Five miles south of the Spokane- Columbia River confluence is a little inlet known as Hawk Creek; a place where people come to spend time doing everything from kayaking to hiking to bird watching. While basking in the sun and fun, it’s interesting to think about the surrounding geology. Such as, when the Okanogan Ice Lobe melted and started receding toward the north, the surrounding bodies of water began to expand to form glacial Lake Columbia. During the Ice Ages, thinly bedded clay deposits were deposited in the glacial lake. Then eventually the ice dam holding water in glacial Lake Columbia released water down Grand Coulee. Gravel layers in the area indicate that glacial Lake Columbia existed during the Ice Age floods (Atwater, 1986). However, a large amount of the erosion in the area likely happened during the draining of the glacial lake and allowed Hawk Creek to eat away at the fine grained sediments and retreat south from the Columbia River, eventually leaving a waterfall that is located near the Hawk Creek Campground (Kiver et al., 2016). Hawk Creek branches off the Columbia River to the east, snaking around the basalt that the water was unable to cut through, therefore requiring it to change its path, until it eventually makes its way to the base of a waterfall.
Focusing on the formation of this waterfall, Hawk Creek cut through the sediment of the flood deposits, forming a waterfall. A plunge pool has formed at the base of the falls created by the force of falling water and rocks and turbulent eddies carved away at the finer-grained sediment. Once the plunge pool has carved away enough rock, the rock above it loses its support and breaks off, leading to the recession of the water back into the surrounding rock. The rock surrounding the falls is Grande Ronde Basalt of the Columbia River Basalt Group (WA DNR 2016). In order to confirm this, Dr. Pritchard cut the rock sample and smoothed the cut surface, preparing it to be x-ray fluorescence. We then analyzed the rock to determine elemental concentrations which can be used to help identify it. Using a Bruker Tracer III portable x-ray fluorescence gun, we confirmed that the rock sample was a member of the Grande Ronde formation, which is about 15 million years old (Reidel and Tolan, 2013).
So as we’re hiking or kayaking or birdwatching, it’s interesting to think what type of rock we are hiking on, why this creek is curved instead of cut straight and why is the waterfall nestled back into the rocks like that. And with this knowledge of the geology of the surrounding area, we know it’s the basalt and the flood deposits we are hiking on, it’s the waters inability to cut through the stronger basalt that forced it to carve a curvy path through the rocks, and it’s the recession and the plunge pool of the waterfall, a complex interaction of geology over the last 15 million years made the features we enjoy today.