A dam at this point is apparently of no conceivable use to improve the lake for beaver occupation. It has one feature, also, in which it differs from other dams except those upon lake outlets, and that consists in its elevation, at all points, of about two feet above the level of the lake at ordinary stages of the water. In all other dams, except those upon lake outlets, and in most of the latter, the water stands quite near their crests, while in the one under consideration it stood about two feet below it. This fact suggests at least the inference, although it may have but little of probability to sustain it, that it was constructed with special reference to sudden rises of the lake in times of freshet, and that it was designed to hold this surplus water until it could be gradually discharged through the dam into the great space below. It would at least subserve this purpose very efficiently, and thus protect the dam below it from the effects of freshets. To ascribe the origin of this dam to such motives of intelligence is to invest this animal with a higher degree of sagacity than we have probable reason to concede to him, and yet it is proper to mention the relation in which these dams stand to each other—whether that relation is regarded as accidental or intentional.
As before, we have here to commend the caution displayed by the closing sentence; but, as useless dams are not found in other places, the inference clearly is that the dam in question, both as regards its exceptional position and exceptional height, can only be explained by supposing the structure to have been designed for the use which it unquestionably served. That is to say, if we do not entertain this explanation, there is no other to be suggested; and although in any ordinary or occasional instance of the display of animal intelligence in such a degree as this I should not hesitate to attribute the facts to accident, in the case of the beaver there are such a multitude of constantly recurring facts, all and only referable to a practical though not less extraordinary appreciation of hydrostatic principles, that the hypothesis of accident must here, I think, be laid aside. To substantiate this statement I shall detail the facts concerning the beaver-canals.
As Mr. Morgan, who first discovered and described these astonishing structures, observes,—
Remarkable as the dam may still be considered, from its structure and objects, it scarcely surpasses, if it may be said to equal, these water-ways, here called canals, which are excavated through the low lands bordering their ponds for the purpose of reaching the hard wood, and for affording a channel for its transportation to their lodges. To conceive and execute such a design presupposes a more complicated and extended process of reasoning than that required for the construction of a dam, and, although a much simpler work to perform when the thought was fully developed, it was far less to have been expected from a mute animal.
These canals are developed in this way. One of the principal objects served by a dam thrown across a small stream, is that of flooding the low ground so as to obtain water connection with the first high ground upon which hard wood is to be found, such connection being convenient, or even necessary, for the purposes of transport.
Where the pond fails to accomplish this fully, and also where the banks are defined and mark the limits of the pond, the deficiency is supplied by the canals in question. On descending surfaces, as has elsewhere been stated, beavers roll and drag their short cuttings down into the ponds. But where the ground is low it is generally so uneven and rough as to render it extremely difficult, if not impossible, for the beavers to move them for any considerable distance by physical force. Hence the canal for floating them across the intervening level ground to the pond. The necessity for it is so apparent as to diminish our astonishment at its construction; and yet that the beaver should devise a canal to surmount this difficulty is not the less remarkable.
The canals, which are made by excavation, are usually from three to five feet wide, three feet deep, and perhaps hundreds of feet long—the length of course depending on the distance between the lodge and the wood supply. They are cut in the form of trenches, having perpendicular sides and abrupt ends. All roots of trees, under-brush, &c., are cleared away in their course, so as to afford an unobstructed passage. These canals are of such frequent occurrence that it is impossible to attribute them to accident; they are evidently made, at the cost of much labour, with the deliberate purpose of putting them to the use for which they are designed. In executing this purpose there is sometimes displayed a depth of engineering forethought over details of structure required by the circumstances of special localities, which is even more astonishing than the execution of the general idea. Thus it not unfrequently happens that when a canal has been run for a certain distance, a rise in the level of the ground renders it impossible to continue the structure further from the water supply or lodge-pond, without either incurring a great amount of labour in digging the canal with progressively deepening sides, or leaving the trench empty of water, and so useless. In such cases the beavers resort to various expedients, according to the nature of the ground.
Mr. Morgan gives an interesting sketch of one such case, where the canal is excavated through low ground for a distance of 450 feet, when it reaches the first rise of ground, and throughout this distance, being level with the pond, it is supplied with water from this source. Where the rise begins a dam is made, and the canal is then continued for 25 feet at a level of one foot higher than before. This higher level reach is supplied with water collected from still higher levels by another dam, extending for 75 feet upon one side of the canal and 25 feet on the other, in the form of a crescent with its concavity directed towards the highlands, so as to collect all the drainage water, and concentrate it into the second reach of the canal. Beyond this larger dam there is another abrupt rise of a foot, and the canal is there continued for 47 feet more, where a third dam is built resembling the second in construction, only having a still wider span on either side of the canal (142 feet), so as to catch a still larger quantity of drainage water to supply the third or uppermost reach of the canal. We have, therefore, here presented, not only a perfect application of the principle of 'locks,' which are used in canals of human construction, but also the principle of collecting water to supply the reaches situated on the slope by means of elaborately constructed dams of wide extent, and of the best form for the purpose. There is thus shown much too great a concurrence of engineering principles to the attainment of one object to admit of our attributing the facts to accident. On this structure Mr. Morgan observes:—