These are two inlet gates (at the bottom of Big Slackwater) letting water in for the canal. These are just to the "east" of guard lock No. 4. The two paddles (raised and lowered by the rods in the picture) are buried under the ground. Originally there was a pond (coming from an inlet weir) that fed these inlet gates, but the whole thing is now buried. These fed a tunnel which fed the canal just after guard lock no. 4.