This dam is known as a Wicket Dam, a style that is common on both the Illinois and Ohio rivers. It is a European design that was popular in the late 1800s and early 1900s. The dam consists of a series of paddles (wickets) that are attached to the river bottom with a hinge on one end. The paddles can be pivoted upwards to create a blockage in the river to raise the river level. Braces snap into place to hold the paddles in place when they are raised. Since the paddles actually block the river and create a waterfall, navigation traffic needs to use a lock to pass the dam. The lock at LaGrange is the standard size for the Upper Mississippi, 110 feet wide by 600 feet long.
During periods of high water, the river can completely submerge the paddles. In this case, the braces are unlocked, and the paddles hinge back and rest on the river bottom. This allows the high water to flow freely. Since the river is no longer blocked, navigation traffic can bypass the lock and sail past the dam using the main river channel.
Wicket Dams require a great deal of effort to make small changes in the water level of the pool behind the dam. Each paddle has to be individually adjusted to raise or lower the pool water level. At some point in the past, a Tainter gate was added to this dam. A Tainter gate is a large piece of steel that can completely block the river for the width of the gate. The steel plate is mounted to pivot arms, and the plate can be pivoted up to open the gate to allow the river to flow. A mechanism is used to raise a Tainter gate, and a lock mechanism is used to keep the gate in a given position. When the lock is released, the weight of the gate caused it to lower into the fully closed position.
Tainter gates are easy to raise and lower and can be set at nearly any position between fully open and fully closed. The result is that the wicket dam is set for one water level, 10 feet of waterfall, and the Tainter gate is used for fine tuning the water levels. This has resulted in a huge labor savings and gives a better control over water levels and flows.