Continued reading:

Lewiston Old Bridge, 1940

Lewiston Old Bridge during an historic flood. The highest flow calculated by USGS in Lewiston was 71,600 cfs on December 22nd, 1955. Photograph by Boni DeCamp, provided by Kenneth DeCamp.

Reason for Action
The Trinity River Division of the Central Valley Project began flow regulation and water diversion on the Trinity River in 1960.  Floods from winter storms and spring snow-melt had previously acted as the ‘pulse of the river, refreshing the channel, keeping banks from clogging with riparian vegetation, and enabling the river to reform itself following the large-scale mining that had severely manipulated the river. 

Upon flow regulation, 70 – 90 % of the water was exported to the central valley. The river lost its floods, began filling-in with sand, and becoming encroached with vegetation. These changes eliminated much of the salmon habitat leading to salmon population decline of up to 96 %.
Pulse Graph

(click graph for better view)

TRRP Restoration Flows

The Trinity River Flow Evaluation Study of the 1990s demonstrated 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.

High Flow 2010 Alders tipping over, May 2010, Lewiston Dam release of 6,000 cfs.
Trinity Dam Release 11000 cfs Outflow from Trinity Dam during the historic 11,000 cfs release in 2011 – the largest in 37 years and the largest authorized for restoration purposes.