Stanley A. Schumm

EVOLUTION OF DRAINAGE SYSTEMS AND SLOPES IN BADLANDS AT PERTH AMBOY, NEW JERSEY

To analyze the development of erosional topography the writer studied geomorphic processes and landforms in a small badlands area at Perth Amboy, New Jersey. The badlands developed on a clay-sand fill and were morphologically similar to badlands and areas of high relief in semiarid and arid regions. A fifth-order drainage system was selected for detailed study. Composition of this drainage network conforms to Hortons laws. Within an area of homogeneous lithology and simple structure the drainage network develops in direct relation to a fixed value for the minimum area required for channel maintenance. Observed relationships between channel length, drainage-basin area, and stream-order number are dependent on this constant of channel maintenance which is in turn dependent on relative relief, lithology, and climate of any area. Other characteristics of the drainage network and topography such as texture, maximum slope angles, stream gradients, drainage-basin shape, annual sediment loss per unit area, infiltration rate, drainage pattern, and even the morphologic evolution of the area appear related to relative relief expressed as a relief ratio, the height of the drainage basin divided by the length. Within one topographic unit or between areas of dissimilar but homogeneous lithology the relief ratio is a valuable means of comparing geomorphic characteristics. Hypsometric curves are available for a series of 11 second-order drainage basins ranging in stage of development from initial to mature. Relief ratio and stream gradients attain a constant value when approximately 25 per cent of the mass of the basin has been eroded. Basin shape becomes essentially constant at 40 per cent of mass removed in accord with Strahlers hypothesis of time-independent forms of the steady state. Comparison of the drainage pattern as mapped in 1948 with that of 1952 reveals a systematic change in angles of junction and a shift of the entire drainage pattern accompanying changes in the ratio between ground and channel slope. Field observations and experimental studies suggest that badland slopes may retreat in parallel planes and that the rate of erosion on a slope is a function of the slope angle. The retreat of slopes may not conform to accepted concepts of runoff action as a function of depth and distance downslope. Runoff occurs as surge and subdivided flow which may be closely analogous to surficial creep. Rills follow a definite cycle of destruction and reappearance throughout the year under the action of runoff and frost heaving. At Perth Amboy, slopes are initiated by channel degradation and maintained by runoff and by creep induced through frost heaving. Runoff or creep may form convex divides, and both parallel and declining slope retreat are important in the evolution of stream-carved topography. Hypsometric curves reveal that the point of maximum erosion within a drainage basin migrates upchannel and that the mass-distribution curve of any basin has a similar evolution to that of the longitudinal stream profile. Comparative studies in badland areas of South Dakota and Arizona confirm conclusions drawn at Perth Amboy and show the importance of infiltration of runoff on topographic development and of subsurface flow in slope retreat and miniature pediment formation.

River Response to Baselevel Change: Implications for Sequence Stratigraphy

Baselevel is the imaginary horizontal level or surface to which sub-aerial erosion proceeds. It is sea level. Controversy surrounds the effect of baselevel change on river behavior, the rejuvenation of landscapes, and the delivery of sediment to the shelf-slope depositional system. The effect of baselevel change depends upon many factors, such as rate of change, amount of change, direction of change, river character, and dynamics and erodibility of the sediment source area. In most cases the effects of baselevel change will be moderate, and they can be accommodated by changes of channel pattern, width, depth, and roughness. Therefore, the delivery of large amounts of sediment to a shoreline or continental shelf probably reflects not only baselevel lowering, but significant uplift of the sediment-source area and perhaps climate change.

Experimental Study of Channel Patterns

A series of experiments was performed in a large flume to determine the effect of slope and sediment load on channel patterns. Sediment loads and slopes were closely related, and as slope and sediment loads increased, threshold values of these variables were encountered, at which channel patterns altered significantly. At a very low slope and sediment load, the channels remained straight, but at a discharge of 0.15 cfs, a meandering-thalweg channel formed at slopes greater than 0.002. With increased slope and sediment loads, thalweg sinuosity increased to a maximum of 1.25. At slopes greater than 0.016, a braided channel formed. The model channels responded to increased sediment loads by maintaining steeper gradients and by major channel pattern changes, but at very gentle slopes and at steep slopes, the channel could not be forced to develop a meandering thalweg. These experiments suggest that landforms may not always respond progressively to altered conditions. Rather, dramatic morphologic changes can occur abruptly when critical erosional and (or) depositional threshold values are exceeded. The meandering-thalweg channel was not a meandering channel. A truly meandering channel with a sinuosity of 1.3 formed when a suspended-sediment load (3 percent concentrations of kaolinite) was introduced into the flow. The clay stabilized the alternate bars, and scour and deepening of the thalweg resulted. This in turn lowered the water level at constant discharge, and the alternate bars emerged o t form point bars. A meandering-thalweg channel was thus converted to a meandering channel by the type of sediment load change that has accompanied climatic and hydrologic changes of the recent geologic past.

Geomorphic thresholds: the concept and its applications'

Geomorphic thresholds were defined initially as the condition at which there is a significant landform change without a change of external controls such as base level, climate and land use. Landforms evolve to a condition of incipient instability following which change or failure occurs. Subsequently, through usage, the definition has been broadened to include abrupt landform change as a result of progressive change of external controls. Therefore, it is now appropriate to recognize both intrinsic and extrinsic geomorphic thresholds. The threshold concept has practical significance. If the threshold conditions can be recognized, not only will different explanations for some landforms emerge but also the ability to identify incipiently unstable landforms and to predict their change will be of value to land managers and engineers. For example, the development of gullies and fan-head trenches can be explained by the depositional steepening of valley floors and fan-heads to threshold slope. As a consequence, as yet ungullied but potentially unstable areas can be recognized. In addition, channel pattern variations and the conversion of meandering channels to braided ones, and of braided channels to single-thalweg sinuous ones can occur naturally at pattern thresholds. Such changes can also be accomplished artificially, when it is recognized that a channel is near a pattern threshold. Sediment yield variations will be related to these periods of instability. Recognition of this will aid in the explanation of some hydrologic, sedimentologic, and stratigraphic anomalies.

Speculations Concerning Paleohydrologic Controls of Terrestrial Sedimentation

The relations that have been recorded among modern climatic, phytologic, and hydrologic data are used to speculate about the effects of evolving vegetation on the hydrologic cycle. At present the peak of erosion rates occurs in semiarid regions, whereas during prevegetation time erosion rates rose to a plateau, the magnitude of which depended upon the erodibility and weathering characteristics of the rocks. With the appearance of terrestrial vegetation and its colonization of the earths surface, erosion rates decreased, as did runoff and flood peaks. A review of the relations existing between the morphologic and hydrologic characteristics of river channels demonstrates that fluvial sedimentary deposits are significantly different depending upon the nature of the sediment load moved through the channel. Combining the conclusions obtained from an analysis of hydrologic relations with conclusions concerning effects of type of sediment load upon river morphology, it is possible to speculate on the changing nature of the land phase of the hydrologic cycle before and during the colonization of the landscape by vegetation. During prevegetation time, bed-load channels moved coarse sediments from their sources and spread them as sheets on piedmont areas. With increased plant cover, alluvial deposits were stabilized, but large floods caused periodic flushing of sediment from the system, thereby creating cyclic sedimentary deposits. The influence of climate change on the volume and type of sediment moved from an erosional system became more pronounced as the effect of vegetation on the hydrologic cycle increased. Finally, with the appearance of grasses during the Cenozoic Era, the relations between climate, vegetation, erosion, and runoff became much as today except for the influence of man.

Sinuosity of Alluvial Rivers on the Great Plains

Data on the morphologic and sediment characteristics of stable alluvial rivers of the Great Plains were collected at 50 cross sections. The channel patterns of these rivers were classified into five types: tortuous, irregular, regular, transitional, and straight. Because no clear demarcation existed between each of the types, the pattern of the rivers was described by sinuosity, a ratio of channel length to valley length. The sinuosity ( (P) ) of these rivers is related to the shape of the channels expressed as a width-depth ratio ( F ) and to the percentage of silt and clay in the perimeter of the channel ( M ) as follows: ![Formula][1] ![Formula][2] Sinuous streams are characterized by a low width-depth ratio ( F ), a high percentage of silt-clay in the perimeter of the channel ( M ), a high percentage of silt-clay in the banks (although the banks of straight channels may also contain large amounts of silt-clay), and a lower gradient than straight channels having the same mean discharge. Discharge itself does not appear to affect the sinuosity of streams. Another possible distinction between straight and sinuous streams is in the proportions of the components of total sediment load. In a wide, shallow channel much of the sediment transported is bed-material load. In a narrow, deep channel most of the sediment transported is wash load. On the Great Plains both straight and sinuous streams may flow on the surface of alluvial valley fills at about the same valley slope. The departure of a stream from a straight course down the alluvial valley results from changes in both the caliber of the sediment load and in the relative proportions of bed-material load and wash load during the post-Pleistocene alluviation of these valleys. When during this alluviation the proportion of wash load increased, most probably by a decrease in bed-material load, the stream adjusted itself by decreasing its gradient through the development of a sinuous course. Recent changes in stream sinuosity in response to changes in the proportions of bed load and suspended load support this hypothesis. [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif

Geomorphic and sedimentary response of rivers to tectonic deformation: a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings

Abstract Rivers are extremely sensitive to subtle changes in their grade caused by tectonic tilting. As such, recognition of tectonic tilting effects on rivers, and their resultant sediments, can be a useful tool for identifying the often cryptic warping associated with incipient and smaller-scale epeirogenic deformation in both modern and ancient settings. Tectonic warping may result in either longitudinal (parallel to floodplain orientation) or lateral (normal to floodplain orientation) tilting of alluvial river profiles. Alluvial rivers may respond to deformation of longitudinal profile by: (1) deflection around zones of uplift and into zones of subsidence, (2) aggradation in backtilted and degradation in foretilted reaches, (3) compensation of slope alteration by shifts in channel pattern, (4) increase in frequency of overbank flooding for foretilted and decrease for backtilted reaches, and (5) increased bedload grain size in foretilted reaches and decreased bedload grain size in backtilted reaches. Lateral tilting causes down-tilt avulsion of streams where tilt rates are high, and steady down-tilt migration (combing) where tilt rates are lower. Each of the above effects may have profound impacts on lithofacies geometry and distribution that may potentially be preserved in the rock record. Fluvial sedimentary evidence for past tilting is traditionally based on the assumption that depositional features reminiscent of modern fluvial tectonic effects are evidence for past tectonic effects where it is closely associated with historically active structures, or where non-tectonic causes cannot be invoked; however, caution must be exercised when using these effects as criteria for past or current tectonic warping, as these effects may be caused by non-tectonic factors. These non-tectonic causes must be eliminated before tectonic interpretations are made.

Gully Erosion, Northwestern Colorado: A Threshold Phenomenon

The widespread occurrence of discontinuous gullies in the oil-shale region of northwestern Colorado is of particular concern because of the resulting progressive destruction of the valley floors. Furthermore, the integration of a semi-arid drainage network can cause a rapid increase in the sediment yield of the basin, with subsequent harmful effects downstream. Field work in the Piceance Creek and Yellow Creek drainage basins indicates that these discontinuous gullies developed on oversteepened segments of the valley floors. Although the critical slope of entrenchment is probably related to magnitude of run-off, discharge measurements are not available; therefore, drainage-basin area was selected as the most representative measure of discharge. An inverse relation between drainage-basin area and critical slope of entrenchment applies, and the lower limit of scatter of the data establishes a critical slope-area relation, which can be used to identify potentially unstable valley floors. This relation can help the land manager determine areas of instability where preventive measures can most economically and successfully be undertaken. It is stressed that this particular quantitative relation is applicable only to the Piceance Creek and Yellow Creek drainage basins. In more heterogeneous basins, other variables will need to be included in the analysis; however, the general theory of valley stability will remain applicable.

Experimental Study of River Incision

Experiments in a 60-ft-long tilting, recirculating flume were conducted to study river incision in simulated bedrock, which was a mixture of sand and kaolinite. Slope, sediment feed, and water discharge were controlled during the development of four channels. After an increase of slope at constant discharge, the following sequence of erosion occurred: (1) development of longitudinal lineations, ripples, and potholes; (2) enlargement of the lineations into prominent grooves; (3) coalescence of the grooves into a single, narrow, and deep inner channel. The inner channel was incised below base level and a sequence of bedrock lows and highs formed. Bedrock scour lows had a weakly regular spacing during incision and a randomly clustered spacing following aggradation. Incision around stabilized alternate bars in a sinuous sand-bed channel resulted in destruction of the bars and maximum scour where the flow was locally constricted. In an initially sinuous bedrock channel, scour depth was greater at bends than at crossings. Provided all of the available sediment load was entrained, the bed was eroded more at convex banks of bends than at concave banks. However, after deposition occurred, the maximum erosion shifted to the concave bank. These results indicate that lateral or vertical incision at bends of incised meandering streams is controlled by the amount of available sediment load entrained by channel-forming discharges. The results also suggest that incised meanders superposed from an earlier pattern on a peneplain should rarely occur in nature, if epeirogenic tilting caused the incision. Representing the locus of deepest scour by a bedrock stream, inner channels may be the locations of heavy mineral concentrations as well as gravel deposits. The experimental results help to explain inner channels discovered at damsites, provide an explanation for some paleochannels in California and South Africa, and suggest that, like the Dalles type of river channel, bedrock floors of valleys will be uneven in both transverse and longitudinal sections.

Ephemeral-stream processes: Implications for studies of quaternary valley fills

Abstract Three unstable ephemeral-stream channels (arroyos), which drain source areas that have high sediment yields ranging from predominantly sand (Arroyo Calabasas) to a mixture of sand, silt, and clay (Sand Creek) to largely silt and clay (Sage Creek), were resurveyed to provide data on the rates and mechanics of erosion and sedimentation processes during periods ranging from 14 to 22 yr. Channel morphology changed significantly. Erosion occurred through nickpoint recession and bank collapse, but erosional reaches are separated by aggrading or stable-channel reaches. In general, sediment that is eroded, as the nickpoint recedes upstream, is trapped in the widened channel downstream. In this manner sediment is transported episodically out of these basins during a series of cut-and-fill cycles. The manner by which the channels aggrade and the morphology of the aggraded stable channels are controlled by the sediment type. The wide and shallow channel of Arroyo Calabasas is filled by vertical accretion of sand-size sediment. The narrow and deep channels of Sage Creek and Sand Creek are created by the lateral accretion of cohesive fine-grained sediment. The channel modification and the cut-and-fill episodes are dependent on high sediment yields, and therefore they are independent of subtle climatic shifts. Cut-and-fill deposits that have been created in this manner should not be equivalent in age from basin to basin, and therefore channel trenching and filling in the semiarid western United States during the Holocene need not be synchronous.