Sediment size matters in high-elevation erosion rates

For the past four summers, Cliff Riebe, a UW associate professor in the Department of Geology and Geophysics, and Claire Lukens, a UW doctoral student majoring in geology, have studied sediment in Inyo Creek, in the High Sierra in California.

The two found that cold, steep, high-elevation slopes with less vegetation produce coarser and larger sediment than low-elevation, gentle slopes. This finding quantifies how sediment production varies with topography and suggests that variations in climate, topography and weathering rates may shape the evolution of mountain landscapes by influencing sediment size.

Riebe is lead author of a paper, titled "Climate and Topography Control the Size and Flux of Sediment Produced on Steep Mountain Slopes," published online Nov. 16 in the Proceedings of the National Academy of Sciences (PNAS).

Both the size and flux of sediment from slopes can influence channel incision, making sediment production and erosion central to the interplay of climate and tectonics in landscape evolution, Riebe says.

"Rivers need tools to cut into their beds," Riebe says. "Water alone can't do the job. And the bigger the sediment is, the easier it is for the river to carve into the landscape. So, when it comes to sediment, it turns out that size really does matter."

"Sediment can be as large as boulders at higher elevations and as fine as sand at lower elevations on the landscape," Lukens adds. "We know this is true from our analyses of sediment in the stream. In effect, we are using geochemistry to interrogate stream sediment about where it comes from and how fast it is eroding."

Erosion rates commonly are measured using cosmogenic nuclides, which serve as tracers of erosion because they accumulate in minerals in the uppermost few meters of rock and soil during the exhumation to the landscape surface. For example, the isotope beryllium 10 is produced from oxygen by nuclear reactions in quartz as the mineral rises to the surface.

Riebe and Lukens combined this technique with another sediment-tracing tool called detrital thermochronometry, which identifies the elevations of hill slopes where sediment was produced by weathering of underlying bedrock. The two used computer simulations to determine the statistical significance of their findings.

"This is the first time these tools have been combined in this way," Lukens says.

For a long time, geologists have been able to quantify how fast sediment is eroding from landscapes. Until this UW research, there has been no complementary method to quantify how the size distribution of sediment particles varies across slopes where the sediment is produced from bedrock by weathering and erosion.