Stream incision and surface uplift in the Kings River drainage, Sierra Nevada, California

Among the awe-inspiring canyons of the Sierra Nevada, the Kings River Canyon stands out. John Muir considered the South Fork of the Kings possibly even grander than Yosemite Valley. In fact, the valley walls are not quite as high as in Yosemite, but nearby peaks are among the highest in the continental United States and give the sense of vast topographic relief. How did this canyon form? Glacial ice had a role in sculpting the valley walls, but stream incision had probably cut most relief prior to Pleistocene glaciation. Recent research suggests that incision is still adding to the topographic relief, if only slowly.

The Kings River Canyon has likely been a major feature of the landscape for as long as the Sierra Nevada has stood near its present elevation (Fig. 1). The timing of the uplift of the modern peaks is controversial and has been debated since Muir's time. The controversy is focused on whether the canyon we observe today formed primarily in the Mesozoic, when the Sierra Nevada was an active volcanic arc, or more recently, during a hypothesized topographic rejuvenation. One indication of more recent uplift would be more recent stream incision. Incision ought to accompany uplift and uplift presumably stopped when Mesozoic subduction stopped, if it were the only cause.

Uplift and incision are linked because potential energy increases as a stream's headwaters rise above its base level. As a stream grows steeper, it has more energy to spend during the trip downstream. In other words, it has more power to transport sediment and incise bedrock. Base level is the point where hydraulic gradient is zero and a stream can do no work, usually at sea level.

Base level for the Kings River has probably been near the Great Valley for its entire history. In the Mesozoic, base level was sea level, and what is now the Great Valley was part of the continental slope. In the Holocene, the Kings River drained primarily into Tulare Lake in the southern Great Valley, a few hundred meters above sea level (Fig. 1). Tulare lake is now dry because irrigation and flood control projects have diverted rivers north to San Francisco Bay, but it was a large, internally drained lake even less than one hundred years ago.

One way to measure incision is to compare the elevation of the present stream channel with its elevation in the past. The bedrock in the Kings River drainage includes narrow pendants of marble, and the marble is soluble enough to form caves. Today, the caves are exposed in vertical sequences hanging in the valley walls above the modern channel. In the past, these caves formed at the level of the stream channel, which was also the level of the groundwater table. Sandy sediment was occasionally washed into the caves by stream action. These deposits were preserved when the stream incised below the cave level, abandoning them.

The age of abandonment can be dated using radioactive isotopes. Isotopes of beryllium and aluminum are produced in quartz when it is exposed to high-energy neutrons in the atmosphere. Isotopes produced in this manner are often called cosmogenic radionuclides. Production stops when quartz is buried (in a cave, for example) and shielded from neutrons. Because these isotopes are radioactive, they continuously decay to different, stable forms. The ratio of cosmogenic beryllium to aluminum depends on the rates of production, the rates of decay, and the time since burial. Burial ages may be calculated by measuring ratio of cosmogenic beryllium to aluminum because the all of the rates are known independently.

Burial dating in the caves on the Kings River shows ages between 2.7 Ma and 0.3 Ma (Fig. 2). Older ages come from higher elevation caves, consistent with progressive incision below cave levels. At least in the location of the cave sequence, the Kings River cut its inner gorge within the last three million years.

A different approach for measuring canyon formation is to compare erosion rates across the landscape. Stream channels must incise faster than the surrounding ridges and hilltops erode in order to create deeper valleys. While there are many options for measuring the spatial variability of erosion rates, detrital tracer methods can be efficient. Tracer methods track particles from bedrock sources downstream into the mobile stream sediment. Differences in the sediment and bedrock statistical populations may indicate spatially variable bedrock erosion.

In a hypothetical drainage, where peaks were granite and valleys were gabbro, rock type would be a useful tracer. A geologist standing downstream would know immediately which stones were eroded from the peaks and which stones were eroded from the valleys. In the Kings River drainage, radiometric ages in apatite are a suitable tracer. The mineral apatite is ubiquitous in the granodiorite that make up most the bedrock, and apatite ages vary systematically across the Kings River drainage bedrock area. The ages are also tens of millions of years old, much too old to be influenced by the time it takes to transport sediment through the drainage system. This transport time is called the residence time and is likely less than about fifty thousand years.

Fifty apatite grains, collected from the modern channel near the edge of the Great Valley, show a surprisingly large number of young ages. In the bedrock, the youngest ages outcrop primarily in the bottom of the Kings River Canyon. As there are more of these young ages in the detrital sediment than a random sample of bedrock would predict, it is likely that the canyons have eroded faster than the surrounding landscape over about the last fifty thousand years. Quantitative comparison of the bedrock and detrital ages indicates that the Kings River Canyon is eroding nearly twice as fast as the adjacent ridges (Fig. 3). Unfortunately, this approach cannot provide an absolute erosion rate on the same timescale.

Together, burial dating and detrital tracers show that the Kings River has accomplished significant incision in the recent geologic past. Although not definitive, these results suggest that the Kings River Canyon formed during topographic rejuvenation, long after Mesozoic subduction ceased.

• McPhillips, D. and M.T. Brandon, 2010, Using tracer thermochronology to measure modern relief change in the Sierra Nevada, California: Earth and Planetary Science Letters, v. 296, p. 373-383.
• McPhillips, D. and M.T. Brandon, 2012, Topographic evolution of the Sierra Nevada measured directly by inversion of low-temperature thermochronology: American Journal of Science, v. 312, p. 90-116.
• Muir, J., 1891. A rival of the Yosemite: the Canyon of the South Fork of Kings River, California: The Century Illustrated Monthly Magazine, v. 43, p. 77-97.
• Stock, G.M., R.S. Anderson, R.C. Finkel, 2004, Pace of landscape evolution in the Sierra Nevada, California, revealed by cosmogenic dating of cave sediments: Geology, v. 32, p. 193-196.