Millions of beaver ponds graced America's wild gardens at the time the
first settlers came. These ragged and poetic ponds varied in length
from a few feet to one mile, and in area they were from one hundred
acres down to a miniature pond that half a dozen merry children might
encircle. These ponds were formed by dams built by beaver, and the
dams varied greatly in size and were made of poles variously combined
with sticks, stones, trash, rushes, and earth.
In the Bad Lands of Dakota I saw two dams that were made of chunks of
coal. This material had caved from a near-by bluff. I have noticed a
few that were constructed of cobble-stones. The water-front of these
dams was filled and covered with clay, and they were the work of
"grass beavers,"--beaver that subsist chiefly on grass, and that live
in localities almost destitute of trees.
It is doubtful if a dam is ever made by felling logs or large trees
across the stream. I have, however, seen a few real log dams, but in
these the logs were placed parallel to the flow of water. One of these
was in the Sawtooth Mountains of Idaho. Here a snow-slide swept
several hundred trees down the mountain. This wreckage was piled on
the bank of a stream. Beaver in a colony a short distance away
accepted this gift of the gods, and of these unwieldy logs built a dam
about two hundred feet downstream from where the avalanche had piled
the logs. This dam was a massive affair, about forty feet long and
eight feet high. It really appeared more like a log jam than a dam,
but it served the purpose intended and raised the level of the river
so that the water overflowed to one side and spread in a broad sheet
against a cliff and through a grove of aspens, which the beaver
proceeded to harvest.
The majority of dams are made of slender green poles which are placed
lengthwise with the flow for the bottom, and set braced with the end
upstream a foot or so higher than the downstream end. With these there
are occasionally used small limby trees. The large end of the tree
is placed upstream, and the small bushy end downstream. If in a
current these sometimes are weighed down with mud or stones. Short,
stout sticks and long, slender poles are deftly mingled in the dam as
it rises. The poles overlie, and many completed dams appear as though
made of gigantic inclined half-closed shears and compasses of poles.
Thus a dam is doubly braced. The weight against it is resisted both by
the end-on poles that are parallel to the flow and by those set at an
angle to it.
[Illustration: A NEW DAM]
The shape and the material of a dam are dependent on a number of
things: the nature of the place where built, the kind of materials
available for its building, the purpose it is intended to serve, and
the relation it may have to dams already constructed. Sometimes a
small dam will be made--that may ultimately become a big one--by
simply digging a ditch across the stream or basin and piling the
excavated material into a dam.
Beaver, like men, are unequal in their skill, both in planning and in
doing work, and the work of most beaver falls short of perfection.
Errors are not uncommon. More than one colony has commenced a dam
apparently without knowing that there was not sufficient available
material to complete it. Others have built in the wrong places, and
have thus failed to flood the area which they desired to reach or
cover with water. Occasionally the difficulties of construction have
been too great for the beaver who attempted it, and the dam has been
abandoned in an incomplete state. Now and then a weak dam breaks, or a
strong one is swept out by a flood.
But why do beaver need or want the pond which the dam forms? They need
it for the purpose of maintaining water of sufficient depth and area
to enable them to move about in safety, and to transport their
food-supplies with the greatest ease. Above all, the pond is a place
of refuge into which the beaver can constantly plunge and have
security from his numerous and ever watchful enemies. The
house-entrance must be kept water-covered. In the water the beaver is
in his element. On the land he is a child lost in the wilds. He has
extremely short legs and a heavy body. His make-up fits him for
movement in the water. He is a graceful swimmer, and in the water can
move easily and evade enemies; while on land he is an awkward lubber,
moves slowly, and is easily overtaken. Water of sufficient depth and
area, then, is essential to the life and happiness of the beaver. To
have this at all times it is necessary, in localities where the supply
is at times insufficient, to maintain it by means of dams and ponds.
Deep ponds are needed around the house; shallow ponds with shores in
near-by groves facilitate far-away logging. Dams are placed across
streams whose waters are to be led away through new channels and made
to serve elsewhere in canals or ponds. Dams are made across inclined
canals to catch and hold water in them. Streams are beaver's avenues
of travel. Along shallow streams in a beaver country it is not
uncommon to see an occasional short dam which forms a deep hole, which
apparently is maintained as a harbor or place of safety into which
traveling beaver may dive and be made safe from pursuit.
Most beaver dams are built on the installment plan. They are the
result of growth. The new dam is short and comparatively low. It is
enlarged as conditions may require. As the trees in the edge of the
pond are harvested, the dam is built higher and longer, so as to flood
a larger area; or as sediment fills the pond, the dam is from time to
time raised and lengthened in order to maintain the desired depth of
water. Thus it may grow through the years until the possibilities of
the locality are exhausted. The dam may then be abandoned. It may be
used for a few years or it may be used for a century. A gigantic
beaver dam may thus represent the work of several generations of
beaver. It often occurs that one or more generations may use a dam and
yearly add something to its size. By and by these beaver may die or
emigrate. The old dam remains, falling to ruin in places. Years go by
and other beaver come upon the scene. The old dam is then used for the
foundation for a new one. The appearance of some old dams indicates
that they have been repeatedly used and abandoned.
New dams, being made largely of coarse materials, appear very unlike
old ones. Decay, settling, repairs, and other changes come rapidly.
The dam is built of poles to-day; it speedily becomes earthy and is
planted by nature to grass, willows, and flowers. On old, large dams
it is not uncommon to see old forest-trees. The roots of these
entangle the constructive materials, penetrate deeply, and help to
anchor securely the entire dam.
In only a few cases are the water-fronts of dams at once plastered or
filled in with mud. This is done only where there is a scarcity of
water. It is the aim of the beaver to raise the water in the pond to a
certain height and there maintain it, the chief purpose of the dam
being to regulate the height or the depth of the water. The water, in
streaming through new dams, deposits therein quantities of sticks,
trash, and sediment, so that in a year or two these choke the holes,
almost stop the leakage of the water, and help to solidify the dam.
The discharge from dams is regulated by the beaver. In some instances
water leaks through a dam in numerous places from bottom to top; in
others it seeps through only close to the top; and in still others the
dam is so solid that the water pours over the top in a thin sheet. In
some cases, however, instead of the water pouring over the entire
length of the dam the beaver force it to pour over in a given stretch
at one end or the other, or sometimes through a hole or tunnel. The
concentration of the overflow at some one point in the dam is commonly
done either for the purpose of using it in transportation or to force
the water to outpour on a spot where it will least erode the
foundation of the dam. Occasionally beaver compel the water to flow
round the end of a dam, which they raise sufficiently high for that
purpose. Sometimes they dig a waste-way for the water.
European beaver appear to have barely developed to the dam-building
stage. Rarely did they build even a small, unimportant dam. Nor did
all the American beaver build dams. At the time the beaver population
was most numerous and widely distributed, probably not more than half
of them used the dam. However, those not using the dam were living in
places where the dam and consequent pond were not needed. Dam-building
enormously increased the habitable beaver area. There were, and are,
thousands of brooks which each year cease to flow for a period, yet
on these brooks are all other beaver requirements except a permanent,
sufficient water-supply. By dam-building water is stored for
to-morrow, or stream-courses changed, and with the assistance of
canals water is diverted to a dry ravine where a colony is
established.
The dam is the largest and in many respects the most influential
beaver work. Across a stream it is an inviting thoroughfare for the
folk of the wild. As soon as a dam is completed, it becomes a
wilderness highway. It is used day and night. Across it go bears and
lions, rabbits and wolves, mice and porcupines; chipmunks use it for a
bridge, birds alight upon it, trout attempt to leap it, and in the
evening the graceful deer cast their reflections with the willows in
its quiet pond. Across it dash pursuer and pursued. Upon it take place
battles and courtships. Often it is torn by hoof and claw. Death
struggles stain it with blood. Many a drama, romantic and picturesque,
fierce and wild, is staged upon the beaver dam.
The beaver dam gives new character to the landscape. It frequently
alters the course of a stream and changes the topography. It
introduces water into the scene. It nourishes new plant-life. It
brings new birds. It provides a harbor and a home for fish throughout
the changing seasons. It seizes sediment and soil from the rushing
waters, and it sends water through subterranean ways to form and feed
springs which give bloom to terraces below. It is a distributor of the
waters; and on days when dark clouds are shaken with heavy thunder,
the beaver dam silently breasts, breaks, and delays the down-rushing
flood waters, saves and stores them; then, through all the rainless
days that follow, it slowly releases them.
Most old colonies have many dams and ponds. A dam is sometimes built
for the purpose of forcing water back and to one side into a grove
that is to be harvested for food. In many cases water flows round the
end of a dam, and in making its way back to the main channel is
intercepted by another dam, then another; and thus the water from one
small brook maintains a cluster or chain of pondlets.
The majority of beaver dams are as crooked as a river's course. Now
and then one is straight. A few are built from shore to a boulder,
from the boulder to a willow-clump, and finally, perhaps, from
willow-clump to some outstretching peninsula on the further shore. It
is not uncommon for a short dam to be built and afterwards lengthened
with additions on each end which may curve either down or up stream.
Sometimes a dam is built outward from opposite shores simultaneously
by separate but coöperating crews of beaver. In swift water these ends
are forced downstream in building, so that when they are finally
joined midstream the dam curves noticeably downstream.
On one occasion I watched beaver commence and complete a dam in
moderately swift water that when finished bowed strongly _upstream_.
This, however, was not the intention of the builders. The material for
this dam consisted of willow and alder poles that were cut some
distance upstream. These were floated down as used. This dam was begun
against a huge boulder near midstream, and built outward
simultaneously toward both shores. Despite the repeated efforts of
the builders to extend it in a straight line to the shore, the flow of
the water pushed these outbuilding ends downward, and when they
finally reached the shore this fifty-odd feet of dam with the boulder
for a keystone had an arch that was about fifteen feet in advance of
the bases.
Not far from where I lived in the mountains when a boy, the beaver
built a dam. This had a slight arch upstream. A few years later the
dam was doubled in length by building an extension on the end which
bowed downstream. It thus stood a reverse curve. Later the dam was
still further lengthened by a comparatively straight stretch on one
end, and by a short, down-bowing stretch on the other. Recent
additions to this dam consist of wings at the end which sweep
upstream. The dam as it now stands reaches about three fourths of the
way around the pond which it forms.
It is not uncommon for a dam to be planned and built with an arch
against the current or against the water which it afterwards impounds.
The most interesting dam of this kind that I ever saw was one across
the narrow neck of a rudely bell-shaped basin that was about two
hundred feet in length. The material for this dam came from a grove of
aspens that extended into one side of the basin. The floor of this
basin was partly covered with a few inches of water. In starting the
dam the beaver evidently knew where they wanted to build it. This was
not by the aspen grove where the materials were convenient, where the
dam would need to be about one hundred and twenty feet long, but was
about fifty feet farther on, where a dam of only forty feet was
required. This dam when completed bowed seven feet against the
enclosed water. The beaver commenced building at the end nearest the
grove of aspens, pulling and dragging the poles the fifty feet to it.
They laid these aspen poles, which were two to five inches in diameter
and from four to twelve feet in length, at right angles to the length
of the dam, and usually placed the large end upstream or against the
current. But the water was shallow, and the transportation of these
poles to the dam was difficult. Accordingly a ditch or canal was dug
from the grove to the place by the dam where the work was going on.
This ditch was about twenty-five inches wide and fifteen deep. The
waters filled it and thereby afforded an easy means of floating or
transporting the poles from the grove to the place where they were
being used. This ditch was carried forward along the upper line of the
dam, and several feet in advance of the spot where the outbuilding
work was advancing. Upon the earth thrown up from this were laid the
upper or high ends of the poles. When the dam was finally completed,
it was approximately eight feet wide on the base and stood four feet
high. As soon as it was completed, the beaver stuffed the water-front
with mud and grass roots, which were obtained by digging from the
construction ditch immediately in front of the dam. In other words,
they enlarged their pole-floating ditch above the dam into a deeper
and wider channel, and used this excavated material for strengthening
and waterproofing the dam.
The longest beaver dam that I have ever seen or measured was on the
Jefferson River near Three Forks, Montana. This was 2140 feet long.
Most of it was old. More than half of it was less than six feet in
height; two short sections of it, however, were twenty-three feet wide
at the base, five on top, and fourteen feet high.
[Illustration: PART OF AN OLD DAM 1040 FEET LONG]