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ID: 2615

Asarian, J. E., K. De Juilio, S. Naman, D. Gaeuman, and T. Buxton. 2023. Synthesizing 87 years of scientific inquiry into Trinity River water temperatures. Report for the Trinity River Restoration Program (TRRP). Riverbend Sciences, Eureka, California. Available:

From the abstract:
We acquired river and tributary water temperature data from tribal, federal, state, and local agencies, consulting firms, and universities. The result is a set of stream temperature data for 252 sites at their original temporal resolution (sub-hourly to daily) compiled into a geographically referenced database containing 396,172 daily values, derived from 13 million original measurements. We also compiled hydrology and reservoir water temperature data in this database. These data were used to analyze existing conditions and develop model scenarios for pre-dam conditions and hypothetical management alternatives. From this effort, conceptual models were updated and used to explore water temperature dynamics on the Trinity River with comparisons made to regulated and unregulated rivers described in the literature.
Prior to construction of the TRD, the Trinity River exhibited complex temperature patterns that are normal to streams in regions with Mediterranean climates. These complex patterns span temporal, longitudinal, lateral, and vertical gradients, which drive the distribution, reproduction, growth, and migration of cold-blooded aquatic species such as fish, invertebrates, reptiles, and amphibians. Trinity and Lewiston dams and impacts from historic land use practices profoundly altered the thermal landscape in these dimensions, and regulated flow releases further exacerbated temperature impacts on the Trinity River. For comparative analysis, we defined four eras associated with management changes: pre-dam era (10/1/1911–1/20/1961), full diversion era (10/31/1963–4/30/1978), transitional era (5/1/1978–9/30/1999), and ROD era (10/1/1999–2019). ROD flows did not officially begin until Spring 2004, but we start the ROD era with hydrologic year 2000 because pre-ROD flows became increasingly similar to ROD flows.
When comparing the pre-dam era to the other eras, the largest impact to water temperatures in the Trinity River occurred at Lewiston. As is typical for rivers below large thermally-stratified reservoirs where water is released from cold depths, seasonal temperature variation has been severely reduced. With-dam seasonal minima and maxima have similar timing (January and August, respectively) as occurred pre-dam, but seasonal range in daily average temperatures has been compressed from 4–23 °C pre-dam to 7–10 °C with-dam. The flow regulation causes the river to be unnaturally warm in winter and cold in summer. Variation among years also declined at Lewiston from thermal dampening by the reservoir water bodies and the relative similarity of flow releases across years, regardless of annual variations in annual water yields or patterns in runoff. The downstream extent and severity of thermal impacts from the reservoirs and flow regulation varies with flows released from the dams. When dam releases are low, the thermal effects diminish more quickly than during high-flow releases when the entire length of the river below Lewiston Dam is strongly affected.
Summary of important findings or confirmation of prior findings:
  • Water released from Lewiston Dam into the Trinity River is nearly always too cold to support optimal juvenile salmonid growth during the primary rearing period (April-June).
  • Elevating Lewiston Dam releases above 300 cfs winter baseflows before April in Dry years and May in Normal and wetter years would provide temperatures that benefit salmonids, invertebrates, and frogs by increasing food assimilation and development rates.
  • April in Dry years and May in Normal and wetter years are the only time periods in spring when it is possible for temperatures in the entire Lewiston to Weitchpec reach of the Trinity River to approximate unimpaired (i.e., pre-dam) patterns.
  • In May of Dry years and June of Normal and wetter years, elevated releases suppress growth of cold-blooded aquatic species.
  • To provide cool water for meeting outmigration temperature criteria at Weitchpec, the ROD required large spring releases but did not consider the deleterious effects of these releases on juvenile salmonid growth in upstream reaches.
  • Trinity Reservoir storage <0.75 million acre-feet in September, October, and November poses a notable risk of water releases that exceed the thermal tolerance of salmonid eggs and can cause mortality.
  • Trinity Reservoir storage is likely to decrease and water temperatures are likely to increase in the future due to changes in runoff patterns from climate change.
  • It is not clear that ROD water management has resulted in increased temperature compliance from the period of record available.
Based on our assessments, we make the following recommendations:
  1. Monitor temperatures in the Trinity River upstream of Trinity Reservoir to better understand the natural temperature regime and its effects on water temperatures in Trinity Reservoir.
  2. Flows in spring should recede beginning in April of Dry and Critically Dry water years and May of Normal and wetter water years to provide water temperatures in the Trinity River upstream of the North Fork Trinity River within the range that provides optimal juvenile salmonid growth (13–16.5 °C) and outmigration (16–18 °C).
  3. Reduced emphasis on meeting ROD temperature targets for smoltification at Weitchpec. Instead, the goal should be a balance between growth, encouraging timely outmigration, and mitigating temperatures in the lower river that approach the thermal limits of juvenile salmonids.
  4. Infrastructure of the TRD should be modified to enable finer flow and temperature management to be implemented for the benefit of the river ecosystem, including:
    1. Installation of a multi-level temperature control device in Trinity Reservoir;
    2. Removal of Lewiston Dam or construction of a new type of conveyance through or around Lewiston.
  5. For Trinity Reservoir, an end of September storage minimum of 0.75 million acre-feet (MAF) should be adhered to following the recommendations of Bender (2012). This should be coupled with a multi-year drought contingency plan that specifies steps taken when reservoir storage is predicted to be less than 1.25 MAF in any year, assuming a multi-year drought is possible at any time.
  6. An assessment of multiyear drought effects on Trinity Reservoir storage levels, water temperatures, and the resulting ability to meet temperature criteria in the Trinity River should be conducted.
  7. Development of a tool for accurately predicting Trinity River water temperatures in summer at flows lower than RBM10’s current lower limit of around 350 cfs.


First Posted: 2023-08-16 21:51:46

Post Updated: 2023-08-16 21:55:41