Edward Keller

Rhythmic spacing and origin of pools and riffles

Quantitative analysis of the spacing of pools in bedrock and alluvial stream channels in California, Indiana, Virginia, and North Carolina suggest that the tendency for streams to meander in the vertical (or third) dimension, as in the horizontal plane, is a fundamental characteristic of many streams that is independent of material type. Simple linear-regression and correlation models reveal that approximately 70% of the variability of the spacing of pools can be explained by the variability of channel width. Analysis of the spacing of 251 pools in eleven streams, utilizing the Kolmogorov-Smirnov goodness of fit test and one-way analysis of variance suggests that the hypothesis that the data from bedrock and alluvial channels are from the same population cannot be rejected at the 0.05 level of significance. Morphologic maps and field observations of stream channels incised in sandstone, limestone, metavolcanic rock, and syenite suggest that although these streams have much in common with alluvial stream channels, there exist considerable differences in certain aspects of channel morphology. This results because bedrock control of morphology locally may be more significant than the effects of general processes that tend to produce rhythmic channel forms such as pools and riffles. However, local controls tend to mask rather than destroy the effects of more general processes that produce the third dimension of meandering streams.

Areal Sorting of Bed-Load Material: The Hypothesis of Velocity Reversal: Reply

The hypothesis of velocity reversal seems adequate to explain the areal sorting of channel material, that is, relatively large material in riffles and finer material in pools. The hypothesis is based primarily on the measured observations that, with increasing discharge, the average bottom velocity of a pool increases faster than that of a riffle until at relatively high flow the average bottom velocity of the pool exceeds that of a riffle. The areal sorting produced by the velocity reversal occurs at flows of moderate frequency.

Chronology and rates of faulting of Ventura River terraces, California

New evidence concerning the chronology of four late Pleistocene terraces of the Ventura River near Oak View, California, that are vertically offset and tilted by reverse and reverse flexural-slip faults provides a means of estimating rates of fault movement and downcutting by the Ventura River. Radiocarbon ages of charcoal contained within terrace deposits date two of the terraces. Correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the Arroyo Parida-Santa Ana fault (not a flexural-slip fault) date the others. Resulting age estimates for the four main late Pleistocene terraces are Qt5b = 30,000 Qt6a = 38,000, Qt6b = 54,000, and Qt6c = 92,000 yr B.P. Vertical-slip rates on flexural-slip faults range from <0.3 to 1.1 mm/yr and apparently are related to the rate, form, and mechanics of folding of the north limb of the Ayers Creek–Canada Larga syncline. Average rates of downcutting of the Ventura River for several intervals during the late Pleistocene, estimated from the chronology and relative elevation of river terraces north of the Arroyo Panda–Santa Ana fault upstream from the zone of faulting, vary from ∼0.5 to 1.3 mm/yr. The range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level, climatic conditions, and/or regional deformation of the western Transverse Ranges. The average rate of downcutting of the Ventura River north of the zone of flexural-slip faulting is ∼0.8 mm/yr, compared with 1.2 to 2.2 mm/yr in the deformation zone immediately to the south. This apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system.

Fluvial sediment transport in response to moderate storm flows following chaparral wildfire, Ventura County, southern California

Chaparral wildfire has a profound effect on erosion and sedimentation in southern California. The Wheeler Fire in July 1985 burned the entire basin of a tributary (drainage area 2.14 km 2 ) of the north fork of Matilija Creek, near Ventura, California. After the fire, fine gravel was delivered to the channel by the process of dry ravel (dry particle-by-particle sliding of debris under the force of gravity) at a rate of 0.29 m 3 /km 2 /month. The first winter flow (2.1 m 3 /s) following the fire deposited 550 m 3 of fine gravel in the 270-m study reach near the mouth of the tributary. At least 90% of this fine gravel was derived from colluvium delivered by dry ravel processes from hillslopes near the channel. The second winter flow (2.5 m 3 /s) eroded the channel to the pre-fire thalweg. A reduction in particle size and critical shear stress associated with deposition of small gravel following the fire allowed these moderate-magnitude flows to transport large volumes of sediment. Deposits of two debris flows were identified in the tributary basin. Radiocarbon dating of these deposits gives dates of 1045 ± 95 yr B.P. for the older deposit and between 295 ± 35 and 385 ± 84 yr B.P. for the younger deposit. These dates indicate that the recurrence interval of large debris flows in the study basin is at least an order of magnitude greater than the recurrence interval of fire in the area.

Geomorphic criteria to determine direction of lateral propagation of reverse faulting and folding

Fault-related folds develop above active faults, and as these faults propagate laterally so do the folds they produce. Geomorphic criteria useful in evaluating rates and direction of lateral propagation of active folds in the direction of propagation are: (1) decrease in drainage density and degree of dissection; (2) decrease in elevation of wind gaps; (3) decrease in relief of the topographic profile along the crest; (4) development of characteristic drainage patterns; (5) deformation of progressively younger deposits or landforms; and (6) decrease in rotation and inclination of forelimb. All these criteria are consistent with lateral propagation, but do not prove it. The presence of more than one wind or water gap formed by the same stream, however, is strong evidence of lateral propagation. Rates of lateral propagation of folding may be several times the rate of uplift and fault slip. Lateral propagation of anticlinal folds allows for a new explanation of how drainage may develop across active fold belts. Development of drainage across an active fold belt is probably a function of relatively long structurally controlled drainage diversion parallel to fold axes and development of relatively short antecedent stream reaches, around the nose (plunge panel) of a fold. Water and/or wind gaps form as uplift, drainage diversion, and stream capture associated with fold growth continue.

Dynamic pedogenesis: New views on some key soil concepts, and a model for interpreting quaternary soils

Abstract Inasmuch as soils are open systems and rarely, if ever, achieve equilibrium with their environments, it is philosophically sound to view all soils as expressing varying levels of polygenesis as that term has been redefined. Soil genesis and resultant morphology may then be viewed in a comprehensive framework of soil evolution that consists of two linked pathways, one developmental and the other regressive, that reflect interactions of both exogenous and endogenous vectors (vectors are factors, processes, and conditions of pedogenesis). Following this philosophy, a model of pedogenesis is framed in an evolutional paradigm that emphasizes the integrated effects of dynamic and passive pedogenic vectors in directing pathways and in controlling rates of soil genesis through time. The dynamic vectors include energy and mass fluxes, frequencies of soil wetting and drying, water table dynamics, organisms, feedback processes, and pedoturbation. The passive vectors include parent materials, soil chemical environment, stability of geomorphic surfaces, and various evolved soil properties and conditions (accessions). Both sets of vectors vary spatially, and the dynamic vectors, more so than passive vectors, fluctuate through time. The model is expressed as S=f(D,P, dD dt , dD dt ) where S is the state of the soil or degree of profile evolution, D is the set of dynamic vectors, P is the set of passive vectors, and dD dt and dP dt denote change through time t. The model helps explain the apparent minimal development and regressed character of some old soils and the rapid and strong development of some young ones.

Geomorphic indicators of active fold growth: South Mountain–Oak Ridge anticline, Ventura basin, southern California

South Mountain–Oak Ridge, near Ventura, California, is an asymmetric anticlinal uplift forming at the present time above the active, buried Oak Ridge reverse fault. Shortening along the Oak Ridge fault accumulated largely in Quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0.5 m.y. Tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth. Stream-gradient indices are relatively high in the northern, eroded fold scarp of the ridge, a pattern consistent with the existence of active, rapid slip on the Oak Ridge fault. Mountain-front sinuosity along the northern slope of the anticlinal ridge roughly decreases from ∼2 to 1 toward the westernmost 10 km of observed surface folding. Valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from ∼1.5 to 0.5. Values of the hypsometric integral along the northern flank increase significantly from ∼0.35 to 0.4 (maximum ∼0.55) from east to west. Drainage density varies from ∼4 to 6 km/km2 along both flanks of the South Mountain–Oak Ridge anticline. Entrenchment of streams into the (southern) backlimb of the fold along the westernmost 9 km of the structure decreases from ∼20 m to <1 m from east to west. Apparent backlimb rotation, as measured by dip of strata along the westernmost 7 km of the fold, decreases from east to west, from ∼35° to 20°. Fold growth of the South Mountain–Oak Ridge anticline occurred during the past 0.5 Ma following deposition of the Saugus Formation. Lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried Oak Ridge fault.

Quaternary rate of folding of the Ventura Avenue anticline, western Transverse Ranges, southern California

Upper Quaternary terraces of the Ventura River, California, are uplifted, tilted, and folded over the Ventura Avenue anticline. Rates of uplift and tilting have decreased since inception of the structure over the past 200 ka. Assuming that the chronology, based on amino-acid racemization, 14C dates, and soils correlation, is approximately correct, then the minimum possible average rate of uplift in the axial region of the fold has decreased from ∼14 mm/yr to 2 mm/yr during the past 200 ka. Interval rates of uplift for the periods 200 ka to 80 or 105 ka, 80 or 105 ka to 30 ka, and 30 ka to present are, respectively, about 20 mm/yr, 9 mm/yr, and 5 mm/yr. The rate of tilting shows a similar trend, decreasing from ∼5.8 urad/yr, 2.5 urad/yr, and 1.2 urad/yr for the same time intervals, respectively. Based on the mechanics of flexural slip folds in stratified sedimentary rocks, these data suggest that the rootless Ventura Avenue anticline is a fold that has been shortening at a relatively constant rate of about 9 mm/yr since its inception.

Development of Alluvial Stream Channels: A Five-Stage Model

Pools, riffles, and point bars are common bed forms in straight and meandering channels. Inflections in meandering channels correspond to riffles in straight channels. Both of these forms are shoals that are symmetrical across the channel for a short distance. Pool-riffle spacing appears to be independent of channel pattern, and as straight reaches merge with meandering reaches no change occurs in the spacing, form, or symmetry of the pools and riffles. Meandering processes which result in increasing channel length may result in the addition of new pools o t keep spacing constant. A five-stage model is proposed to explain the development of alluvial channels. The model is based upon channel morphology, channel morphometry, and qualitative conclusions based on numerous field observations.

Tectonic geomorphology of the San Andreas fault zone in the southern Indio Hills, Coachella Valley, California

Geomorphic investigation of the San Andreas fault zone in the Indio Hills indicates many tectonically produced landforms, including beheaded streams, right-lateral deflected and offset streams, sags, shutter ridges, pressure ridges, and fault scarps. Near Biskra Palms, an alluvial fan-pediment complex has an apparent cumulative offset of about 0.7 km along the Mission Creek fault zone (north branch, San Andreas fault). Many of the tectonic landforms, as well as the fracture pattern that has developed during the Pleistocene, are explainable by simple shear or uplift associated with a small left bend in the main trace of the Mission Creek fault. The ratio of vertical to horizontal displacement in the vicinity of the bend is about 0.04. Exposed in Pushawalla Canyon, 5 km northwest of the alluvial fan-pediment complex, are: (1) a sequence of stream terraces, (2) folded Plio-Pleistocene fanglomerates, and (3) an example of stream capture following a right lateral deflection or offset of several hundred metres. A left step of the Mission Creek fault in Pushawalla Canyon is a probably cause of folds and sporadic uplift that produced the stream terraces. Possible cause of recent stream capture are: (1) juxtaposition of Pushawalla Canyon with a relict canyon moving northwestward along the Mission Creek fault, or (2) right-lateral deflection or offset of Pushawalla Canyon along the fault, with simultaneous headward erosion of a shorter, steep stream flowing toward the Coachella valley. Estimation of a slip rate and identification of paleoseismicity for the San Andreas fault in the Indio Hills is difficult. However, degree of topographic dissection, formation, and preservation of desert pavement, and relative soil profile development suggest that the age of the offset fan may be as old as 70,000 yr but that most likely it is on the order of 20,000 to 30,000 yr. These age estimates for the offset fan indicate a minimum slip rate for the San Andreas fault of 10 to 35 mm/yr, with about 23 to 35 mm/yr the most likely. The pattern of observed offset drainages is complex, but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right-lateral displacement.