WEATHERING PROCESSES AND CLIMATE DIFFERENT, BUT CONVEX WATERFALL MORPHOLOGY SIMILAR IN BASALT (KAUAI) AND SILTSTONE (NEBRASKA)

Convex waterfalls have distinct downstream projections of the waterfall face. The free fall of water from the lip of the falls to the base is interrupted by these projections. Often the falls have two or more distinct points of discharge over the lip. Convexity develops when weathering rates on either side of the waterfall exceed weathering rates on the waterfall face and concavities on either side “migrate” upstream faster. Waterfalls on the spring-fed tributaries of the Niobrara River by Valentine, Nebraska have a convex morphology because discharging groundwater from siltstones of the cliff face and spray from the falling water causes freeze/thaw weathering in the winter and wet/dry weathering ( expansion/contraction of swelling clays) cycles in the summer. The face of the falls is protected from these weathering processes by the falling water. Eventually, as the convexity of the falls increases, the projection of the face of the falls becomes unstable and fails, starting a new cycle. Smith Falls is a classic example of convex development

Basalts on Kauai weather in the presence of water and as the result of vegetation. Where both are present the effect is much more than the sum of the parts. If the falling water interferes with vegetation development on the waterfall face, weathering rates are much higher on each side of the falls leading to convexity. Groundwater discharge and waterfall spray provides the water source in the concavities. Often concavities on either side are not symmetrical because of groundwater flow directions, especially on the “dry” lee side of the island. Impact of falling water has less of an effect than weathering in this setting. The discharge of the Kauai waterfalls is much greater than that of the Nebraska waterfalls. Regardless, in each setting, weathering rates control the morphology of the waterfalls. Waipo'o Falls (dry side) and Opaeka'a Falls (wet side) are examples of convex development.