Impact scales of fluvial response to management along the Sacramento River, California, USA: transience versus persistence

Most large rivers in industrialized nations are managed carefully to maximize their benefits (e.g., water supply, hydroelectricity), while limiting their hazards (e.g., floods). Management strategies employed in lowland river systems such as large dams, levees, and bypasses affect flow regimes, sediment supply to channels, and the net flux of sediment through river reaches fairly soon after construction. Therefore, equilibrium approaches to fluvial geomorphology are typically inadequate to characterize the effects of anthropogenic activity on management timescales (10–102 years). Each human alteration to the fluvial system has an ‘impact scale’ in time and space, and these impacts may manifest as persistent (steady, localized influence) or transient (dying away with distance and/or time) landscape responses. The cumulative effects of transient and persistent fluvial responses influence flood risk, the state of aquatic and riparian habitat, and the fate and transport of contaminants. Whereas some persistent impacts are straightforward to anticipate (e.g., reduced flood peaks), transient impacts may result from emergent behavior in fluvial systems and are not easily predicted. This chapter outlines the differences between these divergent landscape responses to perturbations in managed fluvial systems using examples from the Sacramento River in California. The discussion focuses on: (1) persistent local signals of altered flow regimes below large dams that attenuate in lowland valleys, (2) transient longitudinal sediment redistribution due to changes in sediment supply by dams, (3) transience in the magnitude and frequency of flow over flood control weirs into flood bypasses, and (4) persistent overbank sedimentation in localities that favor the export of sediment from channels to floodplains. The chapter shows that persistent and transient fluvial processes coexist and interact in large, lowland river basins subject to anthropogenic perturbations in a manner that can produce unanticipated outcomes that are relevant to aquatic and riparian ecosystems, river management, as well as to human communities living in lowland floodplains. It suggests the need for more careful examination of the impact scales of river management to clarify trajectories of landform evolution