HR: 0800h
AN: H51C-1129    [Abstracts]
TI: Focused Weathering Control of Convex Waterfalls Along the Niobrara River in Cherry and Brown Counties, Nebraska
AU: * Mason, L J
EM: lmason2@bigred.unl.edu
AF: University of Nebraska-Lincoln, Department of Geosciences Bessey Hall, Lincoln, NE 68588 United States
AU: Pederson, D T
EM: dpederson2@unl.edu
AF: University of Nebraska-Lincoln, Department of Geosciences Bessey Hall, Lincoln, NE 68588 United States
AU: Goble, R J
EM: rgoble@unl.edu
AF: University of Nebraska-Lincoln, Department of Geosciences Bessey Hall, Lincoln, NE 68588 United States
AB: More than 200 waterfalls exist along the southern spring branch tributaries that feed an approximately twenty-five mile section of the Niobrara River, east of Valentine, Nebraska. Many of these waterfalls posses a convex shape in the horizontal plane and are buttressed. This morphology is controlled by focused, season-specific weathering along the escarpments adjacent to the waterfall face and a lack of stream erosion on the actual waterfall face. The waterfalls are composed of the Rosebud Formation, a poorly indurated siltstone that should be easily eroded by stream flow. The spring creeks are ineffective at significantly eroding the waterfall face mainly due to their relatively low discharge, three to five cubic feet per second, and low sediment load. The erosive power of the streams is further reduced at the site of the waterfall by the buttressed shape spreading the flow into a thin sheet. The buttressed shape of the waterfall develops in response to stress relief. The only areas of the waterfall face showing stream erosion and lack of diatom cover is where free falling water is impacting the waterfall face. Large, loose talus slopes at the base of the waterfall escarpments further support that the weathering processes operate at a faster rate than stream erosion. Observable groundwater seepage from the escarpments on either side of the waterfalls exposes the faces to season-specific weathering processes. The moisture content of the escarpments varies with exposure to sunlight and changes in air temperature. Cyclic differential expansion and contraction of clays and minerals as well as precipitation and hydration of salts operate on a daily and seasonal basis. These repeated stresses give the escarpments a flaky, shingle like appearance and can cause rapid deterioration of the escarpment. During the winter, the seeping groundwater and waterfall spray form large ice flows on either side of the waterfall face. Freeze-thaw processes operate on a seasonal and shorter cycle due to the variable regional weather. These weathering processes are focused on areas of the escarpment lacking stream flow. The stream protects the waterfall face by preventing cyclic moisture level changes during the summer and freeze-thaw cycles during the winter. With increasing convexity, the stream flow is diverted to the sides of the waterfall face. The waterfall face is then exposed to moisture level changes and freeze-thaw cycles due to lack of stream protection. The waterfall face experiences major failure and erodes upstream, whereby it is protected by stream flow again and begins to regain a convex shape.
DE: 1815 Erosion and sedimentation
DE: 1818 Evapotranspiration
DE: 1824 Geomorphology (1625)
DE: 1863 Snow and ice (1827)
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting