Mohawk River Bridge Dams: Engineering Landmarks

The Mohawk River has been used as a transportation corridor since the beginning of human settlement. Indigenous people used the river to move east and west, as did the first European explorers and those who followed. The river was shallow and relatively slow flowing. Along the 120-miles between Rome and the Hudson river, there are two waterfalls. The largest of the two is near the eastern end of the river, where the water flows over a 90-foot high falls at Cohoes. At Little Falls the river flows over a series of rapids that are 45-feet in height.

There were other smaller sections of rifts and rapids. In the later 1700s, the Western Inland Lock Navigation Company tried to make modifications to the route by building short canals and locks around these natural obstacles. (For those interested in the topic, I would suggest Robert Hager’s excellent 1987 work; Mohawk River Boats and Navigation before 1820.) When the Erie Canal was constructed, the river valley was used as a natural corridor, but the canal was kept separate from the river by using a man-made channel. In some places, this separation might be the bank of the canal itself, although where possible, the canal ran away from the river so that it would be protected from floods and ice damage.

The construction of the Barge Canal would change all this. Instead of avoiding the river, the canal would occupy it in a process called “canalization”. Dams would be constructed to form 12-foot deep navigation pools and locks would connect these pools. The main issue was what type of dam would best suit the river, which was well known for its floods.

The Mohawk River has a large watershed that stretches north into the Adirondack mountains and south into the Catskills. In addition to its beginning north of Rome near Boonville, the river accepts the flow of the 76-mile long West Canada Creek, which flows out of the southern foothills of the Adirondacks; and the Schoharie River, which has its beginning near Indian Head Mountain, 93-miles south of its confluence with the Mohawk. A large rain storm or snow melt miles away from the Mohawk River can cause these rivers to rise and flood very quickly and without much warning. And in the winter, large blocks of ice would form in the shallow river waters, and in the spring, these ice chunks would create dams that would backup water for miles. Once the ice dam had broken up, the rushing water and ice flows would easily destroy anything they came into contact with. So damming the river was problematic. There were a couple small dams built along the river to impound water which would be fed into the nearby Erie. The dam at the Rocky Rift Feeder was one of these small dams. Large dams were not used.

On a number of rivers, both in the United States and in Europe, movable dams had been constructed that would improve the navigation of the river by creating temporary pools in times of low water. The first dams were designed and built by Charles Antonie Fracois Poiree in 1834. (Although the bear trap dams built along the Lehigh are recognized as the first movable dam, they are built as part of a fixed crest dam, and thus are not a true movable dam.) The Poiree dam consisted of a number of iron frames that were fixed to a foundation on the river bottom. When not is use, the frames lay upon the bottom of the river. When needed, men would lift the frame upright, and lay a wood walkway upon the top of the frame. And then the next frame was lifted and the walkway installed. Once complete, the entire affair looked like a road trestle, and thus the name of the dam is sometimes called the trestle dam. However, at this stage, there is no dam installed yet. The dam was constructed by installing long narrow boards set on end. By installing board after board, a dam is slowly built up and the water is impounded. The height of the pool depends on the height of the trestle and boards. In France, the boards are called aiguilles or needles, and the name of this type of dam is called the Needle Dam. If the pool had to be regulated, boards could be removed. And once the natural condition of the river would allow use without dams, the needles could be pulled and the frames returned to the river bottom. This types of dams were later modified by engineers named Boule and Camere. Boule modified the boards to lay edgewise and connected them together into larger gates. Camere designed a rolling curtain type of dam. Both these continued to use the Poiree’s trestle framework.

Since the canalization of the river was such a departure for the State, they hired as a expert designer David A. Watt. David had co-written the Improvement of Rivers, a two volume book that covered all types of dams and river navigation topics. He worked with the Army Corps of Engineers, and in 1905, he had recently completed projects in Kentucky and Ohio. He realized that for the Barge Canal and the Mohawk River, the big issue was the unpredictable seasonal flooding and ice flows. Although the trestle and needle dam was considered from the Barge Canal, it was realized that the ice flows would quickly damage or destroy the frames. However, David was aware of another type of movable dam was being used in Europe. This was the bridge dam.

At its core, the bridge dam takes the trestle dam and turns it upside down. Instead of mounting the frames to the river bottom, the frames are suspended from an overhead bridge, where they are safe from the ice flows, and easier to maintain. The French had built a bridge dam at Poses on the Seine River in 1885. This dam combined the hanging frames with the Camere curtain dams. Watt knew about this dam, but he took especial note of a new dam being built along the Moldau [Vitava] River in the Czech Republic. This dam combined hanging frames and large steel gates. It also used sections of the Poiree Dam, so it was a hybrid trestle and bridge dam. The dam was built as part of an entire river canalization project that was very similar to what New York wished to build. However, among all the dams along the Moldau, the dam at Mirowitz was the only bridge dam. It is this dam at Mirowitz [Mirejovice] that was often cited by Watt and others as the example for the new dams that would be built along the Mohawk.

Watt would need to build eight bridge dams along the Mohawk between Schenectady and Fort Plain. In this construction alone, he would be doubling the total bridge dams in use around the world. The most notable feature of the dam was the multi-span truss bridge that would cross the river. The locations of the dams placed them squarely in the view of the public, as the New York Central Railroad and the main highways ran along the river. Their successes or failures would be on view for all to behold, especially the politicians who used the railroad to travel to Albany.

Construction began in 1906 and for the most part, all the dams were finished by 1913. The only exception was at Lock 8, where the contractor ran into quicksand and couldn’t complete his contract. For the most part, the structures are alike, although adapted to fit into the environment. All the dams east of the confluence of the Schoharie used three bridge spans as the river was naturally wider with the additional flows from the Schoharie. The dam just to the east of the confluence, at Lock 12, has two very long spans in order to minimize ice damming. All the dams west of the Schoharie have two spans.

Each dam uses frames that are lowered into the river channel and rest upon a concrete sill. With the frames in place, large steel plates are lowered. Each dam has a lower and upper plate that can be moved independently of one another. For the most part, all the lower plates are left in place and if conditions warrant, the upper gates are raised to increase flow through the dam.

In March of 1913, a late winter storm that would bring floods to the midwest and put the Ohio and Erie Canal out of business, also hit New York. The dams at Locks 12 and 13 had been lowered to allow the dredging contractors to get an early start to the season. As the flood-waters rushed through the valley, they collected construction wood from Barge Canal contractors. This material, along with any loose trees and other debris, was quickly caught up on the frames and gates. The result was an impounding of the water behind the dam to a greater depth then the structure was designed for. The pressure of the water caused the plates and frames to twist and bend, chains to break, and supporting members to buckle. It was very apparent to everyone that although the dams were very good in good conditions, in times of flood, they were impossible to raise. Once the debris builds up on the gates, it is impossible to raise the frames. Any adjustment to avoid this from happening has to take place prior to the flood waters reaching the dam. In 1913, instant communication was only a telephone call away, but that depended on intact lines and operators at the switch. It was also apparent that the dams were not up to the stresses caused by flooding waters.

To salvage the dams, all the bridges were given a third, and sometimes a fourth, truss. In all cases, the downstream truss was twinned to provide additional strength. The large steam powered winches that were used to raise and lower the dams were rebuilt to run on electricity. This lowered the stresses placed on the cantilevered sections of the dam. The lifting points were modified so that the winch would travel on tracks only on the downstream side of the dam, a change from the tracks that ran around the dam like a toy train set around the Christmas tree. All these modifications saved the dams and have seen them through many flooding situations, with the last one being in 2011 when Hurricane Irene and Tropical Storm Lee brought damaging floods to the valley. Although the storms resulted in damages over 50 million dollars, the dams held.

There were other bridge dams built as part of the Barge Canal project. At Herkimer, a single span bridge dam was built in 1918 to replace a Poiree Dam that was first built there in 1910. At May’s Point in Seneca County, a single span bridge dam forms a navigable pool for Lock 25. On the Genesee arm of the canal at Rochester, a single span bridge dam was used to create the pool that makes the Rochester harbor. This dam was replaced in 1926 when the power company took control of the works and replaced the dam with a taintor gate dam. The last bridge dam was built in 1927. This dam at Rocky Rift on the upper Mohawk was a smaller three span truss dam. This dam replaced a fixed crest / Boule trestle dam that was difficult to control.

The bridge dam fell quickly out of favor for the reasons seen here. They are difficult to operate under poor conditions, and once debris begins to build up on the frames and gates, they are impossible to open. The dams along the Mohawk might be the last of their kind, although the Emergency Swing Dam at Sault Ste. Marie and the Camere dam on the Red River north of Winnipeg are close cousins in the bridge family tree. All these are engineering landmarks.

Photos, from above: The three span dam at Lock 8 in Scotia in 2013, the eastern most bridge dam on the Mohawk; The dam at Lock 9 in Rotterdam Junction; and The “mother dam” in the Czech Republic in 1905, the model used for all the bridge dams in New York.