A Journey of Ten Million Years...the Salmon of the Sierra Nevada

Chinook Salmon attempting to enter the fish ladder at Camanche Dam on the Mokelumne River

They've been coming here for at least ten million years. Every year, without fail. The lands changed, but still they came. If one waterway was blocked, they eventually found another. Sometimes they were isolated, and could never return to the sea, but they survived anyway. They are the salmon and trout of the Sierra Nevada.

Stanislaus River at Knight's Ferry

Anadromous fish are those that live much of their lives in the oceans, but which return to freshwater streams to reproduce. One might wonder why they would have such a complicated breeding scheme. In all likelihood, it had to do with the survival of the young fish. Rivers and streams tend to offer more hiding places than open ocean, and the young have a chance to grow large enough to survive. The most familiar of these fish are the various species of salmon and Steelhead Trout. I had several opportunities to view the November migration of the Chinook Salmon this week on three different rivers: the Mokelumne, the Tuolumne, and the Stanislaus.

In historical times, the fish ranged far into the interior of the Sierra Nevada and Cascades Range, being stopped only by cascades and waterfalls too high for them to jump. They numbered in the millions. As European and American colonizers replaced the Native Americans across the state, the fish began a steep decline.

One of the first and worst events was the Gold Rush of 1848. Miners tore up miles and miles of river gravels and disrupted the flow of the rivers with their placer mines (sluices, long-toms, and cradles). Hydraulic mines (water cannons) ripped away billions of cubic yards of gravels from the hillsides and choked riverbeds with egg-smothering silt and clay. Vast amounts of water were diverted from river headwaters to feed the hydraulic mines through a system of flumes and pipelines.

Somehow the fish survived this onslaught, but then something more insidious happened. The dam-makers arrived in the late 1800s and early 1900s to build reservoirs to divert water for irrigation purposes. The dams themselves were barriers to the upstream movement of the fish, but even worse was that water diversions left the rivers to small and warm for their survival. The mega-dams were constructed in the 1960s and 1970s, diverting even more water. One river, the San Joaquin, ceased to flow most years over a stretch of about sixty miles. By the time the last dam was built, more than 95% of the historical breeding grounds for the fish had been made inaccessible to their migration (see the map below).

The blue portions of the rivers have salmon. The historical range is shown in black. Source: https://noaa.maps.arcgis.com/apps/MapJournal/index.html?appid=ceebefd9685143daa5bf30d5a7e0c7fa

Millions of years ago, this landscape was much different. The Sierra Crest was lower than it is today, but the summit region was covered by volcanic complexes not unlike the Lassen Peak complex and other parts of the Cascades. Periodic eruptions sent steaming lahars (volcanic mudflows) down the river canyons and onto the floor of the Great Valley (which may have actually been a shallow sea in this area). The rocks from this time period, 5 to 12 million years ago, are called the Mehrten Formation and they can be found throughout the Mother Lode foothills.

The Mehrten Formation has yielded up a treasure trove of fossil species. At Turlock Lake fossils were found of Giant Tortoises and Oncorhynchus rastrosus, the Spike-toothed Salmon. These remarkable salmon were as long as eight feet. They apparently used their "tusks" to fight for territory. Otherwise their lives were similar to the salmon of today. The riverbanks were populated by horses, camels, bison, antelope, giant ground sloths, mastodons, and carnivores, including the ancestors of the bears and wolves. The woodlands were dominated by sycamore and oak. For an excellent overview of the fossil record, check out the technical article by Sankey, Biewer and others, or read their excellent book The Giant Spike-Toothed Salmon and Other Extinct Wildlife of Central California.

Oncorhynchus rastrosus, the Giant Spike-toothed Salmon. Artist: Jake Biewer

Erosional processes steal nutrients from the land and carry them to the sea. It has been remarked that salmon and other anadromous fish return the gift. After they fight their way up the streams and rivers, and after they lay and fertilize their eggs, the fish die. Their bodies provide food for a host of carnivores and scavengers. Long before I found any fish in the Mokelumne River the other day I sensed the lurking presence of several dozen Turkey Vultures. I wasn't actually thinking about fish at that moment, but wondered why so many vultures were hanging around the river.

Moments later the reason was clear, as many were already feasting on the dead fish. The alert eyes that followed the movement of the fish are of an ancient lineage as well. Turkey Vultures are the most common of the avian scavengers, but their ancestors and Condor relatives patrolled these rivers millions of years ago.

These fish have survived for at least ten million years, and it has taken only a century and a half to threaten their very existence. Even now intense controversy follows the negotiations over how much water to devote to agriculture and how much to preserve the future of the salmon and the entire ecosystem that they inhabit. It's not a fish versus people proposition as some have portrayed it. It is a larger question of whether we want to preserve healthy river habitats for clean water, a diverse ecosystem, and for our own recreation and inspiration. Agricultural interests in the drainage of the San Joaquin showed in the 1940s that they were more than willing to destroy a river to apportion every drop of water. Things began to change in the 2000s as agreements were reached to restore flows to the lower river and bring back viable populations of Chinook Salmon. I hope we can be as wise in the other water conflicts around the San Joaquin Valley.

These videos are from the Mokelumne River below Camanche Reservoir, the end of the road for the Chinook Salmon. I had never visited the area before, so I was exploring the trails at the day use area below the fish hatchery. There were lots of fish in the river, and large numbers of them were struggling to break through the gates of the fish ladder, which was closed. I followed the fish ladder into the hatchery grounds, wondering if it actually provided access to the reservoir, but it didn't. It led to holding ponds that were already full of salmon. When the fish are ready, the eggs are harvested and fertilized, providing the stock for the hatchery. The young fish are later released into the river at the hatchery and other locations downstream.



These hatcheries are one way of dealing with the devastating loss of habitat for the salmon, but it seems it would be better if we could provide access to their ancestral breeding grounds upstream. I don't claim any expertise in these matters, but there have to be better answers than what we see happening today. They've been here for at least ten million years, and deserve a chance to be around for a few more.

Driving Through the Most Dangerous Plate Boundary in the World: A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

Mt. Shasta is a big mountain. A really big mountain. Reaching an elevation of 14,179 ft (4,322 m), and rising nearly 10,000 ft. (~3,000 m) above the surrounding terrain, it has a volume of around 100 cubic miles. As such, it is the biggest volcano in the Cascades volcanic arc, and probably is on the short list of largest stratovolcanoes in the world (shield volcanoes like those of the Hawaiian Islands have a different origin and are much, much larger). The volcano is the surface manifestation of the sinking of the Pacific lithospheric plate beneath the North American continent along the Cascadia Subduction Zone. As the plate sinks deeper, superheated water is liberated, which lowers the melting point of the deep rocks, forming magma chambers that sometimes erupt at the surface.

The thing is, one would think that this massive edifice would be made of lava and volcanic ash, but a large percentage of the volcano is actually composed of...mud. Simple mud. Well, more properly, mudflow deposits, or lahars, as they are called by geologists. This isn't the kind of mud you find on the bottom of a lake or the sea. It was mud born of violence. The mud includes massive boulders, showing that the formation of the lahars involved the rapid melting of snow and ice during violent eruptions. The mixing of water, ash and rock fragments forms a slurry that moves quickly down the flanks of the volcano. To humans, they are dangerous. A lahar in Colombia in 1985 killed around 23,000 people in a matter of minutes.

On our journey through the most dangerous plate boundary in the world, we've explored the accretionary wedge and forearc basin of the massive subduction zone that once existed off the California coast, and now we've reached the base of the most visible part of the system: the magmatic (or volcanic) arc . The arc is extinct today in this part of California, but one is still active in the Cascades of Northern California and the Pacific Northwest.

Driving through the foothills of the Sierra Nevada in places like Knight's Ferry or LaGrange, just north of the Yosemite National Park region, it's hard to visualize the violent past of this landscape. The gentle slopes belie the intensity of the floods and flows that carried the boulders for forty or fifty miles from the volcanic centers that once existed near the crest around Sonora Pass and the Dardanelles. The rocks are called the Mehrten Formation, and they were deposited around 5 to 7 million years ago, when the subduction zone was still active at this latitude. Similar mudflow deposits cover much of the northern Sierra Nevada.

Of course, the volcanoes weren't always erupting. Decades or centuries may have passed between eruptions, and the tattered ecosystem would recover. Petrified wood has been found in the formation, and fossils of horses, camels, and other animals that lived on the river floodplains emerging from the mountains above. A few years ago I became aware of some of the most unique fossil species to be discovered in the formation, gigantic tortoises, and six-foot-long salmon...with fangs! The Sabertooth Salmon (Oncorhynchus rastrosus) once swam the ancestral Stanislaus and Tuolumne Rivers, right here in my own back yard. That's almost as cool as living in a place that once had Mosasaurs (oh yeah, they lived here too).

A former student of Modesto Junior College (and current geology major at CSU Stanislaus), Jake Biewer, is working this summer on a new exhibit at the Great Valley Museum highlighting his research on the Sabertooth Salmon and the large tortoises of the Mehrten Formation. I'm looking forward to seeing the results!

In our next post, we'll discover what it's like to be living beneath the volcano. In the meantime, check out this video of a lahar at Mt. Shasta.

What Strange Fossils are Found in Your Region? We have Oncorhynchus rastrosus, the Sabertooth Salmon

Drawing by Jake Biewer of CSU Stanislaus

I've got a delightful new project these days. As I've mentioned many times, our region has a wonderful new natural history museum in our Science Community Center. The Great Valley Museum has existed for more than thirty years, but most of that time it was in a dilapidated former house and bookstore for the east campus of Modesto Junior College. Wonderful people labored over the years to educate our children about the marvelous natural history of the valley but the buildings were not up to the task. At long last we have a facility that is deserving of the incredible talents of the volunteers who have kept the museum going over the years.

The museum opens its doors officially on April 4, but school tours of the museum began this week. The planetarium has been offering shows for a year now. But there are still important tasks to complete. There is a vacant space on the north side that should become an outdoor nature laboratory with a pond and river environment with native vegetation. And there is a big blank wall that is destined to become a paleontology/geology exhibit. And that is what I am working on these days.

A paleontology exhibit of course would emphasize the fossils found in the local region, and we have a rich record of fossil discoveries from the Mesozoic and Cenozoic eras (the so-called "age of dinosaurs" and "age of mammals"). I've written about these fossils numerous times, but now we need to think of how to bring these fascinating creatures to the attention of our children and community members.

I brought up the topic with the paleontology professor at CSU Stanislaus, Dr. Julie Sankey, and that led to an impromptu tour of the work underway at the university. One of my former students, Jake Biewer, has been researching some of the creatures found in the Miocene Mehrten formation at Turlock Lake Reservoir. They include giant tortoises, and one of the strangest creatures of all, a Sabertooth Salmon (Oncorhynchus rastrosus). These salmon were found along the west coast of the United States, and lived from the Miocene to Pliocene (13 million to 4 million years ago). They reached lengths of 3 meters (that's nine feet). Despite the menacing teeth, they probably consumed plankton. The teeth probably were used in competition with other males.

What child couldn't be impressed to find that creatures like these used to live on the land their town occupies? Be ready to hear about some more of the creatures that might be part of an exhibit at our wonderful new museum.

Fossil specimens of Oncorhynchus rastrosus from U.C. Berkeley paleontology collection.