The Failure of Beaver Dams and Resulting Outburst Flooding: A Geomorphic Hazard of the Southeastern Piedmont

David R. Butler

Department of Geography University of Georgia Athens, Georgia 30602

ABSTRACT

Beaver are geomorphic agents through their building of dams and cre­ation of pond environments. In the southeast United States, the successful reintroduction of beaver has vastly in­creased the number of ponds im­pounded by beaver dams. These dams may fail, producing rapid and potentially catastrophic draining of beaver ponds, a geomorphic hazard which has not gained sufficient notice.

This paper examines several cases of known dam failure and subsequent rapid drainage of beaver ponds. Reconstruc­tions of the discharge from these ponds was accomplished using standard hy­drologic relationships of stream dimen­sions or pond volume to discharge. Peak flood discharges may be catastrophi­cally high . The maximum hazard for beaver-dam failure and pond outburst­flooding occurs on the Piedmont, where periodic intense rainstorms combine with rapid surface runoff and limited surface infiltration to produce high-discharge floods.

KEY WORDS: beaver dam, flooding, geo­morphic hazards, Piedmont, southeast­ern United States.

INTRODUCTION

The historical presence of native bea­ver (Castor canadensis) throughout the southeastern United States was of suf­ficient number to strongly impress early settlers in the region . Names such as Beaver Creek, Little Beaver Dam Creek, and Great Beaver Dam Creek appear on maps of Georgia that date back to 1780 (Parrish 1960). However, native beaver in the region were nearly exterminated during the late nineteenth and early twentieth centuries (Moore and Martin 1949, Parrish 1960, Johnson and Aldred 1984). In some areas in the southeast, beaver had in fact disappeared (Ala­bama Department of Conservation and Alabama Forest Products Association 1967, Taylor 1985). In the late 1930s through the early 1950s, southeastern state wildlife and conservation agencies successfully reintroduced the beaver. The conversion of many areas in the south-

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east from croplands to forestlands since World War Two has also played a sig ­nificant role in expanding potential bea­ver habitat (Anonymous 1986). Today, the beaver repopulation efforts in the south­east have proved so successful that many private and corporate landowners now consider the rodent to be a destructive pest, responsible for massive dollar amounts of timber destruction and flooding of cropland (Shipes et al. 1979, Forbus and Allen 1981 , Johnson and Aldred 1984, Anonymous 1986).

The reintroduction of the beaver in the southeast was done in part at least be­cause of its geomorphic capabilities to reduce stream erosion and sediment transfer through the damming of streams and creation of pond environments, and to locally elevate the water table and re­duce the effects of seasonally-fluctuat­ing water table levels (Ruedemann and Schoonmaker 1938, Wilde et al. 1950, Apple et al. 1985, Johnston and Naiman 1987, Remillard et al. 1987). In areas of appropriate lithology (i.e., limestones and dolomites ), beaver ponds may act as agents of karstification by providing lo­calized concentrations of water for so­lut ional work and capture by under­ground streams (Cowell 1984).

Although the literature cited above has provided some examination of the geo­morph ic effects of the beaver and bea­ver dam construction, little attention has been given to the potential for natural hazards created by beaver-dam failure and subsequent catastrophic flooding . This paper examines that potential for rapid beaver-dam failure and pond drainage in the southeastern U.S" and presents historical examples and hydro­logic reconstructions which illustrate the nature of the hazard .

Beaver Dams and Other Natural Dams

The formation and failure of natural dams have been recently reviewed by Costa and Schuster (1987). Obstructive natural dams include landslide dams, moraine dams, volcanic dams, glacier dams, fluviatile dams, coastal dams, eolian dams, and organic dams; this last category includes log and vegetation

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dams, and beaver dams (Costa and Schuster 1987 p. 2-3).

Floods from the failure of natural dams are usually much larger than floods of rainfall or snowmelt origin, but little is known of the processes of natural-dam failure (Costa 1985). However, through an examination of many cases of dam failure, Costa (1985) derived a series of regression equations to predict peak dis­charge for human-built, glacial, and landslide-dam failures. If, for example, the volume of water (V) which escapes a landslide dam is known, peak dis­charge (Q max) can be modelled as :

Q max = 672 Va.56 ; r2 = 0.73

Beaver dams are organic dams con­structed normally of logs and brush weighted down with mud and stones (Warren 1932, Ives 1942, Neff 1959). Dam size and shape vary considerably, but a typical shape is a concave-upstream arch dam (lves 1942). Excellent photographs and schematic drawings of typical bea­ver dams may be found in Dugmore (1914) and Warren (1927) .

Beaver dams will break when stream discharge exceeds a critical strength threshold (Parker et al. 1985). Warren (1927 p. 50) mentioned that "the wash­ing away of dams by floods are com­mon happenings on our western streams, if not elsewhere," and Dugmore (1914) presented a newspaper account of an actual beaver-dam outburst flood. This flood caused landslides resulting in blockage of the main line of the Cana­dian Pacific railroad in eastern British Columbia. Dugmore (1914 p . 148) re­ported that :

"the slide, which was 300 feet wide and 30 feet deep, was caused by the bursting of an old beaver dam high up in the mountains ... the dam burst under the pressure of heavy rain storms last week .. . . Huge trees were brought down with the slide and boulders nearly as big as a box car made the job of clearing the track a difficult one. Some of the trees that came down bore the marks of the lit­tle animals' teeth, and the supports of

the dam erected by the beavers were plainly marked as such by the bleach­ing of their upper ends and the lower points coated with mud and slime."

Unfortunately, little recent attention has been given to beaver-dam failures. Costa (1985) did not examine them in any detail in his work, so no regression equation is available for beaver-dam failures . Costa ' s (1985) equat.j ons for landslide-dam failure provide the best substitute analogy for modelling dis­charge from beaver-dam failure.

Beaver-Dam Sites and Distribution in the Southeastern States

Beaver are present in every coastal southeastern state, here defined as North Carolina southwestward to and includ­ing Louisiana . They are concentrated primarily in the Piedmont and upper Coastal Plain physiographic regions, al ­though they are found to a lesser extent in the Appalachian highlands and lower Coastal Plain (Parrish 1960, Shipes et al. 1969, Johnson and Aldred 1984, Taylor 1985). The low number of beaver in these latter regions is attributable to sites which are geomorphically less favorable. In the lower Coastal Plain, stream beds con­sisting largely of loose sand with little plasticity offer poor foundations for es­tablishment of dams and burrows (Par­rish 1960). Beaver ponds do occur in the Coastal Plain along second and third or­der streams with wide floodplains (Shipes et al. 1979). where some finer-grained alluvium exists.

Beaver are also limited in number in the Appalachian Mountains in the south­eastern states . There , swiftness of streams frequently precludes successful dam construction and maintenance, and an absence of mud in some areas of limestone lithology further reduces the likelihood of successful beaver-dam pro­duction (Parrish 1960, Taylor 1985).

Although actual statistical data are lacking from several states, areal cov­erage of beaver ponds formed by dam construction is impressive. An aerial pond survey in Alabama revealed over 10,000 beaver ponds covering over 95,000 acres, and affecting over 13,000 stream miles

(Alabama Department of Conservation and Alabama Forest Products Associa­tion 1967). Mississ ippi contains about 24,000 acres of beaver ponds, with four northeastern counties alone accounting for over 10,000 acres. Pond acreage es­timates are not ava ilable for other states, although beaver are reported f rom "most of the Coastal Plain and Piedmont" in North Carolina (Taylor, 1985), in 28 of 46 counties in South Carolina (Sh ipes et al. 1979). in " virtually every county" in Georgia (Anonymous 1986), and in every county in the Florida panhandle (Ala­bama Department of Conservation and Alabama Forest Products Associ at ion 1967). Impoundments in Louisiana are believed to cover approximately one­eleventh the amount of their Alabama counterparts (Johnson and Ald red 1984).

Beaver-Dam Failures and Outburst Floods-Case Studies

Beaver ponds may lose water by evapo ration, percolation through the pond floor (Cowell 1984). " leakage through dams resulting from activ ity of semiaquatic mammals" such as otters (Reid et al. 1988), and leakage through or failure of dams as a result of water erosion . The last case, as discussed be­low, is hazardous and typically related to intense, short-term thunderstorms over localized areas.

Five cases of beaver-dam failure and flooding are known from northeast Georgia and western South Carolina, and are described below (Figure 1). Two of these cases occurred in separate inci ­dences in 1969 (sites 1 and 2, Figure 1). Two other ponds burst near Comer, Georgia, in the watershed of the Broad River (Site 3, Figure 1). after a period of very heavy rain in 1986 (R. Rogers, per­sonal communication 1988). The final case took place on 7 September, 1987, when a beaver-dam outburst flood killed four people in Oglethorpe County, Geor­gia (S ite 4, Figure 1).

The 7969 Cases

Pullen (1971) examined several bea­ver ponds in Georgia and South Caro­lina during 1969. In the course of his in­vestigations, he periodically revis ited

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o 500 I , I I

Kliomelers

D Coastal Plain D Piedmont ", \ Undifferentiated

Appalachian Highlands

~Other

Figure 1. Physiographic provinces of the southeastern states, and the locations of beaver-dam outburst floods . 1. Dyer Pond. 2. Railroad Pond. 3. Comer ponds. 4. Mill­stone Creek site.

several ponds. Two ponds, Dyer Pond and Railroad Pond, almost completely drained as a result of dam breakage be­tween revisits. Dyer Pond is in .the drain­age basin of Greenbrier Creek, in the Oconee River basin southwest of Ath­ens , Georgia . Ra i lroad Pond , in the drainage basin of the Savannah River, is east of Augusta, Georgia, ne?r Aiken, South Carolina (Figure 1).

Dyer Pond was orig inally 4 acres in size. On an unspecified date in spring, 1969, but prior to a revisitation on 14 July, the pond 's water level lowered 3 feet and the acreage covered shrank to 0.2 acres " due to an unrepaired break in the dam" (Pullen 1971 p. 17). Railroad Pond 's water level dropped 2 feet in " the fall of 1969," and its acreage shrank from 3 to 0.2 acres.

As Pullen (1971 p. 27) noted, beaver dams in stream channels are very sus­ceptible to being washed out by flood­waters. In order to determine the me-

32

teorologic conditions most likely to cause dam failures and drainages of Dyer and Railroad Ponds, I consulted local -area weather records for the appropriate pe­riods . In both cases, precip itation events of unusual intensity occurred, but only once, during the season when dam fail­ure was known to have taken place (U.S. Department of Commerce 1969a, 1969b, 1969c).

Dyer Pond, known to have failed in spring, 1969, probably failed on or im­mediately after 15 April. Athens, Geor­gia, received between 2 and 3 inches of rain on that date (U.S. Department of Commerce 1969a), with almost 3 1/ 2 inches falling over the period of the 15th and 16th (Athens Daily News 19 April 1969) . Local rivers and streams pro­duced "flash flooding in several areas as numerous streams overflowed their banks" (U.S. Department of Commerce 1969a p. 39). The absence of alternative heavy precipitation events allows tenta-

tive acceptance of the 15 April storm as the causal agent for the breaking of the dam on Greenbrier Creek containing Dyer Pond.

The maximum discharge of the Dyer Pond flood was calculated using Costa's (1985) formula for landslide dam failure. Volume reduction of the pond was cal ­culated at approximately 14,000 cubic meters, using Pullen's (1971) data on areal shrinkage and depth reduction (af­ter conversion to metric for compatibil ­ity) . With Costa's equation,

Q max = 672 Va.56

where V = approximately 14,000 m3;

therefore,

the Dyer Pond Q max = approximately 62 m 3 sec- '.

Railroad Pond, South Carolina, likely failed on or soon after 3 September, 1969. Consultation of weather records for Au­gusta, Georgia, and Aiken, South Caro­lina (1969b 1969c) revealed a heavy downpour on that date, with 2.57 inches recorded on that date in Aiken, and over 4 inches in Augusta. As with the case of Dyer Pond, no other storm of suc~ in­tensity occurred during the general tlme­frame specified by Pullen (1971) .

The maximum discharge from the draining of Railroad Pond was calcu ­lated in the same fashion as for Dyer Pond. Railroad Pond underwent a vol­ume reduction as a result of dam failure of 6900 cubic meters, which provides (via Costa 's equation) a Q max of approxi­mately 41 cubic meters per second.

The Comer, Georgia, Cases

After a period of very heavy rain in 1986, two beaver dams failed and caused rapid drainage of adjacent pond.s. These ponds are within the town limits of Comer, Georgia (Figure 1). The larger of the two ponds was estimated as ap­proximately 75 m x 30 m in area, and about 1.5m deep (R. Rogers, personal communication 1988). giving a volume drainage of 3,375 cubic meters. Based on Costa's (1985) landslide-dam failure formula, a maximum discharge of al­most 28 cubic meters per second resulted.

The Mil/stone Creek Tragedy

The abandoned Echols Mill site on Millstone Creek is located in Oglethorpe County, Georgia, in the Broad River wa­tershed approximately 30 km east of Athens, Georgia (Figure 1). The nearest recording weather station is in the county seat of Lexington, Georgia, 14 km south­west of the mill site. Heavy rains fell in the area of the headwaters of Millstone Creek on 7 September 1987, the Sunday of the Labor Day weekend. Lexington re­corded 3.18 inches on this date (Na­tional Oceanic and Atmospheric Admin­istration 1987).

Numerous beaver ponds formed by dams exist in the small tributary streams in the headwaters of Millstone Creek, and evidence of beaver activity is present throughout the area and along the course of the stream (Figure 2). Millstone Creek itself is typically a very placid stream, usually "two feet across and no deeper than the hubcaps of a car" (Ford and Veal 1987).

In order to more accurately quantify the typical stream discharge at the ford crossing where the disaster occurred, channel width, depth and velocity were measured at three positions along se­lected transects (Figure 3). Velocity was measured by timing a floating object over a 3 m course. Three velocity measure­ments were recorded and averaged. The channel depth measurements were also averaged. With measurements of width (W). depth (D). and velocity (V), dis­charge (Q) can be calculated using Leo­pold and Maddock's (1953) formula :

Q = WDV;

this calculation provided a discharge, as measured during a "typical" period of flow on 1 March 1988, of 0.28 cubic me-ters per second. . .

The creek flows in a channel which IS

essentially "perched" on solid granite bedrock which precludes effective inci­sion. At several points along its course above the mill site, water depths never exceed 10 cm. Along the flanks of the creek, soil depth is typically less than 15 cm to granite bedrock, with granite out-

33

Figure 2. Students examine a beaver-downed tree amidst flood debris on north side of Millstone Creek, upstream from the disaster site. Stream depth varies from 6 to approximately 20 cm in this reach .

cropping at the surface in many loca­tions. Under such conditions, the natural stream flows almost as if in a flume, with no infiltration into the stream bed or banks.

On the night of 7 September 1987, at approximately 10:00 P.M. local time, six people in a Datsun automobile who had been picnicking at the abandoned mill site began to cross the creek at the ford site. Suddenly, rising waters stopped the car midstream (Ford and Veal 1987). The couples climbed out of the car and onto a large rock, where they shouted for help. The creek "swelled into a raging river," according to an eyewitness who had re­sponded to the cries for help. A strong surge of water subsequently came along, which hit the rock, "made a big wave and washed them off" (Ford and Veal 1987 p. 6). A "wall of water" washed their car from where it was parked on the granite outcrop (approximately in the position of the truck in Figure 3) . When rescue

34

crews arrived they found a "raging river." None of the victims were in sight. Large granite blocks from an adjacent quarry, some up to 1 m in diameter, were "being moved like cotton balls" (Oglethorpe County Sheriff's Department, personal communication Lexington, Georgia, 9 March 1988). Deputies from the Sheriff's Department found the car downstream about 100 yards from where it had been parked (Veal 1987). Two of the six peo­ple survived the flood. One of these was found "clinging rigidly to a tree more than 12 feet above the ground, where he told rescuers the waters tossed him" (Ford and Veal 1987 p. 1). The bodies of the four victims were found between a quarter and a half mile downstream from where they had been washed off the rock. The flood was attributed by the Sheriff to bursted beaver ponds from upstream, with the dams broken by the heavy La­bor Day weekend rains.

Beavers can repair failed dams rap-

Figure 3. Stream depth considered typical by local authorities, as measured on 1 March 1988. Three measurements here revealed an average depth of 6.6 cm. Truck parked on exposed granite would have been underwater at height of flood.

idly (Reid et al. 1988), so rather than en­gage in a potentially futile search for a drained beaver pond from which vol ­ume and discharge could be calculated, I instead calculated the Millstone Creek flood discharge by measuring the areal cross-section (W x 0) of the flooded channel. The lateral limits of the flood were determined by walking along the channel and identifying where flood de­bris had been deposited in and around the bases of trees and other obstruc­tions (Figure 4). Included within the flood debris were many logs and sticks which

had been completely denuded of their bark by beaver, possibly debris rem­nants from the collapsed dam upstream. Several logs which were covered with beaver teethmarks were recovered in the flood debris.

After locating the lateral margins of the flooded zone, the total width of the flooded area, and the depth of inunda­tion at several locations along the width­measurement transect were measured. The margins may represent a minimum level only, because debris at higher lev­els may have been lost or removed dur-

35

Figure 4. Flood debris on normally-dry granite slopes adjacent to Millstone Creek, 100 m downstream from the disaster site. Student in background would have been completely underwater during flood maximum.

ing the flood. Average depth and chan­nel width were then multiplied by upper and lower velocity estimates derived from sediment competence curves, using the 1 m-diameter granite blocks as mea­sures of the creek's erosive capability. Determination of discharge (Q) via W x D x V, with 2 m/second as the low es­timated velocity, provided a minimum discharge of 72 cubic meters per sec­ond; determination with 9 m/second as the higher (and probably more realistic) estimated velocity gave a discharge of almost 325 cubic meters per second! In either case, it is clear that beaver-pond outburst floods from dam failures can produce flood peaks of dangerous proportions.

CONCLUSIONS

Beaver dams are both beneficial and potentially hazardous geomorphic agents. The dams create wetlands and assist in retarding surface runoff and erosion.

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However, beaver-dam failures and re­sultant floods are real geomorphic haz­ards in the southeastern United States, and also probably in other environ­ments where beaver have become re­established in the last few decades. In the southeast, locally heavy precipita­tion events and relatively steep topog­raphy produce a hazard that is probably greatest in the Piedmont region. Fewer beaver exist in the areas of steepest gra­dient in the Appalachian Moutains, so although environmental conditions there would produce maximum flood danger, the relative absence of beaver do not produce a significant hazard. On the Coastal Plain, soils possess a greater in­filtration capacity. Streams there are less likely to be subjected to the flashiness of rapid drainage into a beaver pond ca­pable of bursting a dam (Woodruff and Hewlett 1971). In the Piedmont, the haz­ard is at its maximum, particularly in those areas such as along Millstone

Creek, Georgia. Here, granite-floored stream channels produce natural flumes without infiltrational capabilities. Sur­face runoff into beaver ponds is also very rapid in these areas where infiltration capacity is limited.

The following conditions maximize beaver-pond outburst flooding: 1) oc­currence immediately after a locally­heavy thunderstorm which has deliv­ered 3+ inches of rain in less ·than 24 hours; 2) in areas of granite or other im­pervious rock outcrops where surface runoff is rapid and infiltration is limited; and 3) along relatively steep stream courses in the Piedmont.

ACKNOWLEDGEMENTS

wish to thank the students in my Geography 816 seminar for firing my in­terest in the beaver as a geomorphic agent, and for accompanying me to the Millstone Creek site during initial recon ­naissance. Christi Lambert was espe­cially instrumental in the early class dis­cussions on this topic, and Joe Nicholas of this group assisted in the fieldwork. The Department of Geography, Univer­sity of Georgia, provided logistical and financial support for the study.

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Anonymous 1986. The Eager Beaver-Forest­ry 's Continuing Nuisance. Georgia For­estry, 39(1) : 6-7.

Apple, Larry L. , Bruce H. Smith, James D. Dunder, and Bruce W. Baker 1985. The use of beavers for riparian / aquatic habitat res­toration of cold desert, gully-cut stream systems in southwestern Wyoming. In : In­vestigations on Beavers (ed. by G. Pilleri), Brain Anatomy Institute, Berne, Switzer· land, p. 123-130.

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Pullen, Thomas M ., Jr., 1971 . Some Effects of Beaver (Castor canadensis) and Beaver Pond Management on the Ecology and Utilization of Fish Populations along Warm­Water Streams in Georgia and South Car­olina. Unpublished Doctoral Dissertation, School of Forest Resources, University of Georgia, Athens, Georgia, 84 pp.

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Wildlife Agencies, 33 : 202- 211 . Taylor, Mark, 1985. The Beavers are Back.

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Veal, Karen, 8 September 1987. 4 Die as Water Sweeps Car Down Vesta Creek. Athens Daily News, p. 1, Athens, Georgia.

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