BEAVER INFO.doc

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BEAVER INFO

The beaver (genus Castor) is a primarily nocturnal, large, semiaquatic rodent.

Castor includes two extant species, the North American beaver (Castor canadensis)

(native to North America) and urasian beaver (Castor !ber) (urasia)."#$ %eavers

are &nown 'or building dams, canals, and lodges (homes). They are the secondlargest rodent in the world (a'ter the capybara). Their colonies create one or more

dams to provide still, deep water to protect against predators, and to oat 'ood and

building material. The North American beaver population was once more than *+

million, but as o' #-- was *#/ million. This population decline is the result o'

extensive hunting 'or 'ur, 'or glands used as medicine and per'ume, and because

the beavers0 harvesting o' trees and ooding o' waterways may inter'ere with other

land uses."/$

Beaver

Temporal range: 240 Ma

North American beaver(Castor

canadensis)

Scientific classification

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Rodentia

Family: Castoridae

enus: Castor
https://en.wikipedia.org/wiki/Megaannumhttps://en.wikipedia.org/wiki/North_American_beaverhttps://en.wikipedia.org/wiki/North_American_beaverhttps://en.wikipedia.org/wiki/Taxonomy_(biology)https://en.wikipedia.org/wiki/Animalhttps://en.wikipedia.org/wiki/Chordatehttps://en.wikipedia.org/wiki/Mammalhttps://en.wikipedia.org/wiki/Rodenthttps://en.wikipedia.org/wiki/Castoridaehttps://en.wikipedia.org/wiki/North_American_beaverhttps://en.wikipedia.org/wiki/Taxonomy_(biology)https://en.wikipedia.org/wiki/Animalhttps://en.wikipedia.org/wiki/Chordatehttps://en.wikipedia.org/wiki/Mammalhttps://en.wikipedia.org/wiki/Rodenthttps://en.wikipedia.org/wiki/Castoridaehttps://en.wikipedia.org/wiki/Megaannum


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Construction

%eavers use roc&s 'or their dams when mud and branches are less available as seen

on %ear Cree&, a tributary to the Truc&ee 1iver, in Alpine 2eadows, Cali'ornia

A minimum water level o' +.* to +. metres (/.+ to 3.+ 't) is required to &eep the

underwater entrance to beaver lodges 'rom being bloc&ed by ice during the winter.

4n la&es, rivers and large streams with deep enough water, beavers may not build

dams and instead live in ban& burrows and lodges. 4' the water is not deep enough

to &eep beavers sa'e 'rom predators and their lodge entrances ice'ree, beavers

build dams.

%eavers start construction by diverting the stream to lessen the water0s ow

pressure. %ranches and logs are then driven into the mud o' the stream bed to 'orm

a base. Then stic&s, bar& ('rom deciduous trees), roc&s, mud, grass, leaves, masses

o' plants, and anything else available, are used to build the superstructure. The

average height o' a dam is about #.- metres (5. 't) with an average depth o' water

behind the dam o' #./ to #.- m. The thic&ness o' the dam is o'ten around # m or

more. The length depends on the stream width, but averages about 6.5 m long.

%eavers vary the type o' dam built and how they build it, according to the speed o'

water on the stream. 4n slowmoving water, they build a straight dam, whereas in

'astmoving water they tend to be curved. 7pillways and passageways are built into

the dam to allow excess water to drain o8 without damaging it. 9ams are generally

built wider at the base and the top is usually tilted upstream to resist the 'orce o'

the current. %eavers can transport their own weight in material: they drag logs

along mudslides and oat them through canals to get them in place. ;nce the dam

has ooded enough area to the proper depth to 'orm a protective moat 'or the lodge(o'ten covering many acres), beavers begin construction on the lodge."#$

Trees approaching a diameter o' + centimetres (3.+ 't) may be used to construct a

dam, although the average is #+ to 3+ cm. The length depends on the diameter o'

the tree and the siaws are so power'ul they can cut a #.5 cm sapling in one bite."#$

2aintenance wor& on the dam and lodges is particularly heavy in autumn.


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2.

4n areas where the natural water level is too low, beavers will construct a dam. "6+$;nce a beaver chooses a dam site, almost nothing can ma&e him abandoned it.

They have even been &nown to incorperate beaver traps in the construction o' their

dams? "6#$ A minimum water level o' / to 3 't. (.* to . m) is required to &eep the

underwater entrance to their lodge 'rom being bloc&ed by ice during the winter. "6/$

"63$ The average height o' a dam is about * 't. (#.- m) tall, with an average depth

o' water behind the dam o' 6 to * 't. (#./ to #.- m). The thic&ness o' the dam is

o'ten around 5 't. (#.5 m) or more. The length will depend on the stream width, but

averages about #5 't. (6.5 m) long. "66$ "65$ The longest beaver dam on record is

/,#6+ 't. (*5/. / m) long, #6 't. (6.3 m) high, /3 't. (@ m) thic& at the base, 'ound in

Three or&s, 2ontana. "6*$ %eavers will vary the type o' dam built and how they

build it, by the speed o' water on the river. 4n slowmoving water they may build a

straight dam. Bhereas in 'astmoving water they tend to be curved. "6@$ %eavers

start construction by diverting the stream to lessen the water0s ow pressure.

%ranches and logs are then driven into the mud o' the stream bed to 'orm a base.

Then stic&s, bar& ('rom deciduous trees), roc&s, mud, grass, leaves, masses o'

plants, and anything else available, is used to build the superstructure.

Beaver cuts

4t ta&es a beaver about /+ minutes to cut down a * in. (#5./ cm) wide aspen, by

gnawing a groove around the trun& in an hourglass shape. "*#$ "*/$ %eavers are as

sensible to the danger o' a 'alling tree as a human lumber>ac&. As the tree begins to

'all, the beaver will run? "*3$ A beaver0s >aws are so power'ul they can cut a hal'

inch sapling in one bite? "*6$ The beaver0s pond is necessary 'or their 'ood and

housing. 4' the dam brea&s, they will rush around 'rantically to repair it. 4' the they

do not hurry, they could lose all o' the water in their pond. This would expose the

underwater lodge entrances to predators. "*5$ very member o' the colony will

wor& constantly to &eep the dam maintained and 'unctioning. "**$ 2aintenance

wor& on the dam and lodges is particularly heavy in autumn. "*@$ ;nce the dam has

ooded enough 'orest area to the proper depth to 'orm a protective moat 'or thelodge (o'ten covering many acres), the beaver will begin construction on the lodge


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Beaver Dam Construction

4 have seen temporary dams built o' green corn stal&s, soybean plants,and

gravel. 4 have even encountered dams built within city limits that contained wooden

lawn 'urniture, hobbyhorses and parts o' pic&et 'ences as part o' the construction

materials. 2y observation o' hundreds o' permanent beaver dams tells me that

most are made up o' mud with some woody material holding them together.

%eavers pre'er to build their permanent dams on a claytype soil. Clay can be

pac&ed with the woody material, leaves and grasses to hold a dam together.ravely or sandy soils can0t be pac&ed to hold water.

%eavers have good reason 'or building their permanent dams on a clay bottom.

Bhen the beaver uses stic&s, poles, limbs or other woody material, the heavy end

o' this wood is placed on the dam and the smaller ends are on the lower side o' the

dam. ;n the heavy end there is usually some heavy mud pac&ed on it to help hold it

in place. Bhen the water is high and there is 'orce against the upper side o' the

dam, the smaller ends o' the poles and limbs dig into the bottom on the lower side

o' the dam and help hold the dam in placeD 4' the bottom is solid stone or shale, the

water pressure behind the dam during periods o' high water will push the dam o8

its 'oundation.

4' the bottom under the dam is gravely or mostly sand, the turbulent high water

will erode the 'oundation 'rom under the dam construction. %y selecting a clay

bottom, beavers can be sure o' a strong dam.

4t is on the upper side or deep water side o' the dam where the animal digs the

clay 'or the dam construction or repair. This removal o' mud and clay also deepens

the water close to the dam.

%eavers seem to have a pre'erence 'or waterlogged wood gathered 'rom the

bottom o' streams or other bodies o' water 'or dam construction. This material

doesn0t oat because o' its waterlogged condition and the heavy clay 'rom the

bottom used by the beaver helps hold this wood in position on the dam.

Eeavy material

Notice that both o' these materials are heavy. The waterlogged wood is heavier

than ordinary wood and the clay is heavier than ordinary soil. The heaviness o'

these materials helps to &eep the dam in place during periods o' high water. ;'


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course any oating stic&s, 'allen branches or other wood will be used in dam

construction or repair.

Another thing 4 have observed about beaver, and don0t 'ully understand, is that all

material used in dam construction or repair is brought downstream. 4 have torn

holes in their dams, and deposited the material 4 removed 'rom the dam only a 'ew'eet away on the downstream side. The beavers repaired the dam, but they did not

use any o' the material that was only a 'ew 'eet away. They sometimes travel quite

a distance to gather repair material upstream 'rom the ruptured dam. Bhy they

donFt use the material that was removed 'rom the dam and is only a 'ew 'eet away

remains a mystery to me. 4t seems the longer 4 am in this trapping business, the

more questions arise that 4 have no answers to.

;ddball dams

4 have seen some oddball beaver dams during my trapper career. At one place

there were about a hal' doust started to build a dam. 2any o' the shoes oated to the dam and

the beavers used them as building material. This happened several years ago, and

the 07hoe 9amH is still in place.

4 have also learned that beavers will move and bury an animal carcass i' it is

close to a dam. 4t was in the early #*+s when 4 !rst observed this. 4 was using a

beaver dam to cross a deep stream, and had been using it 'or several days. 9eerseason was in progress, and 4 noticed that hunters were also using the dam.

The day a'ter deer season, 4 saw a deer carcass oating about /+ yards upstream

'rom the dam in deep water. 4t was a small deer, and 4 assumed it had been

wounded and too& to the water to escape the dogs. (9ogs were legal to use 'or deer

hunting.) The deer carcass was there 'or a couple o' days, then it was missing. 4

couldn0t imagine what had happened to it. 4t would have been almost impossible 'or


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a human to retrieve the carcass because o' the deep water and oating debris. The

only sign on top o' the dam were my own boot trac&s 'rom the previous 'ew days.

4 hadnFt gone more than /5 steps when 4 came to a 'resh pile o' mud and

decaying vegetation on top o' the narrow dam. 4t was easy to see that it had been

made by the beavers. 4 had never be'ore seen where beavers had made such alarge pile o' debris on top o' their dam. 4 studied it 'or a 'ew seconds, then stepped

on it to continue across the dam. Bhen 4 stepped on the pile o' debris, some o' it

slid o8 into the water, and le't part o' a deer carcass exposed. Bith my shovel 4

scraped most o' the mud and debris 'rom the carcass. %eavers had moved the

carcass 'rom deep water to the top o' the dam and covered it. The next day the

carcass was covered over again.

The 'ollowing day the sign showed that several dogs had 'ound the carcass and

pulled it to higher ground and devoured it. All that was le't was some hair scattered

around and trac&s le't by several di8erent si


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StreamflowThe illustration below depicts how beaver dams in stabilize stream flow rates. The

illustration shows a horizontally compressed cross section between two streams, and

how groundwater charge keeps the stream flowing. The river channels are the U

shapes and the water flows out of the page towards you. Groundwater charge is the

reason streams continue to flow without inputs such as rainfall. Water will continue to fill

the stream from the reserve until the downstream level below the dam is reached (black

triangle). The top half of the illustration shows the surface profile, and the groundwaterlevels for typical rainfall conditions with no beaver dam.


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Bea

ver Dams elevate water tables forming an underground lake to keep streams flowing

The bottom half of the illustration shows the elevation of the groundwater table under the

same typical conditions with a beaver dam present. Beaver dams naturally leak, so thestream will continue to be fed until the downstream level of the black arrows are

reached. Notice that the typical reserve is greater in the bottom illustration, and that

an additional storage buffer exists for storing water. This water reserve can be thought

of as an underground lake, keeping the stream flowing. This wet condition buffer is

represented by the white area full dam reserve and provides storage for flood

mitigation. The blue area is the water charge, and the curved top is caused by rainfall.

The effects of beaver dams in increasing the charge of aquifers reaches (sideways)

across to the next watershed, and upstream as far as the pool is raised! The increased

typical reserve behind a beaver dam is of significant benefit to wildlife and fish during

periods of drought. The benefits are also seen downstream since beaver dams

inherently leak as do charged aquifers. Water springs flowing from the ground are the

result of water flowing out of charged aquifers. These springs also flow from the stream

bed. They tend to be moderated in temperature to the average seasonal ground

temperature.
http://www.beaverdam.info/wp-content/uploads/2013/10/image004.jpg


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The volume of groundwater greatly exceeds the volume of the pond. Also the thermal

mass of the groundwater greatly exceeds the thermal mass of the pond. So, with a

beaver dam, streams keep flowing longer at more moderate temperatures. The surface

of water in the pond may be warmer, but as water flows from the cool charged aquifer

into the stream edges and bottom the temperatures are moderated. Cool springs

percolate into the pond and stream bed from below.

Structure believed to be the

world's largest beaver damlocated, thanks to Google


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TORONTO A anadian!based ecologist said "rida# that he has located the

world's largest beaver dam in northwestern anada using Google satellite

technolog#$

%cologist &ean Thie located the ,())!foot *)+!meter- dam using Google

%arth and NASA technolog# while researching the rate of melting .ermafrost

in the countr#'s far north$

Situated in northern Alberta's /ood 0uffalo National 1ark, which straddles

the Alberta!Northwest Territories border, the dam stretches more than eight

football fields long, Thie said$

23 couldn't believe it when 3 saw it !! it's a vast, vast area$ There ma# be longer

dams out there, but this, b# far, is the largest 3 have seen so far$ And, it would

not have been .ossible to view it without something like Google %arth,2 Thie

said$


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Thie told the Associated 1ress that the detailed satellite .rogram hel.ed him

conclude the dam was the work of beavers !! which, incidentall#, are the

countr#'s national s#mbol$

Thie discovered the dam in (, but he said it onl# recentl# caught media

attention after someone at a 0ritish .a.er s.otted his findings on a blog and

ran a stor# reiterating Thie's claim that the dam is visible from s.ace$ Thie

stakes that claim because he used satellite technolog# to detect the dam$

23t might be hard to believe, but there are a few things that are visible from

s.ace, and beaver dams are among the few animal!made structures that are,2

Thie said$

1ark s.okesman 4ike 5ei6er said the dam sits in a corner of a .ark 2the si6e of

Swit6erland2 in an area surrounded b# heavil# forested marshland$

5ei6er flew over the beaver dam, but said there was no safe .lace to land

an#where nearb# because it's either overl# bogg# or the foliage is too dense$

7sing .ast images and .ark aerial .hotogra.h#, Thie concluded that the eager

beavers began their work in the 89(s and that generations of the rodents have

worked on it since$

2This was the work of e:tended families,2 said Thie, who is the .resident of

%co3nformatics, a science research com.an#$

%eaver 9ams

%eavers ood areas 'or protection 'rom predators, 'or access to their 'ood supply,

and to provide underwater entrances to their den. looded areas also wet the soiland promote the growth o' 'avored 'oods (ig. 3).

%eavers living on water bodies that maintain a constant level (e.g., la&es, large

rivers) do not build dams.
http://www.canadianaconnection.com/cca/beaver.htmhttp://www.canadianaconnection.com/cca/beaver.htmhttp://www.canadianaconnection.com/cca/beaver.htmhttp://www.canadianaconnection.com/cca/beaver.htm


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9ams are constructed and maintained with whatever materials are availableJwood,

stones, mud, and plant parts. They vary in si

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BEAVER INFO.doc