Time and the persistence of alluvium: River engineering, fluvial geomorphology, and mining sediment in California

River managers need to understand fluvial systems as they change through time. Many river systems are presently in a state of flux as a result of substantial anthropogenic changes to water and sediment regimes and channel hydraulics. Yet, historical approaches to understanding river systems rarely receive adequate attention because historical methodologies are not conducive to the application of quantitative analysis. While there is limited precision in most historical reconstructions, the information derived from these studies constrains other interpretations and is essential to a full understanding of the behavior of fluvial systems. Geomorphology provides a perspective on river systems in which time — at various scales — is interwoven into practical and theoretical aspects of scientific inquiry. Thus, geomorphology is important to our understanding of not only physical systems but also fundamental concepts of time. tudy examines channel morphological changes in the Bear and American basins brought about by two episodes of sedimentation from hydraulic gold mining. The primary event was the production of more than 1 billion m3 of sediment throughout the northern Sierra Nevada from 1853 to 1884 which caused aggradation in many channels across the Sierra foothills and Sacramento Valley. Assumptions by both engineers and geomorphologists that morphologic responses to this event were ephemeral, that sediment loads have returned to previous levels, and that deposits have stabilized, are not borne out by field and historical data in the Sacramento Valley. A secondary sedimentation event, not previously studied, was the production of at least 24 million m3 of sediment during a period of licensed mining from 1893 to 1953. This episode of sedimentation has been largely overlooked as a geomorphic, hydrologic, or water quality event. Yet, channel morphologic responses in phase with mining during this period are demonstrated. Systematic changes in stage–discharge relationships reflect channel morphological changes that are relevant to flood risk assessments, stability of engineering structures on floodplains, and geomorphic interpretations.