7 Hydrogeology, Environmental and Engineering Geology (Posters)

SOLID ROCK OR SHIFTING SAND . . . IT MAY NOT MATTER: SHORELINE DEGRADATION AT NEBRASKA'S LARGEST MANMADE LAKE

Paper number: 7-1
Program: Technical Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

Lake McConaughy (35.4 km long) was formed by Kingsley Dam across the North Platte River in 1941. Peak storage was reached first in 1951. The lake has become a major recreation center (properties now valued at >$70 million) and development continues. The shoreline has been changed since 1941 by wave erosion and mass wasting (high stands), and by fluvial and eolian processes (low stands). The shoreline probably retreated about 266 m at one site since the filling of the reservoir. Putative retreats of 20-80 m are the rule. By November 2000, drought, reduced snowmelt and runoff, and discharge from the lake lowered pool level 10 m below its highest recent stand, exposing nearly continuous bedrock exposures and lake shelf along the southeastern shore. These exposures revealed wave-cut bedrock terraces, low cliffs, stacks, and caves along the former high-water line. The original dendritic shoreline is slowly being smoothed. The steeply sloping lake shelf exposed at low lake levels is veneered by up to 2m+ of sand and sandstone or siltstone pebbles to boulders which have been eroded from the variably cemented, but generally weak, bedrock of the upper Miocene Ash Hollow Formation, Ogallala Group.

Strong northwesterly winds across the long fetch (> 7 km) of the lake produce waves > 1 m in amplitude which affect the southeastern shore. The waves undercut scarps and cause collapse or toppling of bedrock and soil-bedrock blocks up to 30 m3. Widely spaced fractures parallel and perpendicular to cliff faces develop landward as shoreline recession continues. Without drastic and probably fiscally impossible engineering actions being taken, shoreline mass wasting and erosion will continue indefinitely until some geomorphic equilibrium is reached. Gullying (exceeding 1 m in maximum depth) of lake shelf sediments by major rainfall events and wave erosion of pocket beaches between bedrock terraces at low stand have also been observed. Shoreline retreat, coupled with beach and lake-shelf erosion at low stand, is clearly endangering some high-value structures (residences, docks, landings, beaches, etc.). Public concern over the results is emerging.