SALT WEATHERING OF WATERFALL ESCARPMENTS ALONG THE NIOBRARA RIVER NEAR VALENTINE, NE

Many of the over 200 waterfalls along the spring-branch tributaries of the Niobrara River have salt efflorescence coatings in alcoves, developed on either side of the waterfall face. Groundwater seeps slowly through the Rosebud Formation, a poorly indurated siltstone that composes the escarpments on which the waterfalls develop. Although the escarpments in general face north, in the summer direct sunlight can fall on the alcoves. Evaporation of seeping groundwater is greatest in the afternoon when air temperatures are highest and humidity lowest. Because the alcove on the eastern side of the waterfall face is most exposed to the afternoon sun, the formation of salt efflorescence is greatest at this location. Also, the overall deposit increases and decreases in a daily rhythm, depending on rate of groundwater seep and evapotranspiration rate. The coating is first apparent on the edges of small flakes and protrusions on the cliff face. The flakes are arranged in a shingle like array. The differential swelling and shrinking of smectite clays and other minerals also occurs with changing moisture level of the alcoves.

The precipitation of and hydration of salts along the waterfall escarpments operates on a daily and seasonal cycle. The cyclic nature of salt hydration and precipitation as well as hydration and dehydration of swelling clays and other minerals causes differential stresses within the rock which eventually leads to failure and rock spalling. Rock escarpments experiencing this kind of weathering tend to be rounded-out and alcove or cave-like with loose talus slopes below.

In this system, running surface water acts to protect the waterfall faces from the effects of salt weathering. The presence of constantly flowing water prevents the cyclic hydration and precipitation of salts as well as the swelling and shrinking of clays and other minerals by maintaining a relatively constant moisture content keeping smectite clays hydrated and preventing and removing precipitated salts. Due to the relatively low discharge of the spring tributaries, they are ineffective agents of erosion and erode slower than salt weathering. This is supported by the presence of the loose talus slopes and protruding rock beneath the flowing surface water.