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Replacement table for Appendix D-9 Bedload Data, TRDC Bedload Samples included in zip file as of 2021-08-24.

 

This report presents the results of Water Year (WY) 2019 sediment transport monitoring efforts on the Trinity River near Weaverville, California. As part of an ongoing study for the Trinity River Restoration Program (TRRP), GMA Hydrology (GMA, formerly Graham Matthews and Associates) conducted bedload and suspended sediment sampling at four monitoring locations. Secondary hydrologic and geomorphic data such as water surface slope, streamflow measurements, cross section surveys, pebble counts and turbidity were collected as part of the study. New for 2019 was: (1) scheduled times for crews to take bedload samples, (2) no suspended sediment at the Lewiston site, (3) pressure transducers installed at water surface elevation reference stations to acquire 15-minute continuous stage data, and (4) partitioned bedload data were provided for all sites to support hydroacoustic computations. WY2019 marks the 14th year (2004-2013, 2015-2017, 2019) of Trinity River sediment transport monitoring by GMA Hydrology for the TRRP.

 

Historic impacts to the Trinity River and their effects on anadromous fish habitat are described in the Introduction (Section 1.0) of this report and are documented extensively elsewhere (e.g. McBain and Trush, 1997). In short, physical changes in the channel resulting from mining, increased sediment loads related to logging and flow reduction due to impoundment resulted in a severely degraded channel in the mainstem while the Trinity and Lewiston dams cut off anadromous salmonids from the upper 700 square miles of the watershed.

TRRP’s restoration strategy involves a combination of Spring Flow Releases, fine and coarse sediment management, and mechanical channel rehabilitation. This 2019 study is conducted as part a sediment budget approach to understanding sediment-related habitat issues. We provide sediment load estimates at four locations along the river: at Lewiston (TRAL), near Grass Valley Creek (TRGV), at Limekiln Gulch (TRLG), and near Douglas City (TRDC). The sediment load estimates inform gravel injection strategies (locations and volumes) and hydrograph development for Spring Flow Releases.

 

Suspended sediment and bedload sampling were conducted from cataraft platforms attached to temporary cableways. Samplers (US D-74 for suspended sediment and TR-2 with 0.5mm mesh for bedload) are lowered and raised using cranes equipped with winches. Each station is located near a stream gage operated by either US Geological Survey (USGS) or GMA. Continuous turbidity was collected for use as a surrogate for suspended sediment concentration (SSC) at three sites. Cross section surveys and pebble counts were conducted before and after the 2019 Spring Flow Release.

 

WY2019 was determined to be a “Wet” water year (TRRP 2019). The approved spring release hydrograph included three high flow peaks above 8,000 cfs during April and May and three additional smaller peaks, less than 5,000 cfs, as flows were dropping through May, June and July. All of the peaks were planned to be instantaneous and not contain a bench. The three high flow peaks were planned to have peak values of 9,850 cfs on April 17; 10,900 cfs on April 29; and 9,000 cfs on May 19. The three smaller peaks were planned to be 4,500 cfs on May 24; 3,800 cfs on June 7; 2,450 cfs on June 23; and 3 3,200 on June 26. A gradual tailout started on June 27th, with the hydrograph dropping back to summer base flow (450 cfs ) on August 5th.

 

GMA sampling crews collected suspended sediment and bedload measurements for eight days over the three large hydrograph peaks between April 17 and May 20, 2019. [...]