Sediment Connectivity: Punctuated Anthropogenic Sediment Production and Dynamic Sediment Delivery Ratios

Forecasting fluvial sediment yields involves several assumptions about catchment dynamics that should be explicitly realized. For example, sediment delivery ratios (SDR), the ratio of sediment yield to sediment production (Y/P), are often assumed constant and used to translate upland sediment production to downstream deliveries or estimate yields from erosion models.  Constant SDR values may be lumped basin-wide or applied in distributed grids or sub-catchment response units. The assumption of constant SDRs, however, tacitly subsumes an assumption of approximately constant sediment production rates averaged over time. Yet, SDRs may vary substantially and systematically over time in response to changing land-use, climate, or other factors that govern sediment production. With human disturbances, it’s common to have a punctuated sediment production followed by a systematic peak and decline of yields as the legacy sediment is translated downstream through time. 

            Hydraulic mining in California is an extreme case of punctuated sediment production where massive amounts of hydraulic mining sediment (HMS) were produced between 1853 and 1884 and then abruptly stopped. This research used geomorphic change detection and DEM differencing in two small, mountain catchments in the mining districts to document sediment production, initial storage, and recent storage of HMS. Total sediment production from mining in the two catchments was 41.3 and 23.5 10⁶m³, which was used to compute SDRs for the various periods. Average sediment yields were calculated from the changes in storage between periods. Changes in sediment yields show the dynamics of SDRs through time. Initial SDRs were high in the late 19^th century but increased further as cumulative deliveries increased. On a catchment scale, the two basins had SDRs of 62 and 70% in 1884 when mining ceased, but this had increased to 88 and 84% by 2014 as the cumulative yields increased. Lateral connectivity played an important role in changing yields as easily eroded sediment became less accessible, but yields also decreased in response to channel gradient declines and bed armoring. Longitudinal connectivity was also important within the catchments through the emergence of bedrock gorges that trapped large pockets of sediment for some periods. The dynamics of delivery ratios calls for a knowledge of the history of geomorphic changes in catchments in order to recognize systematic changes in sediment production.