One Hundred and Fifty Years of Sediment Manipulation on the Trinity River, CA

This paper explores the history of gold mining, gravel mining, and river restoration activities on the upper 42 miles of Trinity River, CA between Lewiston Dam and the North Fork Trinity River. Newly developed quantitative estimates of the sediment supply impacts associated with these activities are presented. Intensive hydraulic mining since the 1860’s has contributed vast amounts of sediment to the Trinity River. Contributions from the La Grange mine alone were sufficient to aggrade the valley bottom by over 3 feet. The longitudinal profile shows preliminary evidence of a persistent sediment wave resulting from hydraulic mining activities. Subsequent dredger mining overturned more than 70 percent of the floodplains. Historic gravel mining operations likely extracted between 60,000 and 125,000 cubic yards of gravel from the channel. A surprising result of this research is that river restoration activities over the last 40 years have extracted nearly as much gravel from the channel as they have augmented – roughly 90,000 cubic yards. Repeat bulk sampling between 2001 and 2009 showed reductions of up to 50 percent in the sand content of the substrate. Additional studies are underway to better understand the legacy impacts of mining and river restoration activities on the fluvial geomorphology and channel complexity of the Trinity River.