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Abstract:

Sediment transport data collected in the Trinity River over a 12 year period are used to identify temporal and spatial patterns in how sediment fluxes have responded to flow and sediment management, evaluate potential causes of changes in the sediment transport regime, and to assess which components of the sediment monitoring program are most valuable for guiding management decisions. Most sediment monitoring results have thus far been presented only in year-specific reports, and the data is distributed among hundreds of separate spreadsheets. A major objective of this report is to synthesize these data into a coherent summary of what is known about sediment transport in the Trinity River has how it has changed in response to management.

Fine bed material loads at the four sediment monitoring locations along the Trinity River between Lewiston Dam and the North Fork Trinity River increases with downstream distance from the Dam. Gravel loads at the monitoring location nearest the dam, however, exceed the loads at the next two locations downstream by about 35%. The elevated gravel loads at the most upstream location can be attributed to gravel augmentations that have been implemented upstream from that location on a nearly annual basis over the past decade. Transport data suggests that gravel augmentations are affecting gravel supplies at the second monitoring location downstream from the dam, but not at the third location. The largest gravel loads are observed at the most downstream of the four monitoring locations.

To first-order, sediment transport rates are controlled by the magnitude of water discharge in the river. But numerous secondary factors strongly influence relationships between stream flow and transport rate, leading to large fluctuations in sediment transport characteristics over time. Temporal variations in the transported loads of fine bed material and gravel follow broadly similar patterns, suggesting that the availability of fine bed material exerts a strong control the transport rates of both sediment types. Changes in channel geometry or substrate composition may also have altered transport rates at some locations. Overall, the transport rates of both the coarse and fine bed material fractions appear to have decreased over the past decade at three of four monitoring locations. Comparisons with sediment loads determined from year-specific measurements confirm that this temporal variability renders long-term composite rating curves ineffective for quantifying annual sediment loads. As TRRP management is explicitly intended to change the rate and character of sediment transport processes in the river, existing sediment transport data are of little use for detecting the effects of management actions on future transport rates.

Bedload sample data from individual monitoring sites show some evidence of hysteresis over annual flow releases in about 60% of the datasets evaluated. Clockwise hysteresis is by far the most common type, especially for fine bedload at the two most upstream monitoring locations. In most cases, the fine and coarse bedload fractions display similar types of hysteresis or lack thereof. We hypothesize that changes in the availability of fine bedload during a flow event determines the degree of hysteresis observed in the transport rates of both size fractions. Nearly all instances of anticlockwise hysteresis occur at the most downstream sediment monitoring location. This is likely due to the greater distance of that monitoring location from upstream sediment sources. It appears that, rather than depleting upstream supplies of fine bed material, extended periods of high flow may allow fine sediment derived from sources far upstream to reach the monitoring location.

Gravel monitoring results from 2015 indicate that gravel augmentations slightly exceeded the gravel flux past the most downstream sediment monitoring location that year, demonstrating that the current method for determining gravel augmentation quantities met its intended target. Gravel budget calculations through water year 2015 show that gravel storage has continued to increase in the reaches downstream from Lewiston Dam. Estimates of gravel deliveries from Rush Creek and Indian Creek for water years 2012-2015 were obtained from repeated topographic surveys of tributary confluences. Continued topographic monitoring of these confluences is recommended, as tributary gravel inputs estimated from long-term composite rating curves were found to be highly inaccurate. Continued development of a gravel budget is needed to inform gravel augmentation activities, but the use of gravel storage as a primary management objective is discouraged. The core Program objective to create spatially complex channel morphology is better assessed by direct evaluation of bed topography.