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by California Current evaporative yield, which increases when COMMENTS AND REPLIESthe Japan Kuo Shio (warm) influence dominates and decreases Online: GSA Today, Comments and Replies when Arctic effects prevail on the current. There is evidence Published Online: April 2006 (Dunai et al. 2005; Garner, 1983) that Humboldt Current fri-gidity and related Atacama Desert have continued unchanged since the Oligocene. That current yields virtually no moisture to the land, and when the California Current is coldest, the results are probably the same. Stream discharge and erosion would be much reduced. A warmer current would reverse the effect, so I argue that accelerated stream incision attributed to uplift by Clark et al. (2005) was as probably caused by ocean The non-equilibrium landscape warming.of the Sierra Nevada, California Clark et al.’s (2005) identification of an elevated, slightly dis-sected low relief surface they term a relict landscape is impor-tant because mountains start out low, and low land by a cold H.F. Garner, 202 Merrywood Dr., Forest, Virginia 24551-1104, USA sea is often arid. An increase in water warmth would encour-age adjacent fluvial dissection, and such a history is indicated for the Sierra Nevada relict surface.Clark et al. (2005) describe longitudinal profiles of rivers in the southern Sierra Nevada they (1) take to be graded, (2) assume The Sierra Nevada orogen has been undergoing isostatic to debouch on a well-fixed regional base ...

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Publié par	Ornui
Nombre de lectures	35
Langue	English

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e10

doi: 10.1130/GSATOFe10

The non-equilibrium landscape

of the Sierra Nevada, California

H.F. Garner,

202 Merrywood Dr., Forest, Virginia 24551-1104, USA

Clark et al. (2005) describe longitudinal profiles of rivers in the

southern Sierra Nevada they (1) take to be graded, (2) assume

to debouch on a well-fixed regional base level, (3) assume to

erode better when the surface beneath is being elevated tec-

tonically, and (4) assume that when a river mouth has a change

in base-level target, that information is somehow conveyed

upstream to tributaries that respond by altering contiguous hill-

slopes. Seemingly, water is implicitly assumed to be “on tap”

and stands “at the ready” when erosion is called for by earth

movements—concepts that stem from William Morris Davis

(1899, 1902).

Taking the foregoing assumptions in order: (1) The graded

stream was the elusive unicorn of the Davisian Geographical

Cycle. In a century, no one has ever bragged of finding one

(Dury, 1966; Garner, 1974). (2) Regional base level is not a firm

limit on river erosion (Wheeler, 1964; Garner, 1965). Running

water erosion does not really stop there. And the sea level

target moves up and down 100 m or more during glacial epi-

sodes (as should stream profiles tied to it). (3) The uplift/ero-

sion notion harks back to the stream rejuvenation mythology

of the Davisian era. Running water erosion in streams mov-

ing several feet per second is, however, largely the product

of vortex action along water shears that form downflow from

obstacles, and as such can hardly respond to uplift of a few

millimeters per century. (4) Both the Kern and Kings rivers

flow in extremely deep valleys—and at least the Kern River

channel is armored. For kilometers, its channel is choked with

outhouse-size angular blocks of rock, probably introduced by

mass wasting. They constitute a local erosional base level for

any upstream tributaries and would prohibit any upstream pas-

sage of an erosional signal.

At least as important to Sierra Nevada river behavior as

any channel morphology is discharge, which depends largely

upon evaporation yield from the adjacent California Current.

Like its Humboldt Current counterpart along west coast South

America, the California Current moisture yield depends on its

temperature, which can vary. Sierra precipitation is governed

by California Current evaporative yield, which increases when

the Japan Kuo Shio (warm) influence dominates and decreases

when Arctic effects prevail on the current. There is evidence

(Dunai et al. 2005; Garner, 1983) that Humboldt Current fri-

gidity and related Atacama Desert have continued unchanged

since the Oligocene. That current yields virtually no moisture

to the land, and when the California Current is coldest, the

results are probably the same. Stream discharge and erosion

would be much reduced. A warmer current would reverse the

effect, so I argue that accelerated stream incision attributed to

uplift by Clark et al. (2005) was as probably caused by ocean

warming.

Clark et al.’s (2005) identification of an elevated, slightly dis-

sected low relief surface they term a relict landscape is impor-

tant because mountains start out low, and low land by a cold

sea is often arid. An increase in water warmth would encour-

age adjacent fluvial dissection, and such a history is indicated

for the Sierra Nevada relict surface.

The Sierra Nevada orogen has been undergoing isostatic

uplift due to erosional unloading since Jurassic time. Its posi-

tion close to the California Current subjects it to variable pre-

cipitation and stream discharge, and the tying of each acceler-

ated river erosion episode to uplift does not fit the situation.

The Sierra Nevada orogen will be incised by rivers whether it

is being uplifted or not.

REFERENCES CITED

Clark, M.K., Gweltaz, M., Saleeby, J., and Farley, K.A., 2005, The non-equilibrium land-

scape of the southern Sierra Nevada, California: GSA Today, v. 15, no. 9, p. 4–10.

Davis, W.M., 1899, The geographical cycle: The Geographical Journal, v. 14, p. 481–

504.

Davis, W.M., 1902, Baselevel, grade and peneplain: Journal of Geology, v. 10, p. 77–111.

Dunai, T.J., and González López, G.A., and Juez-Larré, J., 2005. Oligocene-Miocene

age of aridity in the Atacama Desert revealed by exposure dating of erosion-sen-

sitive landforms: Geology, v. 33, p. 321–324, doi: 10.1130/G21184.1.

Dury, G.H., editor, 1966, The concept of grade,

Essays in Geomorphology: New York,

American Elsevier, p. 211–233.

Garner, H.F., 1965, Baselevel control of erosion surfaces: Arkansas: Academy of

Sciences Proceedings, v. 19, p. 98–104.

Garner, H.F., 1974, The origin of landscapes; a synthesis of geomorphology: Oxford,

Oxford University Press, 734 p.

Garner, H.F., 1983, Large-scale tectonic denudation and climatic morphogenesis in the

Andes Mountains of Ecuador,

Gardner, R., and Scoging, H., eds., Mega-geo-

morphology: Oxford, Clarendon Press, p. 1–17.

Wheeler, H.E., 1964, Base level, lithosphere surface, and time stratigraphy: Geological

Society of America Bulletin, v. 75, p. 599–610.

Manuscript accepted 17 November 2005; published online

April 2006.

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