by Frank DeCourten

Terra Sierra: Truckee River Rocks (and Rolls!)

Released from the gates at Tahoe City, the crystalline water of the Truckee River glides under Fanny Bridge like a stream of liquid silver as the trout hover in slack water pockets, waiting for the food pellets to fall from above.  A few hundred yards downstream, the water accelerates into a swift riffle before splitting into placid rivulets separated by gravel bars and bordered by grassy overhangs.  In the summer months, people on rafts drift peacefully along while anglers cast flies over the lucid water and children wade through the shallows along the banks.  Below Bear Creek, the water accelerates a bit as the river bed steepens and becomes choked with rocks and boulders.  Here, where the river rushes and swirls over its rocky bed, it still mesmerizes those drawn to its edge. For some reason, the fluid motion of rivers, swift or serene, has always captivated humanity.  The Truckee River is no exception.

Near its confluence with Donner Creek, the Truckee River swings to the east, passing through a landscape transformed by humanity.   Few communities are more interlaced with a river than is Truckee.  Sharing more than just a name, our town – its physical setting, its history and culture, its economics and ambiance -- has always been shaped by the stream that runs through it.  The river not only informs our sense of place today, but it will also shape our destiny as a community.  The quality of life for the future residents of Truckee will strongly reflect the degree to which we preserve the river and protect it from the adverse effects of our own activities.  In a region of stunning natural beauty, the Truckee River is one of our most cherished treasures.

We each find our own enchantment in the Truckee River.  For some, it’s the fish, or the exhilarating but forgiving rapids, the refreshing coolness of the water, or perhaps the tranquil babble that so soothes the soul.    For me, though, it’s the rocks.   Geologists see rocks as prehistoric treasures, each one documenting ancient worlds through such details as their location, size and shape, composition, texture, and other attributes.  For those interested in the stories told by rocks, there is no place quite like a river.  As rivers flow through their channels, they can move rocks everywhere along their course, flush them downstream, and eventually deposit them wherever and whenever the speed of the water diminishes. Thus, rivers are much like conveyor belts that continuously sample the material from the canyon walls, the river banks, and the channel bottoms.  And, it is in this regard that the Truckee River is an especially fascinating place to explore, for the stories of the rocks suggest a remarkably different river in the prehistoric past, and reveal an ancient world utterly different from the one we know today.  

In the mid-1960’s, while studying the deposits left by Ice Age glaciers and their meltwater streams in the Truckee region, Peter Birkeland was the first geologist to notice something very interesting about the rocks in the Truckee River.   Birkeland observed many large light gray granite boulders in and near the river, even though the bedrock in the region immediately around Truckee is mostly dark-colored volcanic material.  Some of these large granite boulders, clearly rounded by abrasion in the stream channel, were perched as much as 40 or 50 feet above the modern river, and scattered hundreds of yards beyond the river banks.  Birkeland knew that the boulders must have originated far upstream, in the higher terrain where similar granite comprises the bedrock.  But, how did they arrive in their current locations, far beyond the downstream reach of Ice Age glaciers and much too large and scattered to be transported by the modern Truckee River, even during the worst historic floods?  The roundness and abraded surface of the granite boulders, along with their location in sandy gravel beds, clearly indicated that they were transported and deposited by running water.  But, what water? When? How?  Birkeland mapped the large boulders, along with the gravel and sand in which they are embedded as “glacial outwash”- the gravelly debris that is transported and deposited by rivers bred from melting ice.  

Birkeland’s study became even more intriguing as he traced the glacial outwash deposits downstream from Truckee.  Below the confluence of the Little Truckee River at Boca, in the narrow canyon carved by the Truckee River as it swings to the north, only small patches and embankments of outwash deposits were present. In the Verdi area, however, Birkeland recognized extensive outwash deposits containing many boulders 5-10 feet across perched as high as 40 feet above the river bottom.  The largest boulder in the Verdi deposit was a block of rock 40 feet long, 20 feet wide, and at least 10 feet high.  Downstream from Verdi, Birkeland found only scattered outwash deposits across the floor of the Truckee Meadows.  However, on the east side of the Truckee Meadows, near the old settlement of Vista, Nevada, he once again encountered large outwash boulders spread across a wide tract of land where the Truckee River begins to enter the narrow canyon separating the Pah Rah and Virginia Mountains. Additional outwash boulders were observed as far east as Mustang, where the largest measured 36 feet long and 20 feet across.  Downstream from Mustang, outwash deposits were rare and only contained a few boulders, nearly all smaller than 4 feet in diameter. 

In the past 40 years, other geologists have mapped the glacial outwash deposits along the Truckee River in more detail, worked out an approximate chronology for glacial events in the Sierra Nevada and Truckee Meadows, and have added much to our understanding of the Truckee River rocks.  These studies have confirmed the observations and interpretations of Birkeland, who concluded that there must have been some ferocious prehistoric floods in the Truckee River drainage to transport such

large blocks of stone as far downstream as they are now found.  Even the disastrous New Years, 1997, flood of the Truckee River would have seemed insignificant in

comparison to the Ice Age “megaloods”, as geologists now refer to the ancient events.  In fact, the speed and volume of the megafloods can be estimated from the distribution of the outwash deposits, the size and shape of the transported boulders, and the profile of the river canyon in the vicinity of the outwash sediments.  In addition, geologists have learned much about modern floods associated with glacial conditions from the catastrophic cataracts known as jökulhlaups that ravage Iceland periodically.   These great floods are generally the result of the failure of an ice dam that impounds a lake of glacial meltwater. Together, the observations of modern Icelandic jökulhlaups and interpretations of the Truckee river rocks, evoke some frightening scenarios of Ice Age megafloods in our region.

The geological evidence suggests that there were numerous great floods down the Truckee River during the past two million years, when several different cycles of glaciation affected the adjacent parts of the Sierra Nevada. The best documented megafloods appear to have occurred during the Tahoe Glaciation (one of four major Ice Age cold cycles), from about 60,000 to 130,000 years ago.  Many times within this interval, long tongues of ice descended Squaw Valley, Pole Canyon, Deep Creek Canyon, and other drainages leading from the higher terrain to the west.  As these glaciers flowed into the Truckee River Canyon northwest of Tahoe City, they combined to build an ice dam that eventually blocked the river, and raised the level of ancient Lake Tahoe by at least 90 feet.  Some evidence suggests that earlier ice dams may have temporarily raised the lake level by 600 feet!  Even a 90-foot increase in lake level would have impounded some 3½ cubic miles of water behind the plug of glacial ice.  The pressure of so much water against the ice dam would have been enormous.  Also, because ice is slightly less dense than water, the dam became buoyant when the rising lake water neared the crest of the dam.  Floated off its bedrock foundation, the dam began to lift and crack, allowing water from the engorged lake to seep through it and escape beneath it.  Once this underflow began, the dam would have been destroyed in a matter of hours, unleashing a flood of unimaginable magnitude.   Moving up to 50 miles per hour, the raging wall of water must have swept through the canyon, scouring the walls and washing granite boulders downstream to the lower terrain around the site of present Truckee.  Today, those boulders can still be found scattered across the lower terrain.  The flood-washed granite boulders can be distinguished from other common rocks in the area by their large size, light color, and well-rounded shape.

The flood waters would have spread out over the undulating terrain of Ice Age Martis Valley before gathering again to plunge into the narrow canyon leading toward Verdi.  The ruggedness and scoured appearance of this part of the modern Truckee River canyon can, in part, be attributed to numerous Ice Age megafloods.   Bursting out of the narrow canyon near Verdi, the floodwaters would have inundated the valley downstream to a depth of about 50 feet.  Eventually, the water reached the gently sloping expanse of the Truckee Meadows, site of modern Reno and Sparks.  There, the floodwaters were probably about 20 feet deep and, because they were spread out so widely, slowed to less than 5 miles per hour.  How could such slow-moving water transport large boulders across the relatively flat Truckee Meadows all the way to Vista and Mustang?

Even slow-moving water can transport relatively large rocks.  This is because the smaller particles in a river bed—the sand and pebbles—can be washed from under and/or around a large rock, even when the latter remains stationary on the river bottom. The large rock will eventually tilt or roll as the pebbles, sand, or mud beneath it is undermined and washed away.  Eventually, the large rock will slip downstream incrementally in a slow rolling motion.   This intermittent rolling motion can be accelerated if the stream flow increases, as might happen after heavy rains, or during the spring run-off season.  Nonetheless, whenever we envision glacial megafloods “washing” boulders downstream, it is important to remember the slow episodic rolling of large rocks can result in significant movement over time.  Perhaps this, at least in part, explains how such large granite boulders from the Sierra can now be found in glacial outwash deposits on the east side of the gently sloping Truckee Meadows.

After the megafloods reached the Mustang area, they eventually flowed into the upper reaches of Lake Lahontan, an enormous Ice Age lake that inundated much the low ground in western Nevada during the Ice Age.  The boulders and cobbles carried by the flood waters could be transported no farther east into the standing water of the lake. Near Fernly, Ice Age sediments consist almost entirely of fine silt and mud deposited beneath the still lake waters.  The story of Lake Lahontan is fascinating in its own right (we will review it in a later installment), but it was clearly large enough to absorb the great megafloods of the Ice Age Truckee River.

On your next visit to the Truckee River, try to imagine the devastating power of the Ice Age jökulhlaups that ravaged our region many times thousands of years ago.  Look around at the large granite boulders and envision them rolling and bouncing along beneath the roaring flood waters.  Visualize the site of modern Truckee inundated under hundreds of feet of churning water.  And, if such scenes seems like pure fantasy to you, remember that the hard evidence is everywhere around you….the Truckee River rocks!