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