The August 13, 1916 Lake Toxaway Dam Failure: A Retrospective from the Geologic

Record in Gorges State Park, Transylvania County, North Carolina

Wooten, Richard M.

Latham, Rebecca S.

Detailed geologic mapping of Gorges State Park by the North Carolina Geological Survey

identified flood deposits along the Toxaway River attributed to the August 13, 1916 catastrophic failure

of the Lake Toxaway dam. After withstanding two hurricanes in July 1916, the earthen dam failed

following another hurricane that moved inland from the Gulf of Mexico. Rainfall from the third

hurricane may have exceeded 500 mm (~20 in.) in the 24-hour period before the dam failed. The dam’s

failure mechanisms remain uncertain. Accounts of springs near the base of the dam suggest piping may

have been a contributing factor. A report of a "four-foot tide" at the dam near the time of failure indicates

floodwaters overtopped the dam.

Dr. S. W. McCallie, Georgia state geologist at the time stated, "An estimated 5,376,548,571

gallons of water changed hands." Although there were no reported human fatalities from the dam failure,

the flood destroyed several homes and permanently changed the nature of the Toxaway River gorge.

With the lake gone, the resort on Lake Toxaway closed, hurting the local economy for many years to

come. Rebuilt in 1960-1961 the present dam is very near the original dam site.

Floodwaters from the dam failure torrent scoured 3.5 km (2.2 mi) of the gorge down to bedrock

from Lake Toxaway downstream to Wintergreen Falls. Below Wintergreen Falls discontinuous boulder

levees and sheet deposits occur along a 6 km (3.7 mi) reach of the river. Individual boulders up to 18 m

(60 ft) long, and imbricated boulders confirm the deposits’ floodwater origin. Crests of boulder levees

stand 2-10 m (~6-30 ft) above the present river level, recording minimum floodwater elevations. Some

boulder levees have two subparallel crests, with the lower crest nearer the river, while others have a lower

terraced surface, also on the river side of the crest. Pulsed flow caused by debris dams forming and

bursting as the floodwaters progressed downstream may have produced these composite overbank

deposits. Post-1916 floods also may have modified the original geometry of the levees. A valley-fill,

cobble-gravel-sand facies of the flood deposit underlies Lake Jocassee, and extends into South Carolina

over 11 km (6.8 mi) downstream from the original dam. The flood deposits attest to first-hand accounts

that a "30-foot wall of water thundered down the valley, and rocks as large as train cars rolled and

tumbled down the mountain."

Subsequent studies have focused on estimating the velocity and discharge of the dam failure

torrent by examining relict geologic and hydrologic evidence. Estimates of the outflow velocity made at

four locations downstream from the dam range from ~85 km/hr (~53 mi/hr) to ~48 km/hr (~30mi/hr).

Estimates of outflow velocities were calculated using the superelevation angle of the flow around channel

bends, along with the channel’s radius curvature and gradient. At a location just below the dam the

outflow discharge was calculated to be ~8,316 m

m

ft

Other impacts of the flood torrent are still visible. Erosive floodwaters truncated colluvium and

tributary alluvial fan deposits along toe slopes, triggering rockfall, debris slides, and a still-active 1.6-

hectare (4 acre) weathered-rock slide. The 1916 boulder deposits locally constrict the channel of the

Toxaway River and divert its tributaries. A 51,200 m

upstream of the present confluence of Bearwallow Creek with the Toxaway River may have diverted the

original confluence 60 m (200 ft) or more downstream along the Toxaway River.

Tree ring analyses of increment borings from Tsusga canadensis (eastern hemlock), Pinus strobus

(white pine) and Pinus rigida (pitch pine) support a 1916 origin for the flood deposits. Of the five large

trees sampled that were growing on the flood deposits, the oldest beginning growth year was 1917.

Increment cores extracted from a curved Tsuga canadensis and an Acer rubrum (red maple) on the active

1.6 hectare (4 acre) weathered-rock slide, probably triggered by the flood torrent, indicate a period of

slide movement from about 1965-1974. Tree rings show a decreased relative tree growth rate during the

mid-1960's through the mid-1970's for both slide trees, but not in a nearby control tree (Tsusga

canadensis) off the slide, suggesting a significant period of sliding occurred during this time. The slide

trees record slide movement as a decrease in growth ring thickness due to a decrease in water and nutrient

uptake caused by disruption of the root-soil interface during slide movement. Conversely, the increased

relative tree growth rate in the control tree during the same time period corresponds to a period of above

average precipitation that probably triggered this episode of slide movement.

The flood deposits and features preserved in Gorges State Park provide a modern analog for

sedimentological and hydrologic studies of ancient catastrophic flood events in mountainous terrain. The

N.C. Division of Parks and Recreation plan to incorporate some of these geologic and hydrologic features

into the Park's interpretive programs to help illustrate man’s impact on the landscape.

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