Just what is it that they do not want me to notice.?





The Complex World of Locks, Gates, and Dams

Anyone who learns about the canal comes across terms like lift lock, river lock, inlet or outlet lock, stop lock or stop gate and guard lock or guard gate. The fact that these terms are frequently misapplied adds to the confusion most people have about them. Additionally, there is not always agreement on the best term for a given structure, and the nature of these structures sometimes makes more than one term appropriate. I’ll try to sort them out, but, as always, I invite anyone who so desires to add their opinion or further information.
Although most people assume they know what a lock is, a minimal description is important to explain some of the distinctions made below. So: minimally, a lock is a chamber that holds water between two gates that allow the water level within the chamber to be changed and accessed by water craft. Locks also allow for a change of elevation from the water level at one end of the chamber to a different level at the other. That change in elevation represents the “lift” of the lock.
Lift Locks

While all locks provide some lift, the term “lift lock” is usually applied to those that are on the line of a canal and serve primarily to overcome its change in elevation. On the C&O Canal, we generally consider the consecutively numbered locks from Lock #1 in Georgetown to Lock #75—the last lock before Cumberland—as the C&O lift locks. The average lift of these locks is 8 ft. These 74 locks overcome the approximately 608-foot difference in elevation between the two termini. (There are only 74 locks in this series because Lock #65 in Tunnel Hollow was not built, resulting in the amusingly-numbered locks #63 1/3rd, #64 2/3rds, and then #66.)
Other locks on the canal could raise and lower boats, although the term “lift lock” is not usually applied to them. One example is TIDELOCK, at mile 0 next to the Thompson Boat House. It allowed boats to pass between the Potomac and the basin formed by a dam across the mouth of Rock Creek, which created a pool about 4 ft. above the mean level of the river. Another example is the RIVER LOCKS category, described in a separate section below.
Inlet, Feeder, or Guard Locks
Inlet locks are so named because they let water into the canal, and these structures (in one case, actually not a lock but a gate) are given the number of the dam with which they are associated. Five inlet locks (#3, #4, #5, #6 and #8) are located on the pool behind their respective dams. Inlet lock #2 is downstream from Dam #2 at the end of the channel bringing water from the pool behind the dam. It is also located adjacent to Lock 23 (Violettes Lock). The Dam #1 inlet structures are also below the dam near the end of their inlet channel, and they will be discussed later under gates. Inlet Locks #3 and #6 also have inlet channels, but the locks are behind the dam with a feeder channel below the locks to carry water to the main channel of the canal. The inlet or guard locks are sometimes also called river locks although that term is more properly used for a type of lock that will be discussed later.
The first lift locks at the upper end of Big and Little Slackwater (Lock #41 and #45 respectively) are often called inlet locks because they admitted boats back into the canal as it resumed. However, these locks are included in the lift lock numbering sequence and—in addition to passing boats between the canal and slackwater areas—their major task is to quickly raise the canal above the mean river level and thereby protect it better from high waters. Finally, please note that the term “inlet” is also applied to structures that let water into the canal, while Locks #41 and #45 locks allow the water that has come down the canal to flow out into the river.
Dams
The Canal Company built 6 dams (#1, #2, #4, #5, #6, and #8) to create pools of water that could usually feed the canal even during periods when river levels were low. (Dam #7 was not built.) The Canal Company was permitted to draw water from the pool behind a pre-existing government dam (designated Dam #3 by the Canal Company). That dam, about 1.6 mi. upstream from Harpers Ferry, had been built to provide water to the US Armory canal on the Virginia (later West Virginia) side of the river. Originally, all the C&O dams were constructed with wooden cribs filled with rubble-stone. In the 1850s and 1860s, however, masonry dams replaced the original Dams #4 and #5. All the C&O dams, except for #4 and #5, are now largely gone, although there are modern government dams short distances upstream from the locations of the old canal dams #1 and #2.
Guard Walls, Guard Locks, and Guard Gates
Because structures that let water into the canal behind dams are at river level, these are points where the canal is extremely vulnerable to high waters. To protect these structures and the canal below them from flooding, they are always associated with a high GUARD WALL through which they pass. The lock or gate at the river end is therefore as high as the guard wall and is an important part of that protective structure, justifying the use of the term guard lock or guard gate.
The inlet structures for Dam #1 do not include a lock, but are rather an upper CONTROL GATE that now serves as the primary inlet structure, and below it, a GUARD GATE adjacent to Lock #5. The channel on either side of the guard gate has substantial walls—those downstream from the gate have the appearance of a lock chamber, but do not serve as such because there is no gate at the lower end. Also, note that the guard wall has a concrete coating between the guard gate and Lock #5 to help prevent the wall from washing out when flood waters overtop it.
At Dam #4, a guard wall extends between the river and the canal from the dam’s abutment to the inlet lock #4, 1.1 mile upstream from the dam. The inlet lock let water into the canal from the pool behind the dam, but also passed boats to and from Big Slackwater. The current path for hikers and bikers is on the guard wall, rather than on the historic towpath that is lower down, beside the canal. (Sections of the true towpath along this 1.1-mile stretch of canal are not maintained and are impassible.) Where the guard lock goes through the guard wall, the wall makes a sharp turn to connect with the hillside. There is also a supplementary water supply inlet that comes from a control gate a short distance upstream from the lock. It passes through a culvert under the guard wall, into a flume on the land side of the lock, and then into the canal. The upper ends of the #4, #5, #6, and #8 inlet/guard locks are now sealed with a concrete wall that significantly alters their historical appearance.
River Locks
Three river locks made it possible for boats to pass between the canal and the river. All three were located at points where considerable commercial traffic was anticipated but where no guard or inlet lock was located. River locks also provide lift, the amount being determined by the difference between the level of the canal at that point and the mean level of the river below. The water to fill these locks came from the canal—although there would always have been some inflow from the river, the amount depending on river levels. At each river lock, a bridge carried the towpath over the entrance to the lock. These bridges had to be high enough to allow boats using the river lock to pass under them, yet not so steep that mules pulling boats along the main canal channel would be unable to go up and over them.
The Goose Creek river locks are 2/10ths of a mile below Edwards Ferry, and are the only STAIRCASE LOCKS on the C&O. In a staircase arrangement, the lower gate of the upper lock serves as the upper gate of the lower lock. Because of their uniqueness on the C&O, it is most unfortunate that these locks have fallen into ruin, making them difficult to interpret and unsafe to study closely. These locks are opposite the mouth of Virginia’s Goose Creek. They provided a connection with the Goose Creek and Little River Navigation system that was intended to serve a large and resource-rich region with many mills. Unfortunately, the system was never completed, and it is likely that the locks were very infrequently used.
The second river lock was opposite Harpers Ferry, just downstream from Lock #33. When it was built, the Canal Company planned to construct a mule bridge across the Potomac so that the C&O freighters could be taken across the Potomac to a wharf at Harpers Ferry. However, this plan failed and the river lock was not well maintained. Due to the tendency of lock walls to slump inwards, the large C&O freighters could not fit into the lock by the time of the Civil War. There is some evidence that it was used by smaller boats coming down the Shenandoah. It was filled in following the 1889 flood, if not before.
The third river lock is upriver from the Boteler Mill dam, which was located about a tenth of a mile downriver from Lock #38. The lock served as an important access point for the shipments from the Botelet establishment, a grist mill that after 1828 turned mainly to the production of hydraulic cement. When the mill was demolished and the dam abandoned, the river fell to its natural level, leaving the lock high and dry.
Stop Gates
There are seven stop gates on the canal. These are frequently misnamed stop locks or guard locks although—like the guard gate beside Lock #5—they are only a gate. These can be very simple structures, consisting of nothing but two walls or abutments with vertical grooves into which heavy planks can be dropped to hold back water in the level behind the gate. Stop gates can limit the amount of water passing, or hold it back entirely in the event of a breach in the section below or a need to drain and repair the lower section.
The first and second stop gates—at Great Falls and Dam #4 respectively—could well be called guard gates, as both provide for the towpath and canal to pass through a guard wall. These structures are for flood protection as well as routine maintenance. The Great Falls gate, located upstream from Lock #16, provides for passage of the canal and towpath through a guard wall or levee that stretches for 500 ft. from the adjacent hillside toward the river. After the 1996 floods, the Park Service strengthened the gate and also its beams, which are stored beside the towpath and placed in the gate when flooding threatens. It is hoped that the gate and guard wall will divert future flood waters back into the main channel of the Potomac, protecting the canal’s vulnerable Widewater area.
The second stop gate, at Dam #4, is the first of five in the Cumberland Valley (which is known as the Shenandoah Valley south of the Potomac). The guard wall that this gate passes through extends from the top of the dam abutment over to the road along the canal berm. The gate has substantial masonry abutments on either side. The Park Service rebuilt a winch house on top of it to house the gate’s beams and the mechanism for lowering and raising them. Today, a bridge around the exterior of the winch house carries pedestrians along that guard wall to the top of the dam abutment.
The other stop gates in the Cumberland valley are: at McCoys Ferry at mile 110.29; at Fort Frederick, below Big Pool where a bridge carries the road from the fort across the canal; upstream from Big Pool, at mile 114.15; and near the lower end of Little Pool, at mile 119.71. These four stop gates occur in a distance of 9.5 miles, bearing testimony to the frequency of berm and prism failures along this especially vulnerable level.
The seventh stop gate is about one mile below the Western Maryland Station in Cumberland, and its primary purpose was to retain water in the terminus basins when the canal below was drained.
-- Karen Gray
NOTES: Two sources for canal terminology are: Thomas Swiftwater Hahn and Emory L. Kemp, Canal Terminology of the United States, Monograph Series Vol. 5, 1998: The Institute for the History of Technology and Industrial Archeology, West Virginia University, Morgantown, WV; and Terry K. Woods, The Ohio & Erie Canal: A Glossary of Terms. 1995: Kent State University, Kent, Ohio. The mileages in this article are from Hahn’s Towpath Guide.
The source of this article is the C & O Canal Association’s newsletter, Along the Towpath, Vol. 38, No. 2, June 2006.