STUDYING SWEET BRIAR'S BEAVERS
Last update: 2000

     One family of beavers usually live in Sweet Briar College's two lakes and surrounding forests. They have created a semipermanent pond and marsh in the northwest inlet of the lower lake, and they periodically dam other campus streams. They do extensive cutting of trees in the forests around the lakes, for winter food, shelter, and dams.

     The beavers have admirers and detractors among the College's human population. Their engineering and woodcutting earn respect and their parental care and evening lake patrols earn affection. By storing water their ponds and marshes dampen fluctuations in the lake's water level during droughts and floods. Sweet Briar's summer choruses of frogs and toads would be thin without these breeding habitats.

     Some people worry that beavers are "bad" for the lake or for the forests: they may kill trees and carry giardia, and introduce woody debris into the lake. They may block culverts and flood trails and roads.

      In fact Sweet Briar students have collected data showing that most trees cut by beavers are not killed, but resprout from their roots. Look closely at the trees around the lakes and you will read a story of repeated cutting and forest recovery. There is no evidence that our beavers are carrying giardia. Beavers do not increase sedimentation and turbidity of the lake, but in fact their dams and ponds help clean the water that enters the lakes. Yes, beaver dams may flood trails and block culverts, but it is not difficult for us to keep one step ahead of them and prevent damage.

     A wonderful natural history of water in North America, Water by Alice Outwater (1996: BasicBooks), describes the role of beavers, and the role of humans in their near-extirpation.

     Watch for our beavers, look carefully in the woods for evidence of their recent or long ago visits.

MAPS SHOWING LOCATIONS OF BEAVER AREAS

Black and white aerial photograph of the lower lake (photo taken spring 1997)
Topographic map of the lower and upper lakes

BEAVER ACTIVITY AT SWEET BRIAR, SPRING 2000

8 February 2000: The Field Natural History class made observations near the beaver dam on the Upper Lake. With several inches of snow on the ground, it was an excellent opportunity to look for animal tracks, but we saw no signs of current beaver activity: no fresh cutting, work on the dam, tracks in the snow, or disturbance near the lodge. Although the Upper Lake was almost completely frozen, the small pond created by the beaver dam within the woods was not frozen. We did notice about a dozen open holes in a line out on the lake, and wondered if these were breathing holes made by the beaver during trips to its underwater larder.

BEAVER ACTIVITY AT SWEET BRIAR, FALL 1999
Surveyed by students in Biol 1 (Introductory Biology Lab), November 1999
SITE (see maps)
RECENT ACTIVITY
OLD ACTIVITY

A: Upstream of culvert on Wetland Way

None

In January-February 1996 a dam upstream of this crossing was destroyed by workmen installing the culvert.
Fall 1997: Beavers dammed the culvert and recreated a marsh upstream. In October students, faculty and staff created an artificial dam upstream of the culvert, to preserve the marsh and allow the culvert to be cleaned. This had limited success, and the area is currently not flooded.
B: Downstream of the culvert, upstream of the beaver marsh.

None. Small dams across the channel show no signs of recent maintenance; the uppermost dam is partially stabilized with sand and grass but is eroding.

Some time before 1997 the stream was channelized by Physical Plant.

1997-99: Several small dams constructed across the stream changed the grade but did not flood outside the channel.

C: Woods south of beaver marsh

None. The major (uppermost) dam is stabilized with sediment, grasses and willows. There are no signs of recent maintenance work. The water level is approximately a foot higher above the dam.
NOTE that this dam is visible in the aerial photograph as the n-s edge to the right of the B.

Winter 1995-96: Heavy cutting. Active work on the major dams.
North of the marsh, where a small stream enters through the wood strip between two fields None. A small dam about 30 feet north of the culvert under the trail has created a small marsh and raised the water level several inches. The dam is in good shape but shows no signs of recent work.
D: Edge of lake, opposite the boathouse
Scattered cut trees
 Heavy cutting (done winter 1995-96)
Old lodge sites.
E

None. Two old lodges are not in current use.

 Moderate old cutting.
Lodges in this area were in active use in winter 1998-99 and 1995-96.
F

Scattered cut trees
Large actively maintained lodge.

 This lodge was smaller and in use in winter 1998-99.
G
One freshly chewed tree
 Moderate old cutting
H

A few freshly chewed large trees

 Heavy old cutting
I

Two large weeping willows next to the boat launch have had their lower bark chewed by the beavers. [Carrie Brown watched one beaver working on one of these trees in early September.]
The stream flowing from the upper lake had a new dam on it near the lower lake earlier this year.

 Not checked before 1999.
UPPER LAKE (see the topographic map)

There was a lot of cutting along the east edge during winter 1998-99 and spring 1999.

 In 1996 the perimeter of the upper lake edge was searched and only old cutting was found.
J

A small new lodge has been constructed October-November 1999. The number of freshly chewed and cut trees along this shore is increasing. One girdled pine near the bank has produced so much white resin that it is visible from the north shore of the lake.

 In early spring 1999 there was no fresh cutting near J or K.
K A new dam was constructed in the woods, on the small creek flowing from the southwest, in November 1999. The dam is about 15 feet long, has raised the water level several inches, and has created a small impoundment. There are a moderate number of freshly chewed and cut trees.  

 

TREE SPECIES SELECTIVITY
Data collected by students in GENERAL ECOLOGY
 
Number Cut
Number Uncut
19 March 1992
Flowering dogwood
41
8
Beech
11
6
Maple
4
2
Tulip poplar
11
19
Musclewood
6
9
Oaks
7

20
Hickory
0
1
Cedar
0
1
Pine
0
19
 
17 October 1995
Tulip poplar
13
9
Sourwood
16
11
Beech
6
3
Hickory
3
2
Oak
11
18
Maple
5
22
Alder
2
3
Sassafras
1

1
Witchhazel
0
1
Locust
0
1
Pine
0
1
Students identified each cut tree based on bark and wood characters as well as leaves or buds if present. They then identified the uncut nearest neighbor with diameter at knee height greater than 5 cm. Not all cut trees were identified.

An incomplete history of beavers and people at Sweet Briar.

Winter 1992-93: Beavers were abundant and active in the lower lake. In autumn 1992 at least one beaver moved to Williams Creek, constructed one small dam and cut quite a few trees, but did not remain in the area.

1993-94: Beavers were trapped out of the lower lake, partly out of concern that they might be carrying giardia. Juli Bechard sampled lake water for giardia for her Biology Junior Honors Research, and found a single cyst after extensive sampling, indicating that fears were exaggerated.

1996: At least four beavers (parents and offspring) were active in the lower lake. Rachel Cooper surveyed beaver activity around both lakes and at Williams Creek as part of her Biology Senior Research. Fionna Matheson and Jenn Hogan produced a video documentary on the beavers entitled Pulp Fiction for Field Natural History.

Spring 1997: The beaver marsh hosted a cacaphony of breeding toads, whose revels were cut short when Physical Plant cut through the major dam. Students in Animal Behavior repaired the dam in an attempt to save some of the millions of toad eggs.

Summer - Fall 1997: Beavers dammed the culvert on Wetlands Way, flooding the area upstream. This became an excellent breeding site for two species of frogs; their tadpoles were studied by Fionna Matheson for her Biology Senior Research. In October a group of students and faculty and the Director of Physical Plant constructed an artificial dam upstream of the culvert, to preserve the wetland and prevent the path from flooding.

Spring 1998: Very few toads bred in the marsh.

Spring 1999: Few toads bred in the marsh, but there were a large number of spring peepers.

URL: //nature.sbc.edu/animals/beavers.html       email:lfink@sbc.edu

This site is maintained by Professor of Biology Linda S. Fink  (434) 381-6436
Last updated: 19 February 2000
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