Tailings fans and valley‐spur cutoffs created by hydraulic mining

Abstract

Sand and gravel tailings from nineteenth century open-pit hydraulic gold mines formed large alluvial fans at tributary confluences in the northwestern Sierra Nevada, California. In the Bear River watershed, several of these fans were so large that they blocked main channels for decades. Some channels not only aggraded deeply, but also moved laterally and cut across the inner bends of valley spurs. Now locked in bedrock channels, these valley-spur cutoffs impose local controls on geomorphic, hydraulic, and sedimentary processes. One cutoff has incised 25 m into bedrock over the past century (25 cm a −1 ) with rapid initial incision rates of up to 50 cm a −1 (1884-1890). Recognition of spur cutoffs in the geological record may help to identify large landslides and provide an analogue for a type of natural earthfill dam spillway not prone to catastrophic failures. Tailing fans, valley-spur cutoffs, and the sediment they trap are described from contemporary accounts and recent field conditions in the Bear River watershed. These anthropogenic changes represent a major shift in the watershed from supply-limited to transport-limited sediment budgets and a change in geomorphic processes away from long-term drainage evolution dominated by ingrown meanders. The large volumes of mining sediment stored in these landforms will be slowly released over the next millennium and could be significant to contemporary ecological and public health issues due to recent findings of high mercury loadings associated with hydraulic mines.

Effects of Tributaries on Main‐Channel Geomorphology

“…Cohen and Brierley, 2000) and how tributary fans deflect the main channel towards the far side of the valley and thus reduce the tributary gradient at the confluence. But no simple model can reproduce some of the other observed consequences of deflection by tributary fans: alternation of single-channel and braided planform (Dawson, 1988), interbedding of fan deposits with main-stem floodplain deposits (Florsheim, 2004) and incision of a deflected main channel into the inner spur of a valley bend (James, 2004).…”

A geomorphological approach to the management of rivers contaminated by metal mining

“…Passive dispersal is in marked contrast to 'active transformation' where the whole fluvial system is transformed through the input of mining material (Lewin and Macklin, 1987). The latter situation can arise either through a dramatic increase in sediment supply to river channels (Gilbert, 1917;James, 1991James, , 2004Gallart et al, 1999), or as the result of phytotoxic heavy metals disrupting riparian vegetation that leads to reduced bank stability (Lewin et al, 1977;Macklin and Lewin, 1989). In actuality, passive dispersal and active transformation represent end members of a continuum of channel change associated with mining.…”

River Courses Affected by Landslides and Implications for Hazard Assessment: A High Resolution Remote Sensing Case Study in NE Iraq–W Iran

“…Most often, the newly created channel is shifted from the main direction of the stream. For instance, the Eureka River (western Canada) cut a new channel around the toe of a 20-meter-high landslide dam formed in 1990, abandoning the pre-landslide channel [16][17][18]. Incision has been rapid (almost 1 m/year) as the landslide was mainly constituted by wet earth and loose rocks.…”