UBC Theses and Dissertations
The environmental history and geomorphic impact of 19th century placer mining along Fraser River, British Columbia Nelson, Andrew David
One possible source of part of the 171,000 to 229,000 m³ of gravel that accumulate annually in the gravel bed reach of Fraser River in the Lower Mainland is sediment dumped into the river by 19th century placer gold mining activity. Historical data suggest that, following the Fraser Gold Rush of 1858 and the Cariboo Gold Rush of the 1860s, a substantial placer gold extraction industry was established and continued into the beginning of the 20th century. Gold production figures and typical gold concentrations can be combined as a proxy to estimate that around 50 million m³ of sediment were excavated by mining activity. Excavations caused by mining are still present in the modern landscape. The areas covered by 456 mine excavations were mapped between Hope and the Cottonwood Canyon along Fraser River. A subset of 58 mines was surveyed and strong regression relations predicting mine volume from mine area were found and used to produce estimates of the volume of excavated material. This allows estimation of the total excavated volume of sediment: 45,900,000 m³. Small mines (<315,000 m³) contributed most of the tailings; and only 30% of the tailings came from hydraulic mining. Grainsize sampling and stratigraphic observations suggest average mine tailings were composed of 14% small cobbles, 32% gravel, 41% granules and sand, and 13% silt and clay. The resulting sediment wave on the Fraser can be classified as a megaslug. Sediment transport calculations suggest that the capacity of the Fraser to transport sediment is substantially higher than the average tailings load, so the key factor limiting downstream movement of sediment and resulting delivery to the aggrading reach is the virtual velocity of the sediment. Annual velocities of between 1 and 5 km a-¹ are probable. These velocities predict 100,000 to 700,00 m³a-¹ tailings are delivered to Hope, which compares favorably with the observed aggradation rate. Sediment from placer mining on the main stem of the Fraser may continue to influence the rate of sediment delivery to Hope for another century or more, nevertheless, the historical aggradation rate may not represent future conditions.
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