『Abstract
Until recently, published rates of incision of bedrock valleys
came from indirect dating of incised surfaces. A small but growing
literature based on direct measurement reports short-term bedrock
lowering at geologically unsustainable rates. We report observations
of bedrock lowering from erosion pins monitored over 1-7 yr in
10 valleys that cut indurated volcanic and sedimentary rocks in
Washington, Oregon, California, and Taiwan. Most of these channels
have historically been stripped of sediment. Their bedrock is
exposed to bed-load abrasion, plucking, and seasonal wetting and
drying that communities hard, intact rock into plates or equant
fragments that are removed by higher flows. Consequent incision
rates are proportional to the square of rock tensile strength,
in agreement with experimental results of others. Measured rates
up to centimeters per year far exceed regional long-term erosion-rate
estimates, even for apparently minor sediment-transport rates.
Cultural artifacts on adjoining strath terraces in Washington
and Taiwan indicate at least several decades of lowering at these
extreme rates. Lacking sediment cover, lithologies at these sites
lowers at rates that far exceed long-term rock-uplift rates. This
rate disparity makes it unlikely that the long profiles of these
rivers are directly adjusted to either bedrock hardness or rock-uplift
rate in the manner predicted by the stream power law, despite
the observation that their profiles are well fit by power-law
plots of drainage area vs. slope. We hypothesize that the threshold
of motion of a thin sediment mantle, rather than bedrock hardness
or rock-uplift rate, controls channel slope in weak bedrock lithologies
with tensile strength below 〜3-5 MPa. To illustrate this hypothesis
and to provide an alternative interpretation for power-law plots
of area vs. slope, we combine Shields' threshold transport concept
with measured hydraulic relationships and downstream fining rates.
In contrast to fluvial reaches, none of the hundreds of erosion
pins we installed in steep valleys recently scoured to bedrock
by debris flows indicate any postevent fluvial lowering. These
results are consistent with episodic debris flows as the primary
agent of bedrock lowering in the steepest parts of the channel
network above 〜0.03-0.10 slope.
Keywords: geomorphology; erosion; neotectonics; rivers; weathering.』
Statement of the problem
Field sites
Olympic Mountains, Washington
Washington Cascades
Oregon Cascades
Oregon Coast Range
California Coast Range
Western Foothills, Taiwan
Methods
Results
Erosion pins
Folia
Strath terraces
Rock strength
Area vs. slope analysis
Discussion
Implications for concavity of long profiles
Conclusions
Acknowledgments
References cited