Placer mining along the Fraser River, British Columbia: The geomorphic impact
We investigate the geomorphic impact of nineteenth century placer mining along the Fraser River, British Columbia, by estimating the volume and grain-size distribution of excavated sediment, evaluating the transport potential for the sediment in the river, and discussing the relation between placer waste sediment and observed morphodynamics of the Fraser River channel. Volume-by-area regression relations applied to 456 mapped mines estimate the total volume of material excavated to be ∼58 × 10 6 m 3 (bulk volume). Sampling of mine scarp and mine lag sediments indicates that the discharged tailings consisted of 54% sand and finer material and 46% gravel and small cobbles. Modern observation and historical narratives indicate that the channel of the Fraser between Quesnel and Laidlaw has been generally stable following placer mining. Application of a sediment transport function indicates that the river is capable of moving annually an amount of sediment comparable to the maximum loading from placer mining. By applying established relations between channel scale and sediment virtual velocity, we estimate the rate of downstream movement of the placer-waste slug to be between 1 and 6 km yr -1 . The predicted rates of sediment migration indicate that peak delivery of placer waste to the lower river below Laidlaw likely occurred early in the twentieth century. This predicted behavior agrees well with observed aggradation on the lower river. The result emphasizes the importance of historical legacy in the appraisal of recent geomorphological processes and shows that widespread small-scale disturbance has affected the Fraser River, with the effect concentrated where the gradient of the river is reduced.