Mound Forms

BEAVER--MASTER MOUND BUILDER

Of the order of rodents the beaver is one of the most cleaver. Nature has equipped the beaver with a set of tools that makes its architectural achievements possible. Just as the materials and methods of construction in human-made architecture enable us to evolve our buildings to achieve great feats of engineering and show us new possibilities in form, so the tools and materials of the beaver create the circumstances by which it can create the mound design of which it is famous.

The beaver is one of the heaviest rodents, weighing up to 70 pounds, and can stay submerged underwater for up to fifteen minutes. Its webbed hind feet and broad tail, which it uses as a rudder, propel and steer the animal through the water and are also tools for construction. Beavers are experts not only in building dwellings but also in hydro-engineering.

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Beavers use strong, stout pieces of wood at the river bottom and lighter cross pieces to build upward.
Additional cross pieces and vertical stakes are often reinforced by rocks and trees for additional resistance from water current forces.
Use of felled trees laterally placed with smaller vertical sections placed at an angle reinforced by river sand and rock.
An alternative approach uses a protruding area of soil, bushes and trees to create the dam.

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Seeking out a place where the river flows noisily, the beaver chooses a level area. The beaver makes adjustments both visually and audibly.
The beaver begins to fill in space with tree sections and sand to make a level but dammed area.
Tree sections of various shapes are rooted into the river bed. These sections are then covered in sand and river bed material to form a highly compressed mass.
Built-up material from the river bottom is packed between the protuding tree sections of dam.

Beavers vary their dwellings according to local conditions. The inner chamber of their mound dwelling is positioned according to the level of the river in which it is situated. Once the mound is built the water rises up to meet the lodge. During construction, they dig an upward sloping tunnel into the riverbank which culminates into a larger subterranean chamber about three feet in diameter and two feet high. There is also usually a feeding chamber located near the entrance. Feeding generally occurs at night inside this chamber or outside on the river's edge, and all leftover scraps from feeding are disposed of directly into the water.

The beaver has designed its own version of an HVAC (heating, ventilation & air-conditioning) system; most of the mound structure is carefully sealed with mud and clay, but part of the upper dome superstructure is left with hollow openings for ventilation. During cold weather small clouds of steam can be seen rising from the tops of beaver mounds.

The specifics of mound building are remarkable. Strong, stout timber is placed onto the bottom of the stream or river bed, creating a "wall" that is built up from the river bottom. Pieces are interwoven throughout and forked branch pieces are then placed between the dam wall and the river bottom supporting the structure against water current. Cross pieces or stakes are often inserted for additional resistance.

An alternative strategy is to anchor the structure to existing trees or boulders and further brace these by heavy stones brought to the site. Gaps in the wall of the dam are filled with small twigs, reeds, leaves and other small materials and covered with mud or clay to make the dam completely watertight. The walls of the underwater entrance are smooth and steep. In front of the entrance is an ingenious deep pit where materials are gathered. The pit also acts as a water current buffer by reducing the speed and destructive force of the water, protecting the structure. The downstream wall is composed of coarse branches anchored laterally for additional bracing. The crown of the dam is hydrodynamically designed to be lower at the edges to allow water to flow over near the banks. Acoustics is a very important consideration in dam construction. For some reason beavers instinctively seek out those places in a river where the water is rushing noisily, for example between stones or where there are dips. A smooth surface is necessary for the water to run over the dam quietly, so the beavers place branches and mud in that space in order to level the area. When the dam is essentially level, the water-now obstructed-rushes past on either side of the dam creating another noisy depression. So the beavers fill in that space with material so that once again, they can make a level surface.The beavers end this process by placing finer materials such as mud and small stones in between the crannies of the twigs for a more compact and smooth surface. When their work is completed, the water level will have risen and it will run quietly around the dam.

When the surrounding water level of rises, threatening its home, the beaver has several options. They can go to the source of the problem-the water- and lower the crown of the dam where the water runs over thus lowering the water level. If this does not suffice, they can gnaw off materials from the ceiling of the structure to build up the floor. Similarly, soil and twigs can be heaped together to create a thicker and higher floor level. If the water level continues to rise, more twigs and mud can be added to the original structure and the main chamber can be dug out further up, keeping the same angle of entry. In some areas, as in shallow, slow moving ponds, the beaver creates an island of branches, twigs and mud.If a hole develops in the dam they immediately find and repair the damage. Their mounds can reach a height of six to ten feet with hollowed-out living chambers inside, and entrances below water level. The largest recorded dam is in the Voronesh region of the former Soviet Union and measured about 3800 feet long, 3 feet high and wide. In the swamps of the Mississippi basin and the Jefferson river in Montana beavers build dams over a thousand feet long which are strong enough to carry a person on horseback! In the winter, when the water surface is completely frozen, beavers have devised a method of controlling their environment. In places where the water is dammed up they make openings in the dam to allow water to flow out thereby lowering the water table underneath the frozen surface. This creates an airspace in the ice where they can breath and swim. The temperature inside is maintained at above freezing even when the outside temperature drops to -95 degrees Fahrenheit. Before winter sets in they cut down trees for storage. They collect large quantities of branches and brush which they anchor in heaps at the bottom of the water near their entrances. This stock pilling solves the problem of transporting provisions from great distances; or land to their living quarters.

Few species have the ability to manipulate the environment to their own ends as the beaver does. The dam, lodge, winter foodstore, canals and runways are all part of a completely designed environment. The beaver and its dam, which at first might seem to be destructive rather than constructive, have wide effects on the ecology of the surrounding countryside. The dam raises the water table and slows drainage at the base of the dam by maintaining a steady flow of water in headstreams. The dam also prevents soil from washing downstream. And the rich soil that builds up soon supports a variety of plants that will attract other animals. Similarly, the systems of dams and pools provide food and habitat to other creatures, which in turn attract predators and the whole web of life is enriched. Once the stream has backed up and collected into a still pond and the lodge has been built, the beavers can regulate the depth of the water.The beavers' main task and principal means of survival, is to monitor the water level and maintain the dam. An enormous responsibility for the beaver when you remember that some dams grow to be a half-mile long! The commitment of labor to this kind of massive project:(cutting trees, trimming spillways, etc....) makes sense for the beaver only because the dam is meant to last forever. Beaver dams are maintained generation after generation by offspring of growing colonies. There are dams today that have been in use for over a century. The beaver is the greatest animal architect and the strength of its structures alone is enough to verify its valued position. The lodges and dams are intelligently designed and well made. They incorporate materials difficult to collect and are combined into a coherent form. The functional mastery of hydro-engineering is unparalleled; certainly no other animal structure in the world has to withstand the rentless, elemental pressures borne by a beaver dam spanning a swift river. These dams and lodges work decade after decade, responding to adjustments, recovering from disasters and holding fast in form, function and purpose.

The dam building engineering of the beaver is another model worthy of study. Although the structural system used by beavers is generally understood and used in human dam building,a person wishing to build an economical dam structure from local materials could learn from the engineering process of the beaver. The use of trees to provide anchoring is a rarely used architectural concept. This method of structural anchoring could be used in tension structures to create suspended roofs or small buildings off the ground. There are quite a few examples of "tree houses" built in rural areas throughout the United States using the beaver's tree anchoring system.

Perhaps the greatest lesson we can learn from the beaver is its understanding of the natural environment and the non-destructive maintenance of that environment. The presence of the beaver dam has positive ecological effects which consider the entire environment as the design--not merely the issolated dam structure. The beaver dam uses only enough materials to serve the purpose, to create adequate structural strength to resist even the most extreme water pressure and flooding situations.

The hemispherical shape of the beaver dam expresses the principle of maximizing volume while minimizing materials and surface area. This shape also distributes stress and strain loads throughout the structure similar to an igloo. This goes hand-in-hand with the hemisphere's energy efficiency and air circulating ability described. The stability of the dome/mound structure is a sensible resolution to the constant water pressure and potential flooding of the area. The den shape is roomy and also durable. As indicated the entire dam structure is built by a few beavers using simple procedures of construction. The beavers readily adapt to the specific features of the site and seem to innately proceed with a construction strategy based upon the immediate locale. As with all of nature's structures the beaver's den is both aerodynamic and hydrodynamic. Air easily passes over the mounds drawing internal air with it and replenishing the dens with fresh air. In the water the dam provides a silent and smooth running mini-waterfall for regulating water levels and flow. The arched profile of the dam provides a perfect form for achieving this purpose.

Note: Reference from Hutchins, Ross E. Hansell, Michael H. and Tsui, Eugene