REASON FOR ACTION
The Trinity River Division of the Central Valley Project began flow regulation and water diversion on the Trinity River in 1960.
Once flow regulation diverted up to 90% of Trinity River water to the Central Valley, the river no longer flooded and it began filling-in with sand and became encroached with vegetation.
These changes eliminated much of the salmon habitat leading to salmon population decline of up to 96%.
Floods from winter storms and spring snow-melt previously acted as the ‘pulse’ of the river–freshing the channel, keeping banks from clogging with riparian vegetation, and enabling the river to reform itself following the large-scale mining that had severely altered the river.
TRRP RESTORATION FLOWS
The Trinity River Flow Evaluation Study of the 1990s showed that high flows are necessary on the Trinity River to maintain or restore salmon populations. In 2000, the U.S. Department of Interior directed that approximately 50% of the river’s water would remain in the river (not be diverted to the Central Valley) and that the Trinity River Restoration Program would recommend how water was to be released for the restoration of the river and its fisheries.
Restoration flows are intended to clean spawning gravels, build gravel/cobble bars, scour sand out of pools, provide adequate temperature and habitat conditions for fish and wildlife at different life stages, control riparian vegetation, and perform many other ecological functions. In order to mimic some of the inter-annual variation that is naturally found within the Trinity Basin. The ROD defines five water-year types along with the a minimum volume of water to be released into the Trinity River for each (see Typical Releases). The annual flow recommendation development process is described on our Current Flow Release Schedule page.
Prior to ROD flows, the Trinity had become a ‘fine sediment impaired river’. Sand volumes that once clogged spawning grounds are now cleared by Restoration Flow Releases. Extensive sediment, bed mobility and scour monitoring conducted on the restoration reach of the Trinity River shows that coarse sediment has increased and fine sediment has decreased in the River near Lewiston, CA. This findings marks important progress in TRRP’s objective of restoration the quantity and quality of habitat for salmon.
The aim to encourage more natural river processes can be measured by the rate at which gravel and cobble are transported through the river. Transport rates vary widely based on flow velocity, channel substrate and form, as well as many other factors, however monitoring shows that since restoration actions began sediments have been mobilized and are forming new features that promote healthy river processes.
High flow releases are intended to scour willows from the banks to prevent riparian berms from forming.