FREEZE THAW EROSION OF WATERFALL ESCARPMENTS ALONG THE NIOBRARA RIVER NEAR VALENTINE, NE

Over 200 waterfalls exist along the southern spring-branch tributaries feeding an approximately twenty-five mile stretch of the Niobrara River, east of Valentine, NE. A significant portion of these waterfalls have a convex shape in the horizontal plane, are buttressed, and formed of the poorly indurated siltstone of the Rosebud Formation. This morphology is controlled by weathering and erosion due to the interaction between groundwater and surface water along the streams. Flowing surface water from the streams appears to be a relatively ineffective erosive agent at the site of the waterfalls. Due to the convex buttressed nature of the waterfalls and the low discharge of the streams, the flowing water tends to spread out over the entire fall face flowing in a thin sheet. Few areas of the waterfall face are lacking diatom cover or show evidence of being eroded by flowing surface water. The only areas on the waterfall face where erosion by flowing surface water can be readily observed are where free-falling water is impacting the rock of the waterfall face. On either side of the waterfall face alcoves have formed producing damp scree. This suggests the weathering processes along each side of the waterfalls are more effective than stream erosion.

A majority of the waterfall escarpments are north facing and during the winter, large ice flows form on either side of the waterfall face. This ice is formed by the freezing of groundwater slowly seeping from the cliff faces and waterfall spray. The northern exposure of the waterfall escarpments and the low angle of solar insolation allow the cliffs to experience a relatively deep freeze. The highly variable nature of the regional weather allows for rapid freeze-thaw cycles. Rock fall from the escarpments can be seen and heard during these warming cycles.

Because the stream water is spring fed the waterfall faces are protected from freezing. It is the concentration of weathering to either side of the waterfall face as well as protection of the waterfall face by the microenvironment of the stream that controls the unique morphology of waterfalls along the Niobrara River.