A WATERSHED PERSPECTIVE OF DAM IMPACTS ON HYDRAULIC MINING SEDIMENT IN CALIFORNIA

The history of hydraulics dates to the birth of civilizations in China, Mesopotamia, and other hydraulic societies. For example, Chinese Emperor Yu rose to power > 4000 yr B.P. as a hydraulic engineer by developing levees along the Huang He. In contrast, the history of hydrology is recent. Ignoring occasional hydrologic insights of Greek philosophers and da Vinci’s secret writings, the first notion of basic hydrologic processes dates to the French hydrologists (Palissy, Perrault, Marriott) about 400 years B.P.; a ten-fold difference in age. This discrepancy belies a chronic myopia regarding runoff and sediment generation and the management of rivers, water resources, and flood hazards. Emphasis on local hydraulic problems and structural solutions has discouraged basin-scale studies and understanding.

Little knowledge of basic watershed processes was available when engineers were faced with controlling > 1.0 10⁹ m³ of hydraulic mining sediment released to Sacramento Valley tributaries. Lowland floodplains had aggraded, navigation was threatened, and flood frequencies soared. Emphasis on foothill dams, Valley levees, and flood conveyance successfully reduced flooding and sedimentation but provided little insight into watershed processes. Gilbert’s classic study introduced a watershed perspective, but few grasped its spatial implications or the significance of countless crib dams and vast sediment deposits stored in upper rivers. Instead, focus on Gilbert’s sediment-wave model based on three lowland gauges led to the erroneous conclusion that most mining sediment is now gone or permanently stored. Misconceptions also allowed hydraulic mining to resume assuming that crib dams would permanently store tailings in mountain streams. Until recently, little concern was paid to mountain watersheds where vast repositories of sediment remain. Modern evidence of these deposits and small detention structures is reviewed. Current studies of removing Englebright Dam to restore salmonid populations must now contend with potential release of these deposits. After 4000 years of water science, the serendipity of unanticipated applications of scientific knowledge should be clear. The watershed perspective is new but this understanding will greatly improve management practices.