Canal and river four miles above Harpers Ferry at upper lock.

The boat from which this picture was taken is approaching lock No. 37, which was called Mountain Lock and is considered one of the most scenic locks on the canal. Note how close the canal is to the river in this area. Where hills come directly down to the river, it was not possible to put the canal back away from it in an effort to make it less vulnerable to flood waters. The engineers would, however, attempt to keep the canal as high above the river as possible. The requirement that the canal be virtually level between locks often made that difficult, as gaining elevation meant building a lock. Determining where to put the locks was one of the major challenges for the engineers that designed the C&O. Some canals elsewhere are built to climb up and down ridges (by locks), but the C&O never does so, instead using 74 locks to raise its level at tidewater in Georgetown the approximately 608 feet to the terminus basins in Cumberland without ever having to climb over and descend from a ridge along the way.