ohio & Erie Canal” by Terry K. Woods.