The point of a canal – a man-made waterway – is to control an amount of water and where it flows so that it can be used reliably for transportation, recreation, irrigation, etc. You need enough water to achieve and maintain the desired depth, but don’t want too much water as that can lead to erosion of the canal walls and potentially the inability to maintain a sufficient amount of water in the canal.
The Ohio & Erie Canal had a prescribed minimum depth of four feet. In order to keep four feet of water in the canal, the walls of the canal had to extend more than four feet above the bottom of the canal. As long as enough water was available and the only water entering the canal was the water allowed in by the man-made controls (gates at lakes and dammed-up rivers), keeping the depth at four feet was not too great of a problem. But “into each life some rain must fall,” as Longfellow said, and precipitation can quickly increase the amount of water entering the canal. How does one manage all that extra water?
Every so often along the canal walls is something called a weir. There are several types of weirs for several different purposes (all involve diverting water somehow), but we’re going to focus one of the most simple of weirs.
Along the Ohio & Erie Canal, the weirs are essentially concrete or stone walls that are built to rise four feet above the canal bed. Weirs are built into the canal walls and allow any water rising above four feet high to pour over the weir and into a spillway that usually leads to a river or stream at lower elevation. That’s how you can usually keep your water level to no more than four feet deep . . . as long as there isn’t too much precipitation.
Weirs help maintain the integrity of the canals by keeping rising water from running over and eroding other parts of the earthen canal walls. Water that runs over the weirs is controlled (as much as possible), as the engineers dictate how and where the water goes.
So next time you’re along a canal and you see a low concrete or stone (or even wooden) wall, you’ll hopefully have a better understanding of “what weir talking about.”