Effects of climatic and anthropogenic change on small mountainous rivers: the Salinas River example

Over the past 50 years rivers have discharged about 18×109 t of sediment annually to the global ocean, mountainous rivers with small drainage basins playing a particularly key role. Mean annual values, however, often mask short- and longer term variations resulting from climatic and anthropogenic factors. Discharge from large rivers generally displays relatively little inter-annual variation, whereas long-term discharge and sediment delivery from smaller rivers often are determined by infrequent events. The lack of adequate monitoring unfortunately means that these event-driven deliveries to the coastal ocean are usually poorly documented and the magnitude of their impact often insufficiently understood. The Salinas River provides one example of the natural and anthropogenic variability occurring in small river systems, as well as the problems associated with understanding sediment transport and discharge based on limited sediment-load data. With half of the historical (70 years) sediment load being delivered in less than 5 weeks, the Salinas is an event-dominated river on which the construction of dams and withdrawal of water have severely changed (and continue to change) its natural flow regime. The use of outdated sediment measurements leads to potentially large errors in load estimates, thereby underscoring the need for newer and more extensive data, particularly during infrequent events.