CONTRIBUTION OF THE LARGEST EVENTS TO SUSPENDED SEDIMENT TRANSPORT ACROSS THE USA

This work analyses the contribution of the largest events to suspended sediment transport on the continental scale. The analysis is based on the United States Geological Survey (USGS) Suspended Sediment and Ancillary database. Data were obtained from 1314 catchments. comprising more than 2 500 000 daily events. The total number of days in the dataset amounts to 10 000 years. Catchments are of different sizes and belong to distinct climatic environments; they are distributed for the analysis according to USA hydrological divisions (HDs). The main objective of the research is to examine the effect of the n-largest event on the total suspended sediment load over recorded periods, and to discuss different behaviour between HDs. To accomplish this, the daily events at each catchment are ranked by magnitude. and then the percentage represented by the a-largest event (e.g. 3-largest. 5-largest, 10-largest, 15-largest, 20-largest, 25-largest) is calculated from the total accumulated load. Results indicate that suspended sediment transported by the 25-largest events represents on average more than 50 per cent of the total load. The California HD, mostly under Mediterranean climatic conditions, accounts for the highest percentage of sediment transport across conterminous USA. whatever n-largest daily events are selected. There. the 3-largest events contribute. on average. 38 per cent of the total sediment load, the 10-largest events represent 61 per cent and the 25-largest events produce more than 76 per cent of the total sediment transport. Overall, the contribution of largest daily events seems not to depend on the climatic conditions in small catchments (<100 km(2)) and, in addition, the percentage of suspended sediment increases over all HDs. while, at the same time, the catchment size decreases. Finally, we discuss differences between catchments across the USA. according to climatic and historical (i.e. land use) factors. Copyright (C) 2009 John Wiley & Sons, Ltd.