Typical Releases

There are four basic types of flow releases to the Trinity River:

1. Releases for River Restoration (including “base flows”)
   • Discussed below.
2. Klamath River Augmentation
   • The Bureau of Reclamation sometimes increases the release from Lewiston Dam to the Trinity River to mitigate late-summer conditions in the lower watershed (usually the Klamath River) for fish health purposes.
3. Ceremonial Purposes
   • The Hoopa Valley Tribe requests that the Bureau of Reclamation increase releases of Trinity water via Lewiston Dam in odd-numbered years to support tribal ceremonies in late summer. These releases typically span only a few days and often form a small peak over the Klamath River augmentation flows.
4. Reservoir Management
   • The Bureau of Reclamation may occasionally need to release water from Trinity Reservoir in order to protect or maintain infrastructure within the Trinity River Division as well as communities downriver. These releases may be triggered for a variety of reasons ranging from dam safety to maintenance of power generation equipment. While these releases often occur in relation to winter storm events, releases were increased during a 2018 wildfire event.

A history of water releases is provided on our Flow Volume Summary page. 

Release for River Restoration

The Trinity River Record of Decision (ROD) in 2000 increased the quantity of water allowed to flow down the river in order to maintain fish health (“base flows”) and provide variable flows for restoration of river ecology (“Restoration Flow Releases”). The ROD was designed toward exporting 52% of the water to the Central Valley Project, versus releasing 48% of the water to the Trinity River (these are long-term averages, as reservoirs are meant to store water from wet years for greater export during dry years). Flows to the river are recommended by the Program, thus quantities (volumes) allotted to the river are known as “Restoration Volumes” even though they include the base flows of the river year-round.  

The ROD specified Restoration Volumes for 5 water year types, which are determined by state forecasts of reservoir inflow (table below). 

The Program uses these Restoration Volumes toward Restoration Flow Releases that are designed to meet specific management objectives aimed at improving ecological function of the river. Every year scientists across the TRRP collaboratively develop one or more flow schedules (hydrographs) that fit within the expected Restoration Volume while achieving multiple ecological objectives.  Flow Schedules are reviewed the Trinity Management Council (TMC) then recommended to the U.S. Department of Interior. The Regional Directors from Reclamation and U.S. Fish and Wildlife Service then approve flow schedules, which Reclamation implements with approval from the Secretary of the Interior.

Restoration Volumes are specified by a California Department of Water Resources forecast and TRRP has no option to store allocated water from one year to the next. The most recently approved schedule can be viewed on the Current Restoration Flow Release page. 

A Shift to Earlier Flows

For many years TRRP was limited to the late spring and summer for scheduling Restoration Flow Releases (above baseflows) due to the timing of a forecast that determines the Restoration Volume.  This volume is determined by the B120 forecast by the California Department of Water Resources which is posted in early April.  However, earlier forecasts have improved in the two decades since the TRRP was formed.  Meanwhile, the ecological needs for elevated flows in the winter and early spring have become clear.  The Program recommends flow releases as early in the water year as December 15.  The amounts are limited to ensure that the water year can be completed within the Restoration Volume without additional risks.  Details of developing this process are available through our Variable Flow Project page. This recommended shift has been partially implemented in two water years, Water Year 2023 and 2025. Monitoring will inform how flows may be implemented in future years. 

• 300 cfs Baseflow (Oct. 15 – Dec. 14)
  • The fall baseflow period of 300 cfs is a management necessity. Restoration Program Scientists believe that adding pulse flows or increasing flow could be beneficial for fish. 
• Synchronized Storm Pulse Period (Dec. 15 – Feb. 14)
  • The intent for this time period is to ‘piggy-back’ a dam release on a significant natural storm event (if one occurs). The trigger is a prediction from the River Forecast Center of 4500 cfs (mostly from tributaries) at the mainstem gage above the North Fork. TRRP will then schedule up to 60,000 acre feet of water to be released and timed for a peak release of 6500 cfs to coincide with the predicted storm. If no such storm flows are predicted during this period, then no synchronization flow will happen. Based on recent history, we expect these to occur in about 1 out of 3 years.
• Wet Season Flood Period (Feb. 15 – Apr. 14)
  • Period 1 (Feb. 15 – Mar. 14)
    • Releases above baseflow will be scheduled in most years with the volume depending on (a) if there had been a synchronization flow, and (b) the February 90% exceedance forecast from the California Department of Water Resources. The only years in which flows will not rise for this time period are those where a synchronization flow has already occurred and the 90% forecast indicates a possibility for Dry or Critically Dry water-year. [The 90% exceedance forecast indicates a 90% probability of meeting or exceeding a particular inflow to the reservoirs.]
  • Period 2 (Mar. 15 – Apr. 14)
    • Depending on the March 90% exceedance forecast, an additional volume of water may be scheduled for release.
• Snow Melt Peak and Recession (Apr. 15 – variable)
  • The April forecast issued by the California Department of Water Resources determines the total Restoration Volume for the water year. Most precipitation has already occurred by this time and the 50% forecast for reservoir inflows is used. Remaining Restoration Volume above baseflow requirements will be scheduled for releases similar to those in the past. The decreased volume for this time period will likely bring the river down to summer baseflows earlier in the year than has been typical.
• 450 cfs Baseflow (Variable – Oct. 14)
  • The summer baseflow period is managed at 450 cfs due to temperature requirements for migrating spring Chinook salmon. 

Benefits

Rivers and estuaries are naturally dynamic ecosystems that rely on periodic changes in flow to maintain their health and productivity.  

Adding a storm pulse and elevating flows in the winter season can…

• Synchronize tributary flow events to help transport harmful sediments from tributaries during the wet-season. If the mainstem is allowed to match the power of tributary accretion during storm events, waters can carry suspended sediments downstream versus allowing them to settle on mainstem redds to smother millions of salmonid eggs.
• Peak floods recondition the streambed by building riffles and gravel bars as well as scouring deep holes that serve as holding and rearing habitat for salmonid species.
• Improved delta formation at the mouth of tributaries.
• With unnaturally low wet-season flows (300cfs) birds and other aquatic species see the river as “ready to habit” leading to the destruction of clutches and egg masses when flows come up during the snow melt peak.
• More habitat for juvenile fish at the right time during their springtime growth phase.
• Better macro-invertebrate production for fish food.
• With unnaturally low wet-season flows (300cfs) birds and other aquatic species see the river as “ready to habit” leading to the destruction of clutches and egg masses when flows come up during the snow melt peak. 
• Natural warming and temperature diversity when flows come down earlier in the early summer benefit juvenile salmonids, frogs, turtles as well as macroinvertebrates.
• Increase in hydropower generation over a multiyear average.
• In wet winters with a full reservoir, the risk of spilling water from the reservoir for safety of dams is reduced because releases are budgeted from allocated restoration water. This allows for extra storm water to be reserved for storage. Of course in some years inflow and storage capacity can supersede restoration allocation releases and Reclamation may still need to release water for safety purposes.

Further Reading

The Trinity River Variable Flow Project Environmental Assessment 

Abel et al. (2022) Trinity River Winter Flow Project 

Restoration Flow Pages

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Current Restoration Flow Release Schedule

Variable Flow Project page

Flow Volume Summary

Flow Release Notifications

River Conditions

Lake Conditions

Flow FAQ

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