The 74 lift locks came in several varieties. A boat traveling the length of the canal would also go through two guard locks: at Big Slackwater and Little Slackwater. Locks were typically 100 feet long, 15 feet wide, and 16 feet deep. They lifted boats between 6 and 10 feet. Typical freight boats were 92 feet long, but the rudder could be turned to fit into the few shorter locks, such as Lock 28 (Point of Rocks), which was 90 feet, 9 inches. The only lock shorter than 90 feet was the troublesome Lock 36 at 89 feet, 11 inches, being the only lock less than 90 feet in length. Locks were often whitewashed to make them easier to see at dawn or dusk. The locks were built on a foundation of 12 inch x 12 inch timbers, spaced about a foot apart, laid longitudinally below the walls, overlain by traverse timbers, also 12 inch x 12 inch. These in turn were overlain with 3-inch planks, with the masonry placed upon them. All timbers were intended to be permanently wet. During construction, the builders left room on the berm side to allow another lock to be constructed alongside, but this never happened. Debris that got into the lock sometimes had to be removed with a pitchfork. If the paddle valve was stuck because of trash, the lock tender had a 20-foot pole with a hook to get the trash free. Lock numbering changed over the years. Guard Lock No. 2 was numbered Lock 24 for a time, which could cause confusion with Riley's Lock, No 24, upstream. Oddly enough, many of the lock numbers correspond to the contour elevatione.g., Lock 26 is at the 260-foot elevation line, and lock 30 is at the 300-foot mark, although it is doubted that things were planned that way, and many locks do not correspond to the elevation: e.g., Lock 72 is at the 580-foot mark. Most of the wicket openings in the upper gate recess have been closed with masonry, but one wicket at Violettes Lock is still intact as of 2013 (see photo). Note that modern replacement gates in locks 1–23, have the wickets in the gates, since the original wickets in the lock masonry are inoperable. This system of routing water through the masonry was abandoned due to the tendency for debris to clog the opening. The 1830 design eliminated the use of culverts in the masonry, and the upper gate is
below the breast wall. Most locks were fitted with V gates, but some locks, particularly around Seven Locks (7, 9, 10, and 12), and also all three North Branch locks (Locks 73–75 ), were re-fitted with mechanical drop gates to speed up traffic in those areas. (Note that lock 6 had been converted to a drop gate, and later converted back to a V gate.) With the mechanical locks, a lever pulls a rod which connects to a crank, connected to another rod which turned the wicket valve which was in the floor (below where the gate would rest when opened) and water flows below the floor and into the lock. Hahn states this brought the time to lock through from about 10 minutes to around 3 minutes.
Double locks In 1875, the company decided to experiment with double boats (one boat tied to another, pulled by a team of mules) allowing them to decrease freight costs by 50%. The company extended 14 locks to do so. Locks 25 through 32 were extended for that reason. 11 locks were extended at the lower end (like Lock 25) and 3 were extended at the upper end, for a total of 14 extended locks on the canal. While since that time the stones from the downstream extensions have often been removed, the early mule rise (i.e. the towpath rise before the lock appears 120 feet earlier than normal) indicates the locks extended at the lower end. Lander Lock (No. 29) is a good example of this. While physical evidence shows that locks 25, 27, 29–33, 37, 38, 43, & 60 also were lengthened, Canal records only mention locks 5-7 being lengthened In May 1864, those who tended these double locks wanted higher pay for them, but were unsuccessful. As a result, one of the lockkeepers, S. C. Rogers, abandoned his post (Locks 45–46) in protest, and was replaced by Obadiah Barger. Later in November 1864, Obadiah complained that he had been removed as locktender of the same locks (45–46) without just cause, and the locktender Susan Newcomer was never present but hired a youth without approval of the company, to tend the locks. After an investigation showed it to be true, she was sacked and Obadiah was reinstated as locktender again.
Lock tender's shanty It was quite common to build a little shanty so that the lockkeeper could look for boats coming, especially during the night or inclement weather.
Increase in depth Some locks were increased in depth (Lock 25, 22, etc. for instance) by bolting timbers to the top of the lock. Some of those bolts are still in the stonework. This was apparently done because of silting in the canal necessitating raising the water level.
Bypass flumes Most locks have a bypass flume, which allows water to bypass the lock to water the level below. If the flume was covered over with concrete, that generally meant that a roadway passed over the lock. Originally the locks 1-27, with the possible exception of Lock 13 did not have bypass flumes, using the culverts to divert water, but later the bypass flumes were put in (which is what we see today).
Composite locks Locks 58-71 are composite locks. Because there was a scarcity of good building stone in the upper Potomac, they made those locks of rubble and undressed inferior stone. Since the stone was undressed, that made a rougher surface, so the interior of the lock had to be lined with wood so as not to damage the boats in the lock. The wood sheathing had to be replaced from time to time. Originally
kyanized wood was used. Ellwood Morris, the Assistant Engineer, suggested using cement instead of stone for the locks to cut cost. On September 25, 1938, Charles B Fisk, the chief engineer, wrote to the board recommending the use of composite locks, such as those already found on the
Chenango Canal. Chief Engineer Charles Fisk was the one who prevailed on the board of directors to remove Lock 65. This resulted in the numbering of fractional locks and , so that the numbering above Lock 65 (now eliminated) could be preserved. On 1 June 1870, Engineer W R Hunton reported that the composite locks were serviceable but not in good condition, with coping out of place, and leaks due to lack of mortar. Some of the locks were rebuilt in 1873–74. By 1886-1888 the locks were in bad shape and in need of repair or rebuilding. Around 1910, because of problems, the wood was replaced with cement (which is often what is seen today). Some of that was because it was impractical to line the gate pockets with wood. Locks 66, and have a 10-foot lift instead of the regulation 8-foot lift.
Lock names Some of the locks were also informally named, usually after a prominent lockkeeper, a nearby town or important geographical feature. Some locks are known by more than one name and some are only known by their number. The names of the lockkeepers changed over the years, and the nicknames also changed. For instance Lock 21 had many lockkeepers: Mr. Fuller (1830), Mrs Susan Cross (1836), Robert C Fields (1839, Fired on 1 May 1846), Samuel Fisher (1846, 1851), hence today the lock is known as "
Swain's Lock". The Swain family had been involved with the canal ever since its construction: John Swain helped build the canal; his sons John, Hen, and Bill Swain were boatmen as well as Jesse Swain (boatman and later locksman), and his son, Otho Swain. Darbey's lock or Darkey's lock was named after Hughey Darkey, who had four or five red-headed girls. Twigg's lock (69) was named after the Twigg family, one of the first settlers in the upper Potomac. John and Rebecca Twigg who settled there in the mid-1700s had two sons, Robert and Fleetwood John Twigg because of whom a
Romeo and Juliet like story (without the tragedy at the end) ensued. Fleetwood John got himself an
Indian maiden, and was rejected, so he built his house on the "other side of the pond". The "Blue Eyed Twiggs" (Robert's children) were not allowed to play with "Black Eyed Twiggs" (descended from F. John), and the feud continued for a few generations until a "Blue Eyed Twigg" fell in love with a "Black Eyed Twigg". Many of the locktenders who were of good reputation, were later promoted to district superintendent. These included Elgin and John Y. Young in the 1830s and 1840s, John Lambie in the 1840s. A. K. Stake began at locks 41–41 from 1847 to 1848, Lewis G. Stanhop at locks 41–42 also in 1848, and Overton G. Lowe at Lock 56 when the canal opened to Cumberlandthese three individuals were later promoted and continued working for the Canal company well into the 1870s.
Incidents There were plenty of incidents with negligent lockkeepers. On September 11, 1895, at Lock 22, the boat
Excelsior arrived, and tried to lock through. The lockkeeper was drunk, and opened the lower gate paddles too early. The boat hit the mitre sill, broke in half, and sank with its 113 tons of coal. Richard A. Moore, the owner of the boat, collected over $1,300 in damages, and the lockkeeper was fired. On January 19, 2026, a sewer line running near the canal ruptured, causing a
spill of 300 million gallons into the Potomac River. The
District of Columbia Water and Sewer Authority (DC Water) installed a bypass system near Lock 10 using a contained section of the canal to carry wastewater 2,700 feet downstream and back into the sewage system, and is carrying out repairs to the
Potomac Interceptor.
List of locks The date is the date when the lock was completed. It often took a year to build the lock, and about $10,000. The lift (in feet) of a lock is listed when known. The locks ranged from 6-foot lift to a 10-foot lift, 8-foot being the most common. Specific information about lock dimensions, materials used, design, etc. can be found in William Davies's book. } ==Stop gates==