Sketch of tree dam which accompanied Colonel Bailey’s report (U.S. War Department 1891-1895:Plate 53-3).
The archaeological investigation of the tree dam was completed in a manner similar to the excavations at the crib structures. But here a trench excavation unit was dug. This trench was 22 feet long and 5 feet wide, and it was positioned parallel to the flow of the river. During these investigations, the river began to rise, and when the excavation unit was finally abandoned, the archaeologists were working about 2 feet below the water level. The field crew was successful in reaching that depth only with the aid of a water pump. Archaeologists had hoped to excavate a slice of the dam completely down to its base, but attempts to excavate deeper were halted when the pump could not keep out the incoming water.
Archaeologist David Kelley drawing map of logs uncovered during the tree dam excavation.
The tree dam excavations revealed that both pine and hardwood logs were used and that the tree bark was left intact. The tree limbs had been cut off, but by observing the knots on the tree trunks, archaeologists were able to note the direction in which the trees were positioned. They found that many of the trees were positioned with their tops downstream, exactly opposite of that shown in historical illustrations. Also, all of the upstream ends of the trees had been trimmed of their branches, and their tips had been pointed with an axe. Spaces between the logs were filled with sand and mud, and the entire structure was covered with brick and stone. Interestingly, the archaeologists also found a hewn, octagonal, wood column among the logs. The upstream end of the column had been rough cut, seemingly to fit into that particular spot in the tree dam. The column was most likely a mast from a riverboat.
Portion of the tree dam exposed during the low water in August 1984.
Together, the crib and tree dam sections did not cross the entire 758-foot riverbed. A 150-foot gap existed between the two dams. To close this gap, four coal barges were used. While the remains of these barges were not found in the archaeological excavations, historical photographs provide a fairly detailed picture of their appearance. These 24-by-170-foot barges were sunk in the gap, lying lengthwise with the current, and more stones, brick, and iron rails were used to anchor them. Braces and ropes, anchored to the riverbanks, were also needed to secure the barges against the rising water pressure.
Bailey directed that the second barge from the Alexandria side be only partially filled with anchoring materials. This was the barge that he hoped either to ram or blast out of the way, creating a flood that the gunboats would ride like kayaks. As it turned out, Bailey’s idea worked, but not exactly as he would have liked.