Pastoral and Monumental: Dams, Postcards, and the American Landscape

Chapter Three: Materials, Design, Construction

PASTORAL AND MONUMENTAL 89 Chapter Three materials, design, construction An array of questions arises when someone sets out to build a dam. Some relate to location, topography, and geology, some to purpose, and many concern what and how. What materials will be used to build the dam? What will be the size and shape? And how will it be constructed? Later chapters deal with the why of dams and the way that people use them; the postcards featured here document the technology itself. Compared to detailed blueprints, contract specifications, or day-to-day sequences of construction photographs, postcards rarely offer a comprehensive record of a dam and how it came to be. Nonetheless, it is striking how much variety and technological nuance is captured in vintage postcard views. Materials Since ancient times, earth, rock, and wood have been used to build dams. In terms of material, the biggest development of the past 125 years has been the replacement of cut-stone masonry with concrete (in the nineteenth century concrete was often referred to as “artificial stone”). With dams, durability is a highly desirable characteristic and stone is well suited to resisting erosion. However, quarrying and transporting cut stone can be both time-consuming and expensive. In contrast, loose rock fill is simpler to use, especially if it can be readily excavated from a nearby streambed or hillside. But maintaining the shape and the structural integrity of loose rock fill—especially when frequent floods cause overtopping—can be a problem. While wood is far less durable than stone, it is often available in MATERIALS, DESIGN, CONSTRUCTION 90 A classic timber crib mill dam in Dover, Maine, circa 1910. Maine is a heavily wooded state, and material for timber crib dams has been close at hand since colonial times. Also note how the structure rests atop a rock ledge stretching across the river bed. A large timber crib dam in Green Lake, New York, upon completion in 1913. By the early twentieth century concrete was in wide use, but traditional materials did not immediately vanish from America’s dam-building lexicon. The choice of a specific design always reflects a wide range of influences and long-term service and permanence (which can entail considerable expense) is not always the most important factor governing design. MATERIALS, DESIGN, CONSTRUCTION 91 Timber crib dams could be quite large and control the flow of major rivers, as with the Watab Dam across the upper Mississippi River near St. Cloud, Minnesota. Absent timber cribbing, rock fill can also be used to create large embankment dams. This postcard, circa 1910, shows the seventy-six-feet-high Escondido Dam north of San Diego formed with more than forty thousand cubic yards of stone. To impede leakage, a double-layer of redwood planking was placed across the dam’s upstream face. MATERIALS, DESIGN, CONSTRUCTION 92 large quantities near rural or village dam sites and can hold its shape for many years before deterioration or rot sets in. From the seventeenth century through the early twentieth century EuroAmericans built a multitude of small dams using stone, wood, or a combination of the two. The most common strategy to provide stability at modest cost was to conjoin stone and timber into structures known as timber crib (or sometimes simply “crib”) dams. In these, a lattice or crib of timber logs provides a basic shape for the dam; then loose rock is dumped or piled within the lattice to increase its mass and durability. To protect against the erosive power of flowing water, wooden planks were commonly placed atop the structure and the downstream face. Timber crib dams were susceptible to washout, especially if hit by heavy overflow. But they were not hard to rebuild, and often times they could be quickly repaired after suffering flood damage. Earthen deposits are often available in large quantities near prospective dam sites and well built earth-fill dams can successfully hold back water for decades, even centuries. To insure the long-term integrity of an earth-fill dam, it is essential that the structure never be allowed to overtop—if water starts flowing over an earth embankment dam, it will quickly erode the downstream face and, in a short time, cause collapse. It is also essential that excessive water be prevented from seeping through the earth embankment and internally erode the structure, precipitating failure from within. It is impossible to build a completely watertight...