Lake Yankton Trail 7 Earthcache EarthCache
Lake Yankton Trail 7 Earthcache
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This earthcache is located along a public hiking trail. Parking is available without a Park Sticker at the National Fish Hatchery parking lot, but access is only available by hiking the trail. Lake Yankton and Gavins Point Dam are located in the Missouri River valley. Lewis and Clark Lake, above the dam, is bordered by high, chalkstone cliffs which were exposed by the excavations necessary to build the dam. The Missouri River and the lake impoundment lay in a valley that is geologically referenced as the Missouri River Trench. Gavins Point dam was built to provide flood control along the river and was completed in September, 1956. The Missouri River Valley averages one mile wide for approximately 100 miles from northwest to southeast in this region. The floor of the valley is essentially silt and lays approximately 300-600 feet below the surrounding bluffs above the river. Practically speaking, the Missouri River valley serves as the deviding line between the plateau of the Great Plains on the west and an area to the east that is referred as the Coteau du Missouri. Geologically, the great dams along the Missouri River basin have caused a basic change in the river bed. Since completion of the dams, the riverbed is eroded much deeper than it previously was. This has resulted in many changes in the riverbed that have resulted from erosion in some area and silting in along other areas. Answer the following questions to receive credit for logging this earthcache: Q1: What is the altitude of the GZ? Q2: What is your estimate the altitude of the bluffs to the south of the riverbed? Q3: What is the geological make-up of the soil at GZ? Q4: Observe to the west and south. What geological features helped make practical the location of Gavins Point Dam at this site?
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