A drainage basin simulation model introduced here incorporates creep and threshold slumping and both detachment- and transport-limited fluvial processes. Fluvial erosion of natural slopes and headwater channels is argued to be dominantly detachment-limited. Such slopes undergo nearly parallel retreat and replacement with alluvial surfaces under fixed base level, in contrast with gradual slope decline for transport-limited conditions. The arrangement of divides and valleys is sensitive to initial conditions, although average morphology is insensitive. Dissected, initially flat surfaces in which downstream concavity is slight exhibit nearly parallel drainage, compared to very wandering main valleys when concavity is great. Steady state is reached after a cumulative base level drop approximately 3 times the final relief. Simulated valley systems are similar to those predicted by a previous model of optimal drainage basins. A critical value of slope divergence normalized by average slope gradient is a useful criterion for defining the valley network.

/pdf/a-detachment-limited-model-of-drainage-basin-evolution-3o7nban85w.pdf

A detachment-limited model of drainage basin evolution

Expression of active tectonics in erosional landscapes

The dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state. This concept is often invoked, but the nature of the steady state is commonly not specified. We identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical-model results and natural examples. These types are (1) flux steady state, (2) topographic steady state, (3) thermal steady state, and (4) exhumational steady state: they refer to the erosional flux, the topography, the subsurface temperature field, and the spatial pattern of cooling ages, respectively. Models suggest that the topography will reach a steady mean form at the scale of an orogenic belt, but perfect topographic steady state is unlikely to be achieved at shorter length scales. Thermal steady state is a precondition for exhumational steady state and in the case of temperature-dependent deformation, topographic steady state. Exhumational steady state is characterized by reset age zones spatially nested according to closure temperature, as illustrated in natural systems from New Zealand, the Cascadia accretionary margin, and Taiwan.

On steady states in mountain belts

The area studied, mostly in the headwaters of the Shenandoah River, Augusta andi Rockingham Counties, Va., includes about 55 square miles of densely forested mountain land and has an average relief of about 1,500 feet. It is part of an area that in June 1949 was subjected to a violent cloudburst which damaged large tracts on slopes and bottom lands. Most of the area is underlain by flaggy arkosic sandstone and interbedded reddish shale of the Hampshire formation of Devonian age. The highest ridges are capped by massive sandstone of the Pocono formation of Mississippian age. In most of the area the rocks dip gently to the southeast but in the northwestern and southeastern parts they are folded into synclines that localize northeastward-trending ridges. Topography is remarkably uniform and the slopes are graded to regular forms that may be described in simple mathematical terms. Because of a high bifurcation ratio between streams of different orders, almost half of the area is composed of firstorder valleys. Each of the first-order valleys may be subdivided! into five different parts based on the convexity or concavity of the contours. Each part, because of its geometric form, receives water downslope at a different rate. Tbe driest area on which the contours are convex outward (from the mountain) is called the "nose" and presumably receives the least moisture by flow from the slope above. The side slope has straight contours and hypothetically receives somewhat greater runoff. The hollow above the stream head is an area of concentration of the drainage lines in which the discharge of storm runoff is inferred to increase downslope at a rate greater than the square of the distance. It merges with the channelway, a narrow strip running down the valley axis between steep side slopes. In the channelway, runoff increases downstream in proportion to the 1.6 power of the distance. The channelway at some places is bordered by a narrow strip of concave-upward slope that because of its geometric form receives somewhat more runoff than the side slopes above. In valleys larger than the first and second order the channelway is so broad that the stream channel itself is bordered by bottom lands many times larger. All of the bottom lands are subject to erosion or deposition during floods of various frequencies. The stony and bouldery soils of the mountain slopes were sampled at many localities by measuring the diameters of individual particles selected at points on a gridi pattern. The mean size of the particles on the surface of the ground was found to vary markedly from one part of a valley to another and to increase roughly with the runoff concentration inferred from the topography. The texture of debris is fine on noses and coarse in channelways and hollows. Mean size ranges from *Johns Hopkins University, formerly of Harvard University. less than 1 mm on some noses to over 250 mm in some hollows. In the channelway the size generally decreases to about 100 mm and remains nearly constant in a downstream direction. Standard deviation or sorting of the particles narrows as drainage area increases. Fields of large, angular quartzite blocks are conspicuous features of some side slopes and noses. They generally are present where massive quartzitic sandstone of the Pocono formation crops out in an exposed location such as a ridge top or nose, and where the sandstone is underlain by relatively soft shale. In these places boulders have slid down, forming a mantle on

Geomorphology and forest ecology of a mountain region in the central Appalachians

[1] Mechanistic theories of fluvial erosion are essential for quantifying large-scale orogenic denudation. We examine the topographic implications of two leading classes of river erosion model, detachment-limited and transport-limited, in order to identify diagnostic and testable differences between them. Several formulations predict distinctly different longitudinal profile shapes, which are shown to be closely linked to terrain morphology. Of these, some can be rejected on the basis of unrealistic morphology and slope-area scaling. An expression is derived for total drainage basin relief and its apportionment between hillslope and fluvial components. Relief and valley density are found to vary with tectonic forcing in a manner that reflects erosion physics; these properties therefore constitute an additional set of testable predictions. Finally, transient responses to tectonic perturbations are shown to depend strongly on the degree of nonlinearity in the incision process. These findings indicate that given proper constraints, fluvial erosion theories can be tested on the basis of observed topography.

/pdf/topographic-outcomes-predicted-by-stream-erosion-models-3x3t191hfy.pdf

Topographic outcomes predicted by stream erosion models: Sensitivity analysis and intermodel comparison

Morphological Analysis of Land Forms

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Morphological Analysis of Land Forms