There are four basic types of flow releases to the Trinity River:
- Releases for River Restoration (including “base flows”)
- Discussed below.
- 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.
- 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.
- The Bureau of Reclamation may occasionally need to release water from Trinity Lake in order to protect or maintain infrastructure within the Trinity River Division. 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 under for of these 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 TRRP, 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).
TRRP 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 that 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 more clear. TRRP is now recommending 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 Winter Flow Variability page. This shift to early flows is being implemented in Water Year 2023 and monitoring will inform how flows may be implemented in future years.
- December 15 – February 14: Flow Synchronization
- 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.
- February 15 – March 14: Elevated Baseflows 1
- 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.]
- March 15 – April 14: Elevated Baseflows 2
- Depending on the March 90% exceedance forecast, an additional volume of water may be scheduled for release.
- April 15 – summer: Spring Restoration Release
- 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.
A decision tree for these times and water volumes is presented in Abel et al. (2022) Trinity River Winter Flow Project.
One side benefit to shifting restoration flows earlier in the year is an opportunity for greater reservoir refill in wetter years by consuming a volume of water that may otherwise have been released for reservoir management.
RESTORATION FLOW PAGES