Current conversations, media and our own experiences point to fire seasons that are far from ordinary. However, from dendrochronology (the study of tree rings) and other data sources, analysis find that prior to Euro-colonization, multiple millions of acres burned in on average in California. California’s ‘worst’ year in recent history saw about 4.5 million acres burned… which when comparing to historic averages would be within the ‘normal’ range (prior to Euro-colonization). In fact, tree ring scars show that many areas burned as frequently as every 5-10 years!  Within the past century, our society along with forest managers have promoted and practiced a prohibition on abundant low-intensity fire, allowing unburned materials to build up in forests and woodlands that along with population increase has set the stage for the complicated relationship now experienced with wildfire.

Most of us who have lived any length of time in the rural west are stressed about wildfire through the summer and well into the fall.   We endure smoke, dramatic headlines, helicopters flying over, evacuations, and too many of us witness damage to places we hold dear, including our own properties.  Forests that have not yet been touched by fire are heavily loaded with dead wood, leaves, and duff ready to become an inferno at any moment. Where fires have burned there is often a heavy load of grasses, frequently mixed with the woody remnants of trees from the last fire. Everywhere we go, organizations involved with fire share dramatic photos of conflagrations consuming tall trees.  And then we see flashfloods over fresh burns like with the McKinney Fire dumping sediments into rivers so thickly that it kills fish.  It seems we are smothered in news of devastation from wildfire!

But let’s step back for a little perspective.  Wildfire is nothing new to the west. Even before the first people set foot on these lands, our forests burned frequently from lightning strikes. These forests evolved with wildfires.  As tribes developed, their people lived with wildfires, found prosperity from them, and learned to manage the land by intentionally setting fires.

This maintained relatively open forests and woodlands, and kept mountain meadows functioning as wetlands to feed headwater streams. Natural wildfires tend to become more intense as they go upslope.  Look to the Trinity Alps where most mountain tops remain open and rocky.  Many peaks have sufficient soil among the rocks to support trees, and some scattered trees growing near the top of Thompson Peak demonstrate that the Trinities have no true elevational tree line.  But trees do grow slowly on those peaks and high intensity fires have historically happened often enough to keep those peaks mostly bare. 

From dendrochronology (the study of tree rings) and other data sources, most analysis suggest that prior to Euro-colonization, multiple millions of acres burned in on average in California. Our ‘worst’ year in recent history saw about 4.5 million acres burned… which would be within the ‘normal’ range prior to Euro-colonization. Tree ring scars show that many areas burned as frequently as every 5-10 years!  But we stopped that abundant low-intensity fire, allowing unburned materials to build up in forests and woodlands, setting the stage for the conflagrations we now see.

Frequent low intensity fires keep those fuels cleared out forests and woodlands.  It also helped keep them from getting too dense, promoting the growth of large older deep-rooted trees while minimizing the number of young upstarts that dry out the surface soils.  By keeping pines and firs out of oak woodlands, these fires promoted habitat for deer and other wildlife.  For these reasons, tribes managed the lands in California with fire.

So one of the big questions of our time is… how do we get back to healthy systems that function well (and safely) with fire?

References and Further Reading

• Asarian, E. 2024. Water temperatures in the Klamath-Trinity Basin: flow, other key drivers, and climate change implications. Presentation on 2024-05-01, Science Symposium of the Trinity River Restoration Program. Riverbend Sciences, Arcata, California. Available: https://www.trrp.net/library/document?id=2647.
• Salmonid Federation Restoration. Fire and Fish Workshop and Information
• Scott L. Stephens, Robert E. Martin, Nicholas E. Clinton, Prehistoric fire area and emissions from California’s forests, woodlands, shrublands, and grasslands. Forest Ecology and Management. Volume 251, Issue 3. 2007. Science Direct.com
• Trinity River Restoration Program Featured Article: Sediment and Summer Thunderstorms. The River Riffle, July 2023.
• Gruell, George E. Fire in Sierra Nevada Forests: A Photographic Interpretation of Ecological Change Since 1849. Mountain Press Publishing Company, 2001
• Fire in California’s Ecosystems. (2018). United States: University of California Press.

The start of summer in Trinity County has been a hot one, with 100° plus degrees for 10 days straight in early July combined with another series of 100° degree days forecasted for the latter half of the month. As warm-bodied land dwellers, we cope with heat by seeking refuge: interior shelter, air conditioning, shade, and of course, water. Refreshing water comes in many forms – pools, sprinklers and creeks, lakes, and rivers – whatever is available to us! Cold-blooded salmon are not so different in this regard. Throughout their various life stages, they too seek water temperatures that provide opportunities for success.

Last August, in the River Riffle newsletter we published “A Brief Introduction to Thermal Ecology of the Trinity River”. The article describes the thermal ecology of local rivers which experience cold, wet winters and hot, arid summers, characteristics of the Mediterranean climate of our region. Salmonids thrive where thermal diversity is available because they can maximize growth and survival by seeking optimal temperatures. For example, a juvenile salmonid’s job is to eat, grow and survive. At this stage, juveniles move from higher velocity areas where they feed in relatively cold water before residing in slower warmer areas of pools to rest and digest. The warm, slow water helps them relax and digest. Moving to take advantage of temperature differences allows them to efficiently digest their food to gain weight and grow larger. Eat, rest, digest, and repeat. The more that young fish can take advantage of these diverse temperature and flow conditions the better they can digest food into growth, which in turn improves their chance of survival in the ocean.

The seasonal, daily, and spatial temperature environment in the Trinity River before dams were constructed differed considerably from what we see today. Year to year variability was driven by winter precipitation and snow accumulation. As temperatures rose in the spring, snowmelt provided a regenerative increase in flow and expansion of habitat. On the journey from melting snow through creeks and tributaries to the mainstem river, water was warmed by the longer, warmer days, often reaching optimal temperatures for growth. As the snowmelt receded and the river returned to summer low flow, water temperatures in large, deep pools would stratify into different layers. In both these ways, temperature variability was provided to not only juvenile salmonids but also native frogs, turtles, and aquatic insects. The diversity of temperatures within nooks and crannies, combined with different depths and sheltered areas of the river formed a robust underwater nursery for the aquatic wildlife below. The timing and magnitude of these transitions and the variability of temperature depended on rain and snow patterns months before, yet the pattern was predictable. Fish and wildlife evolved over millennia to take advantage of this cycle.

The presence and operation of Trinity and Lewiston dams have dramatically altered the temperature regime in the Trinity River. As the spring and summer days heat up, Trinity Reservoir water stratifies to form a large pool of cold water below a depth of about 6 feet. Above this depth, water temperatures are warmed by the sun. Although infrastructure on some dams allow water to be drawn selectively from throughout the water column, Trinity Dam can only draw from its deep cold-water pool.  Cold water that is released from Trinity Dam to Lewiston Reservoir warms as it flows towards Lewiston Dam, but releases to the river are still much colder throughout the summer than what the river experienced at Lewiston before the dams were in place.  While the pre-dam river and other undammed local rivers warm in the spring to provide ideal temperatures for fish growth, the cold-water releases from Lewiston Dam keep the mainstem Trinity River so cold that growth of native salmonids, frogs, and turtles is stunted.

As spring progresses into summer, migration can become limited to cooler times of day, as the adults rest in the cold bottoms of thermally stratified pools by day and resume their upstream migration when the entire river has cooled at night. Finally, when adult spring-run Chinook reach their over-summer holding habitat, they can spend weeks to several months in deep pools with slow water to conserve energy for spawning in the fall.

There are many objectives of the regulated flow releases to the Trinity River from Lewiston Dam.  Summer baseflow releases aim to provide favorable temperatures for migrating and holding spring Chinook who have lost access to habitat in the upper watershed above Trinity and Lewiston dams. This temperature mitigation is achieved by flow releases of 450 cubic feet per second from the end of the spring release until mid-October. A recent research paper published in the Hydrological Processes journal, The mechanics of diurnal thermal stratification in river pools: Implications for water management and species conservation (Buxton et al. 2022) explores the effects of summer flow management on the Trinity River. The research examines pre-dam flow records from 1911-1960 when summer flows averaged 177 cubic feet per second in the Trinity River in comparison to flows since 2000 that measure 2.5 times higher at 450 cfs. The increased flow and cold deep water drawn from Trinity Reservoir create summer temperatures that are around 18°F cooler (10°C) than pre-dam temperatures in summer at Lewiston. The post-dam Trinity River experiences more water and higher velocity, yet its habitat areas are remarkably smaller than the pre-dam environment which received less water and lower velocity.

Above, we mentioned pool stratification in Trinity Reservoir. The stratification occurs because the slow-moving water warms and becomes less dense, which causes it to buoy toward the surface. This stratification of temperatures occurs in river pools that are large and deep enough (typically greater than 9 feet in depth). Size is important because when flow through the pool slows to a very low speed it prevents the warm top water from mixing with the cold bottom water and allows for stratification. This thermal layering in stratified pools is important for salmonids because it provides both adult and juveniles the opportunity to place themselves in the right temperature at the right time of day to best improve their survival and/or growth.

A key component of the pool stratification research was the measurement and modeling of temperatures as well as flow velocities in correctly sized pools for holding spring Chinook both below and above the dams. Data were collected and compared from a control pool in the Trinity River upstream of Trinity Reservoir as well as below Lewiston Dam (Pear Tree Pool). The findings showed that the pool above the dams exhibited stratification, providing the range of beneficial temperatures that follow the annual pattern of sun exposure and summer flows that naturally occur in our region.

In contrast, stratification did not occur in the pool below the dam at Pear Tree. Flows were too fast, mixing all water into one layer of uniform temperature. While temperatures in the Pear Tree pool were suitable for holding adult spring Chinook, the higher velocities increased their energy expenditure, likely taxing their energy supply needed for building and protecting redds and ultimately laying and fertilizing eggs. Unfortunately, the pool also lacked temperature diversity for juvenile salmon. Juveniles were presented with uniform temperatures and higher velocities, also taxing their energy supply needed for digesting their food to put on weight. Not to mention losing the benefits of temperature diversity discussed above.

Temperature diversity combined with variable water depths and velocities, refuge from predators, and plentiful food are important factors determining a young fish’s ability to grow and survive. While we often think “cold water is best,” perhaps a more accurate statement would be “diverse water temperatures are essential”! While that may seem obvious, a little clarification can improve our understanding of the needs of fish and how we may better serve them. Rivers in our region support a multitude of aquatic and land-based animals, insects, birds, and amphibians. Providing habitat diversity – including variable water temperatures – are paramount to meeting the needs of our wildlife community, and Program scientists continue to learn about the complex interactions between temperature and ecosystem health, hoping to better inform management on the Trinity River.

Citations

1. Pinnix, W.D., S.P. Boyle, T. Wallin, T. Daley, and N.A. Som. 2022. Long-Term Analyses of Estimates of Abundance of Juvenile Chinook Salmon on The Trinity River, 1989-2018. U.S. Fish and Wildlife Service. Arcata Fish and Wildlife Office, Arcata Fisheries Technical Report Number TS 2022-40, Arcata, California. [link to download]
2. Buxton, T. H., Y. G. Lai, N. A. Som, E. Peterson, and B. Abban. 2022. The mechanics of diurnal thermal stratification in river pools: Implications for water management and species conservation. Hydrological Processes 36(11):e14749. DOI: 10.1002/hyp.14749. [Link to Download]

References

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 [Link to download].

Naman, S., K. De Juilio, and K. Osborne. 2020. Juvenile salmonid temperature target recommendations. Memorandum to Ken Lindke, Fish Work Group Coordinator. Trinity River Restoration Program, Weaverville, California. [Link to download].

Abel, C., K. de Juilio, K. Lindke, S. Naman, and J. Alvarez. 2021. Shifting a portion of Trinity River spring releases from Lewiston Dam to the winter period: a flow management action to benefit juvenile salmonid habitat availability, growth, and outmigrant timing. White-paper for the Trinity River Restoration Program (TRRP). TRRP, Weaverville, California. [Link to download].

Many Trinity County residents are attuned to the annual water year forecasting prepared by the California Department of Water Resources, also known as the Bulletin 120 or B-120. Every year, the department gathers real time water accumulation information, snowpack data and uses modeling to forecast what to expect for the major snow bearing watersheds in California. The water bean counting starts October 1 (the nominal beginning of California’s wet season) with a final determination April 10 each year. The forecasts are broken up into several regions throughout California with the Trinity River at Lewiston Lake forecast filed under the North Coast Hydrologic Region.  The ultimate goal of the B-120 is to value expected amounts of water inflow to storage locations around the state. These data makes it possible for water managers to make local informed decisions about potential floods, the amount of water that can be released from reservoir systems, as well as what type of dry season residents and fire agencies could expect within their regions.

For the Trinity River Restoration Program, the April B-120 forecast determines the water year allocation for our yearly restoration flow releases, which were outlined in the 1999 Flow Study Evaluation and adopted in the 2000 Department of Interior – Record of Decision. These five water year types that determine the amount of water released to the river from year to year are categorized as Critically Dry, Dry, Normal, Wet and Extremely Wet. You can see the relative allocation for restoration purposes in the table below.

It is interesting to note that the State’s April B-120 has only overpredicted the water year type once, in 2008.  Currently the allocation for river restoration is the only conditioned amount of water released from Trinity & Lewiston Reservoir; where the Restoration Program’s yearly allocation is limited by water year type, the Central Valley Project can divert any amount in any water year type, usually diverting less in wetter years and more in drier years. Safety of dams releases and water releases to the Trinity River for ceremonial purposes or for Klamath River mitigation purposes are not part of the restoration release volume.

As mentioned above the State’s forecast uses a few different methods to determine how much water to expect as inflow into Trinity & Lewiston Reservoir. The most story-worthy data collection type are the on-the-ground, snow surveys which are conducted during a short window every February, March, April and May. The Trinity Alps snow surveys are led by two agencies: The U.S. Forest Service who motor in via snow Cat to several locations in the Trinity Alps Wilderness, and an expert group of backcountry cross country skiers led by The Watershed Research and Training Center. There are nearly a dozen survey courses established throughout the Trinity River watershed and these sites have been measured in exactly the same locations since the 1940s.

Each year The Watershed Center sends out a small team of between two and four backcountry skiers to travel through the Alps Wilderness and measure snowpack at three survey courses: Shimmy Lake, Red Rock Mountain, and Bear Basin. Once the team reaches a survey location, they drive a specialized aluminum tube tool called the Mt. Rose Sampler, into the snowpack until they hit ground. “It takes a few times to get used to doing it,” says Josh Smith who has been conducting surveys in the Alps since 2011, with the first full recorded season in 2012. The surveyors use the tool to measure the height of the snow, then carefully extract the tube from the snowpack and weigh the snow-filled tube using a handheld scale. These measurements allow the surveyors to calculate the Snow Water Equivalent in designated transects within the three courses for which their team is responsible for. The State uses the hand measurements from the snow survey teams to bolster additional data taken from unmanned sensors located across and just outside of the watershed. These data sources together feed into a model that predicts the volume of water that will flow into Trinity Reservoir that year.

That said, the Watershed Center is looking for local Trinity County residents who believe they have a sufficient mental and physical stamina to participate in this long-standing Trinity County tradition. “We get a lot of calls from people who think this is right for them,” Josh continues, “most people only come out once, and then they are done. It’s a real suffer-fest.”

If you’d like to learn more, please reach out to Josh Smith at the Watershed Training and Research Center by calling (530) 628-4206.

Not only is the hyporheic zone an area that supports great biodiversity, in a 2005 study this zone was also coined the “river’s liver” from findings that carbon and nitrogen cycling in the river was “controlled by the live sediments of the central river channel, which thus represent a “liver function” in the river’s metabolism.”[1] So, the hyporheic zone acts as a filtering mechanism for the river, and an area of rich biodiversity. But that’s not all!

The hyporheic zone provides a multitude of functions and it is critically important to the health of our waterways. Because the hyporheic zone acts as a filtration system through its porous sediments, it also promotes higher levels of dissolved oxygen through photosynthesis. Dissolved oxygen in waterways support anadromous fish species like chinook, steelhead and coho salmon by helping them to maintain a healthy respiratory function.

When Trinity River salmon return to spawn, they dig redds (or nests) to lay their eggs in. The female fish flaps her tail sideways into the river bed, digging down around 12″ to 14″ into the hyporheic zone. After the eggs are laid and fertilized, she covers them with rocks. These rocks protect the eggs and newly hatched alevin from predators. The eggs location in the hyporheic zone provides water flow that flushes metabolic wastes during egg development and provide dissolved oxygen for her embryos to breathe.

The presence of dissolved oxygen, protection from predators, and microvegetation for food makes the hyporheic zone a biologic hotspot for macroinvertebrates. While the insects nestle down in between rocks they feed on leaves, algae, and twigs. In a healthy hyporheic zone where flow and sediments are correctly combined the area supports an abundance of food for the macroinvertebrates which in turn supports an abundance of food for juvenile and adult salmon. And that’s not all! Due to the interaction with upwelling cooler ground water, hyporheic zones help to moderate stream temperatures during the lower flow summer and fall months.

The Significance of the Hyporheic in the Trinity River

The hyporheic zone covers the entire streambed and bank areas of a river bottom and the ability to perform its natural functions are influenced by many things. Some influences include whether the stream is straight or meanders, if there are any obstacles in the channel, such as log, boulder, or even the pier support for a bridge. Factors also include whether the stream has a single channel or multiple channels, and also how porous the streambed sediments are. For example, near Lewiston Dam where there are very few fine sediments, large amounts of the river’s flow go subsurface because they are conveyed in the hyporheic zone. This is an unnatural situation because the river requires all sized sediments, from sand and silt up to cobbles and boulders. So while semi-open pore spaces in the bed are desirable, if the pore spaces are fully open do to the lack of fines in the bed, salmon eggs will jiggle around in redds and die from abrasion as well as the speed of flow between rocks in the hyporheic zone will be too fast for microvegetation to grow and macroinvertebrates to live.

Thus, to combat a static river system, since 2000, the Trinity River Restoration Program has focused efforts around replenishing the critical building blocks of a river. This is achieved by gravel additions in the upper river, large wood placements along river banks and a yearly spring snowmelt hydrograph that is released from Lewiston Dam. Also, since the Trinity was heavily impacted by hydraulic and dredge mining the program allocates funds for watershed restoration and gives great attention to the diversification of channels and floodplains with mechanical rehabilitation.

These things combined help the dammed Trinity in healing itself, however, many gains are yet to be made with restoration and management. For example, Todd Buxton, PhD, is starting a three year study of the hyporheic zone in the Trinity River. The study will construct a mathematical model to simulate the flow rates and directions, temperatures, and dissolved oxygen in the hyporheic zone. At the same time, macroinvertebrates in the hyporheic zone will be measured where the study is being conducted at sites within the 40-mile restoration reach. This information will be used to develop a statistical model for predicting the density and species composition of macroinvertebrates based on the above characteristics in the hyporheic zone, since these aspects have a strong role in determining how many and what species of macroinvertebrates may be present. Application of the paired models will then enable scientists to better understand how surface flows and temperatures in the river can be better managed to promote macroinvertebrate populations and increase the availability of food for salmon.

Additionally, the program has made significant gains with increases in outmigrating salmon smolts since 2000, but these fish are not returning home at the rates that were observed before this year. Scientist have identified that the Program’s current management of flow timing could be a critical limiting factor for the fish of the Trinity River. If juvenile fish had more food available to them when they are emerging from their nests they may enter the ocean more robust. If elevated flows from the dam are timed with winter storms, then the Trinity River could add to the power of the tributary flows to increase mainstem water levels to prevent delta creation while simultaneously preventing sediments from smothering redds and other important living organisms within the hyporheic zone.

Understanding this unique area has been a challenge for river scientists because the hyporheic function is expansive. It is also unique to each river system and crosses over many scientific disciplines. The zone not only intrigues those interested in studying the microbiome of a river system, but also includes; ecologists, geomorphologists, hydrologists and environmental engineers, just to name a few. With each discipline and study within, scientists learn more and more about the fascinating world that exists beneath and alongside a river’s bed and how river restorationists can better understand to allow it to flourish.

References

Hyporheic Zone, Wikipedia

Lewandowski, J.; Arnon, S.; Banks, E.; Batelaan, O.; Betterle, A.; Broecker, T.; Coll, C.; Drummond, J.D.; Gaona Garcia, J.; Galloway, J.; et al. Is the Hyporheic Zone Relevant beyond the Scientific Community? Water 2019, 11, 2230. https://doi.org/10.3390/w11112230

The Significance of the Hyporheic Zone, Jana Hemphill, Deschutes Land Trust, 2021.

Citations

[1] Fischer, H.; Kloep, F.; Wilzcek, S.; Pusch, M.T. A river’s liver–microbial processes within the hyporheic zone of a large lowland river. Biogeochemistry 2005, 76, 349–371. [Google Scholar] [CrossRef]