Flow FAQ

Frequently Asked Questions (FAQ) on Flows

Why does it appear you are not following the current flow release schedule?

There are several reasons why it may not appear that the flow releases from Lewiston Dam are following the current release schedule: stream gaging uncertainty, dam operational logistics, or other releases being conducted.

Stream Gaging Uncertainty. Flow releases from Lewiston dam are measured by the Lewiston stream gage located just downstream of the dam. Accurately measuring stream flow is difficult because the shape of the river changes over time, especially during periods of high flow. The real-time flow information reported live on the internet can have significant errors due to changing river conditions, equipment malfunction, or data transmission errors. This real-time flow data is provisional and subject to change until it has been quality controlled and compared to physical flow measurements. The uncertainty associated with the final quality controlled stream flow data typically ranges from 5 percent to 10 percent. That means when flow releases to the Trinity River are 10,000 cfs, the reported stream flow could be off by as much 1,000 cfs. The uncertainty associated with the provisional real-time flow data can be much greater.

One example of stream gaging uncertainty occurred on May 2, 2011. The target flow release that day was 8,000 cfs but the real-time stream gage data showed 10,400 cfs. A physical flow measurement taken that morning confirmed the true flow was 8,000 cfs and real-time flow data was subsequently amended. This data will remain provisional until the U.S. Geological finalizes the flow records later this year.

Dam Operational Logistics. The Bureau of Reclamation operates Lewiston Dam to follow the current release schedule. The water is released through two large radial gates that are raised and lowered to attain the target flow. Small changes in gate height can make a significant difference in the flow rate because the gates are large. However, small changes in gate height are difficult because they are so big and heavy.

A typical flow release change is a multi-step process. First, the gate height is set to the target level. Dam operators then wait for about an hour for the downstream flows to stabilize, then read the flow reported by the Lewiston stream gage. A fine tuning adjustment is then made if the initial gate height setting was slightly too low or too high. Again, the dam operator waits an hour for the downstream flows to stabilize, then reads the stream gage. Additional fine tune adjustments may be required. This process may take several hours during which time the flow release maybe just over or under the target flow release level.

Other releases are being conducted. The current flow release schedule only lists flow releases for the purpose of river restoration. The Bureau of Reclamation routinely releases water to the Trinity River for other purposes such as “safety of dams”, temperature mitigation, or tribal ceremonies (see “What are the Dance Flows?” below). These other releases are described under the typical flow releases page on our website and water volumes are subsequently reported on the water-year summaries page. You may sign up to receive notifications for all flow releases to the Trinity River through our release email list.

What is the highest flow that can be released from Lewiston Dam to the Trinity River?

This question really has two parts: restoration flows, and safety of dam releases.

Restoration Flows. The largest flow that may currently be released for river restoration purposes is 11,000 cfs.  The flow rate is controlled by Lewiston Dam, and measured (estimated) both at the Dam and at flow gages downstream.  Please see the question above, “Why does it appear you are not following the current flow release schedule?” to understand why flows may not match the TRRP release schedule.

Safety of Dam Releases. During the winter, the Bureau of Reclamation maintains lower levels in Trinity Lake to provide a buffer in the event of an extremely large winter storm. The quantity of that buffer is based on several factors, and primarily references many years of hydrologic record for the basin. Maintaining storage space is a very important aspect of flood control operations, and is fundamental in protecting areas downstream of Trinity Dam, as well as the dam itself. As winter storms fill Trinity Lake, the Bureau of Reclamation may need to increase releases to the Trinity River to maintain the lower lake levels. Because these elevated winter releases help protect the dam, they are commonly called “Safety of Dams releases” and may or may not occur in conjunction with actual winter storms.

The largest flow release to date ws 14,400 cfs due to winter storms in 1974.  Lewiston dam has the capacity to release 30,000 cfs to the Trinity River.

What were pre-dam floods like?

Lewiston Old Bridge, 1940
Old Lewiston Bridge, 1940 (peak flow calculated by USGS was 40,300 cfs). Photo by Boni DeCamp.

Storms caused frequent floods on the Trinity River prior to flow control by the dams.  The U.S. Geological Survey has maintained a flow gage at Lewiston below the current dam, since fall of 1911.   The peak flow measured at this gage measured a gage height of 27.30 feet on December 22nd, 1955, which provides an estimate of 71,600 cfs (Flows in the Trinity River at Lewiston would have exceeded 100,000 cfs during the 1964 storm, had Trinity Dam not been built).  From 1911 to 1960 (when the dams began regulating flows), flow measurements exceeded 20,000 cfs 19 separate times with an average yearly peak flow of about 18,500 cfs.

What is the purpose of restoration flows?

Variable flows, particularly when combined with channel rehabilitation and gravel augmentation, are the most effective way to restore the Trinity River.  TRRP restoration flows are minimal compared to pre-dam floods that exceeded 70,000 cfs, but are sufficient to maintain a dynamic and ecologically robust river.  Please see our Flow Overview page and other pages within the Flow section of our website.

How are flow schedules developed?

Based on preliminary water year forecasts in February and March, technical representatives of the Trinity River Restoration Program and program partners develop flow release recommendations to meet various restoration objectives and management targets listed in Chapter 8 of the Trinity River Flow Evaluation Final Report [PDF – 10mb]. Deliberations start with the typical flow release schedule recommended in the Record of Decision that may be adapted to meet specific restoration needs for the current year.  The flow recommendations are then presented to the Trinity Adaptive Management Working Group (TAMWG) and the Trinity Management Council (TMC) in late March or early April for consideration. The flow release recommendation approved by the TMC is then forwarded to the Bureau of Reclamation, which has the final authority over dam releases.

See our Current Release Schedule page for more detail on the flow scheduling process, or our Typical Releases page for more detail on the water volumes available per water year type.

How can I stay informed about current and planned flow releases?

TRRP maintains a Current Release Schedule page.

The public can sign up to recieve informational updates about future releases by phone or email.

Real-time stream flow and flood forecast information for the Trinity River and its tributaries is available on our River Conditions page.

Why are restoration flows in the spring?

The typical flow hydrograph was designed to mimic snowmelt patterns in spring and early summer.  These were historically the most reliable pattern in the hydrograph and have additional management benefits of avoiding scour of fish eggs, potentially controlling seed germination for greater diversity of riparian hardwoods, and for greater certainty of the water year type.

Do restoration flows “drain the lake?”

Trinity and Lewiston Dams were built to divert water to the Central Valley (see Diversion Facilities and Ops, and Diversion Impacts).  In the early years of opperation, 90% of the water collected by the dams was transferred to the Central Valley, leaving only 10% to flow down the river. As the impact of diversion on fisheries became better understood, the ratio gradually shifted.  Presently about 50% of the water coming into the lakes is allowed to flow down the river.  The other 50% continues to be diverted to the Central Valley via the Carr tunnels and Whiskeytown Lake.  Restoration Flows are possible due to decreased water diversions to the Central Valley.  Therefore, restoration flows have negligible impact on the long-term water storage of Trinity Lake.

From late-winter through early summer, the U.S. Bureau of Reclamation provides forecasts of expected levels for Trinity Lake over the next year (see “Water Operations Analysis” on the CVO Reports page).  TRRP provides a graphical view of these forecasts in relation to boat ramp elevations on our Lake Conditions page.

How are the amount of water diversions to the central valley determined?

The amounts and timing of Trinity basin exports to the Central Valley  are determined by subtracting Trinity River scheduled flows and targeted carryover storage from the forecasted Trinity water supply. Trinity River Division operations are integrated with the Shasta Division and other CVP water supply reservoirs. Exports are made after considering many different operational factors including: water year type, minimum flow and water temperature requirements for the Trinity and Sacramento Rivers, storage levels and refill potentials in both reservoirs, as well as demand for water supply and hydroelectric power generation. 

What are the “Boat Dance Flows”?

Tribal nations along the Trinity River have long traditions centered on the river.  These include the Boat Dances of the Hoopa Valley Tribe and the Yurok Tribe.  The Hoopa make arrangements with the U.S. Bureau of Reclamation to maintain their tradition through a release of water from Trinity and Lewiston Lakes, typically in late August or early September of odd-numbered years (2009, 2011, etc.).  Flows are typically reach 2-3,000 cfs with about a 1 week total period for ramping up, holding for a day or two, then ramping back down again to summer base-flow.  These flow releases are separtate and apart from the Trinity River Restoration Program flow release, and do not count toward the restoration water allotment.

High flows caused trees and other woody debris to fall into the river near my house. does the Trinity River Restoration Program plan to remove the debris from the river?

The Trinity River Restoration Program has no authority, and is not responsible, for removing woody debris from the river. Private landowners and public land management agencies make decisions about removing debris that might impair public safety, threaten the integrity of bridges or other structures, or increase the likelihood of local flooding. If landowners elect to remove debris, we urge that landowners are cautious and follow safe work practices.  Landowners should contact the California Department of Fish and Game to obtain appropriate permits.

If left within the river or floodplain, fallen trees and bushes do play an important role in the ecology of the river. Large woody debris provides cover, shade, and structure for fish and wildlife. It also causes local scour that refreshes gravel, and increases the number and size of pools or holes for fish to utilize during low flow periods. The organic materials of woody debris improve soil structure and nutrients on the floodplain.  Large woody debris contributes to the geomorphic processes, resulting in the creation and maintenance of the complex river habitats required by Chinook and Coho salmon and steelhead.  More information is available on our pages about Rehabilitation Concepts and Large Wood Monitoring.