This cut-away shows how canal barges pass through this "bridge for boats.". If it looks tight, it is. There was usually about three inches on either side. The weak part are the walls which were subject to the weight of the water, occasional blows from boats but most of all, floods. Aqueducts were usually built at the mouth of creeks and during flooding, the waters would flow over the aqueduct walls. Anything floating in the creek would hit and weaken the walls.
The Monocacy Aqueduct has been repaired and if you overlook all the external bracing, it's in good shape. The three arches of the Antietam Aqueduct is one of the best preserved with both walls intact. The Seneca Aqueduct has most of an entire arch missing. Conococheage and Licking Creek have only one wall. Tonoloway is missing both walls and has a wooden walkway erected in its place to carry foot traffic. Before its restoration, the Catoctin Creek aqueduct only had part of one arch left and was spanned by a footbridge..