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Measuring and predicting sediment transport in rivers is an important task for sedimentologists, geologists, ecologists, and engineers. The timing and size distribution of sediment transport in rivers can be a key determinant in riverine ecology, affecting plant and animal distribution and population dynamics. Sediment supplied by rivers can adversely affect engineering operations such as dams, both by building up in pools and reducing dam efficiency over time, and by causing erosion of power turbines. Sediment moved by rivers leaves the fluvial regime for estuarine or coastal environments, and can be a significant source of littoral cell sediment and key in beach replenishment and health. Documentation of sediment transport in rivers is clearly important, as is a clear theoretical understanding of the controls on bed and suspended sediment entrainment and transport.
Historical changes in the Columbia River Estuary and along the Washington coast suggest that there have been changes in sediment supply and distribution within the Columbia River littoral cell, which spans the west coast of the U.S. from Tillamook Head, Oregon to Point Grenville, Washington. Previous studies suggest that the Columbia River is the primary source of sediment to the coastal shelf (Sherwood and Creager, 1990; Sternberg, 1986). The importance of fluvial transport of sediment to the estuary and the coast has motivated several efforts to constrain sediment budgets in the region (Gelfenbaum et al., 1999; Sherwood et al., 1990).
We examine historical suspended sediment concentration data to consider changes in sediment transport in the river and its tributaries over the last century. In addition, we use water discharge records to calculate changes in annual sediment load, and attempt to quantify the relative importance of changes in river hydrographs and changes in the sediment/discharge rating curve in reducing total sediment load during this time. While comprehensive data throughout the basin are scarce, we focus on four locations where multiple data sets exist: the Columbia River near Vancouver, WA, the Snake River near Burbank, OR, the Kootenai River near Copeland, ID, and the John Day River at McDonalds Ferry, OR. Sediment transport in the main stem of the Columbia is of primary interest, but consider tributaries because they are critical contributors of sediment to main channels in other large river systems (e.g., Topping et al., 1999). The John Day River is of particular interest because it is the second largest undammed river system in the United States, although it has undergone similar land use and urbanization changes compared to other parts of the Columbia River basin. The overarching goal of the study is to calculate sediment flux in the Columbia River throughout the last century, and to analyze primary controls on significant changes in sediment transport during this period.
Records of water discharge and suspended sediment concentration near the mouth of the Columbia River and in the Snake and Kootenai Rivers demonstrate a decrease in sediment concentration over the past 50-100 years despite an overall increase in mean annual water discharge in all rivers. In addition, the magnitude of annual peak discharges have decreased, a result of flow regulation due to dam operations in these rivers. Only on the undammed John Day River are present-day sediment discharges similar to the early 1900’s, with a marked decrease in sediment flux on the Columbia, Snake and Kootenai Rivers over the same time period. Calculations of sediment load in the Columbia River showed a 35-60% decrease over the past century; changes in the hydrograph due to irrigation withdrawls and flow regulation appear to have twice the effect compared to changes in suspended sediment rating curves in the river.