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Land use activities, including mining, logging, and road building substantially altered sediment regimes in the Trinity River and tributary watersheds beginning in 1848 with the discovery of gold on the Trinity River. These actions increased sediment delivery to streams and caused channel aggradation that exceeded several meters in some areas of the Trinity River. Completion of Trinity Dam and the start of flow regulation in November 1960 eliminated the supply of sediment from the upper watershed before Lewiston Dam and Carr Tunnel were completed for use in diverting Trinity water to the Central Valley of California starting in April 1963. The reduction in Trinity River flows enabled fine sediments delivered by tributaries to accumulate in the channel and in berms along the riparian fringe of the river. In response, fine sediment reductions in the river were attempted with pool dredging, riffle cleansing, and bar scalping in the 1970s to reverse noticeable declines in salmonid populations, but with almost no lasting improvements. Source treatments were also implemented in the 1970s to reduce the fine sediment supply from tributaries, especially Grass Valley Creek. Revegetation, road improvements, construction of Buckhorn dam, and operation of sediment retention ponds near the confluence with the Trinity River were successful and largely eliminated Grass Valley Creek as a fine sediment source. Watershed restoration also reduced or maintained fine sediment inputs from Deadwood, Rush, and Indian creeks within ranges that appear normal for the period that data are available (late 1990s to mid-2000s). Meanwhile, a flushing flow study undertaken in the early 1990s recommended discharges that scour fines from depths that salmonids lay their eggs in the Trinity River. The Trinity River flow study further proposed hydrographs in the late 1990s to restore the Trinity River fishery by promoting ecosystem processes, including bed scour, bar construction, and riparian plant establishment, to name a few. The hydrographs were adopted in the Record of Decision (ROD, 2000) and first implemented in spring 2004 to meet a host of restoration objectives, including re-creation of a gravel channel bed and transporting fine sediment at rates that exceed supplies from tributaries.

These objectives were increasingly met on the restoration reach of the Trinity River between Lewiston Dam and the North Fork Trinity River beginning shortly after the start of ROD flow releases in spring 2004, as indicated by Wolman pebble counts and bulk sediment samples at repeat sites showing strong declines in fine sediment (≤8 mm) and progressive armoring of the bed. Reduction in fines has also been documented in bed load sampled on the Trinity River during high flow releases from Trinity and Lewiston dams in spring (hereafter “spring flows”)

since 2006. The samples show skewness of the grain-size distributions increased (lower skew=fewer fines) along with median bed load diameters, and fine sediment as a proportion of the total load dropped. These changes were most apparent on the Trinity River at Limekiln Gulch (TRLG) and Douglas City (TRDC), and somewhat less apparent on the Trinity River at Lewiston (TRAL), which has exhibited a chronic deficit in fine sediment since 2006, outside of pulse inputs from channel reconstruction projects upstream of TRAL in 2008 and the Carr fire in 2019. Loads for grain-size fractions (i.e., fractional loads) varied between years depending on their supply and discharges available for transport them in the river. Linear regressions fit to fractional loads normalized by the volume of the spring flow release and discharges in excess of stability thresholds indicated that coarse bed load increased at TRDC, fine bed load decreased at TRLG, and suspended loads decreased at TRDC in 2006–2019 and increased slightly at TRAL in 2006–2016. The latter finding likely resulted from reservoir bank erosion in 2014–2016 and not contemporary supplies of fines since turbidity and suspended sediment transport was not measured at TRAL in 2017–2019.

Transport threshold analyses indicate that fine bed load (≥0.5 mm and ≤8 mm) requires higher discharges for entrainment than coarse bed load (>8 mm) at TRAL due to the paucity of fines in the Lewiston reach. The counter-intuitive finding that large grains move before small particles at TRAL results from most fine sediment in the reach being sheltered under a coarse surface layer. This same arrangement explains the higher critical discharges estimated for fine bed load at TRDC since water year (WY) 2012, which suggests a deficit of fine sediment has developed in the Douglas City area. Analysis of the relative mobility of grain-size fractions using a hiding function by Parker et al. (1982) supports these results and shows that higher mobility has alternated between fine and coarse grains through time at TRAL and TRDC, with average results indicating approximately equal mobility since 2006. At TRLG, critical conditions for grain size fractions and threshold analysis indicate fine sediment is substantially more mobile than coarse grains, with threshold discharges for fines averaging 33% of those for coarse grains. Reconnaisence is needed to explain this result, with a possible reason being the apparent supply limitation in coarse sediment in the reach requiring relatively high flows to entrain enough material to reach the dimensionless transport rate for estimating critical conditions.

Annual estimates of Shields stress for the median grain [...]