Web Author's Notes:
Here are two illustrations of the main dam designed by Col. Joseph Bailey and built by the 16th Ohio and other troops just north of Alexandria, Louisiana, on the Red River.  The sketch shows the Federal gunboat fleet passing through the purposeful break on May 13, 1864.

The purpose of the dam was to raise the water level of the river, about 10 miles upstream, to allow the Federal gunboat fleet under Adm. David Porter to cross over the "upper falls" (rapids) and move down to Alexandria, just above the dam. Once the fleet made it past the upper rapids and formed near the dam, as shown below, a small portion of the dam would be broken and the boats could then cross in the high water passing through the break and move past the "lower falls" into the deeper water below.

After some difficulty with the main dam holding during construction, Bailey decided to build some additional "wing dams" near the upper rapids.  This would take some of the pressure off the main dam and assist in the water level at the upper rapids.  The plan ultimately worked and saved the entire gunboat fleet from having to be destroyed to keep it from falling into enemy hands.

Also, see a map of the Red River with Bailey's dams.