David R. Montgomery

Soil erosion and agricultural sustainability

Data drawn from a global compilation of studies quantitatively confirm the long-articulated contention that erosion rates from conventionally plowed agricultural fields average 1–2 orders of magnitude greater than rates of soil production, erosion under native vegetation, and long-term geological erosion. The general equivalence of the latter indicates that, considered globally, hillslope soil production and erosion evolve to balance geologic and climate forcing, whereas conventional plow-based agriculture increases erosion rates enough to prove unsustainable. In contrast to how net soil erosion rates in conventionally plowed fields (≈1 mm/yr) can erode through a typical hillslope soil profile over time scales comparable to the longevity of major civilizations, no-till agriculture produces erosion rates much closer to soil production rates and therefore could provide a foundation for sustainable agriculture.

A systematic analysis of eight decades of incipient motion studies, with special reference to gravel-bedded rivers

Data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size, t* c 50 . Calculated t* c 50 values were stratified by initial motion definition, median grain size type (surface, subsurface, or laboratory mixture), relative roughness, and flow regime. A traditional Shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition; t* c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel Shields curves, with the latter generally underlying the former; values derived from competence functions define a separate but poorly developed field, while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous, rather than time-averaged, critical shear stresses. The available data indicate that for high critical boundary Reynolds numbers and low relative roughnesses typical of gravel-bedded rivers, reference-based and visually based studies have t* c50 ranges of 0.052-0.086 and 0.030-0.073, respectively. The apparent lack of a universal t*50 for gravel-bedded rivers warrants great care in choosing defendable t* c50 values for particular applications.

Channel network source representation using digital elevation models

Methods for identifying the size, or scale, of hillslopes and the extent of channel networks from digital elevation models (DEMs) are examined critically. We show that a constant critical support area, the method most commonly used at present for channel network extraction from DEMs, is more appropriate for depicting the hillslope/valley transition than for identifying channel heads. Analysis of high-resolution DEMs confirms that a constant contributing area per unit contour length defines the extent of divergent topography, or the hillslope scale, although there is considerable variance about the average value. In even moderately steep topography, however, a DEM resolution finer than the typical 30 m by 30 m grid size is required to accurately resolve the hillslope/valley transition. For many soil-mantled landscapes, a slope-dependent critical support area is both theoretically and empirically more appropriate for defining the extent of channel networks. Implementing this method for overland flow erosion requires knowledge of an appropriate proportionality constant for the drainage area-slope threshold controlling channel initiation. Several methods for estimating this constant from DEM data are examined, but acquisition of even limited field data is recommended. Finally, the hypothesis is proposed that an inflection in the drainage area-slope relation for mountain drainage basins reflects a transition from steep debris flow-dominated channels to lower-gradient alluvial channels.

Channel Initiation and the Problem of Landscape Scale

Since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape. This threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head. A field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth, undissected hillslopes and the valley bottoms to which they drain. This finding contradicts assertions that landscapes are scale-independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization.

Topographic controls on erosion rates in tectonically active mountain ranges

The functional relationship between erosion rate and topography is central to understanding both controls on global sediment flux and the potential for feedback between tectonics, climate, and erosion in shaping topography. Analysis of a high-resolution (10-m-grid) DEM transect across the convergent orogen of the Olympic Mountains reveals a non-linear relation between long-term erosion rates and mean slope, similar to a model for hillslope evolution by landsliding in steep terrain. The DEM data also reveal a relation between mean slope and mean local relief. Coarser-scale (1-km-grid) global analysis of the relation between erosion rate and mean local relief reveals different trends for areas with low erosion rates and tectonically active mountain ranges, with the composite relation being well-described by non-linear models. Together these analyses support the emerging view that erosion rates adjust to high rates of tectonically driven rock uplift primarily through changes in the frequency of landsliding rather than hillslope steepness, and imply that changes in local relief play a minor role in controlling landscape-scale erosion rates in tectonically active mountain ranges. - 2002 Elsevier Science B.V. All rights reserved.

LARGE WOODY DEBRIS JAMS, CHANNEL HYDRAULICS AND HABITAT FORMATION IN LARGE RIVERS

Field surveys document the accumulation of large woody debris (LWD) into structurally distinctive jam types in the alluvial channel of the Queets River on the Olympic Peninsula of north west Washington. Calculations, field observations and historical evidence show that these jams can form stable structures controlling local channel hydraulics and providing refugia for riparian forest development over decades and possibly centuries. Distinctive spatial patterns of LWD, pools, bars and forested islands form in association with particular jam types. The deposition of ‘key member’ logs initiates the formation of stable bar apex and meander jams that alter the local flow hydraulics and thereby the spatial characteristics of scour and deposition leading to pool and bar formation. Historical evidence and the age structure of forest patches documents the temporal development of alluvial topography associated with these jam types. Bar apex jams, for example, are associated with a crescentic pool, an upstream arcuate bar and a downstream central bar that is the focus of forest patch development. Experimental and empirical studies in hydraulic engineering accurately predict channel scour associated with jams. Individual jams can be remarkably stable, providing long-term bank protection that creates local refugia for mature forest patches within a valley floor environment characterized by rapid channel migration and frequent disturbance. Processes controlling the formation, structure and stability of naturally occurring LWD jams are fundamental to the dynamics of forested river ecosystems and provide insights into the design of both habitat restoration structures and ecosystem-based watershed management.

Where do channels begin

The closer channels begin to drainage divides, the greater will be the number of channels that occupy a unit area, and consequently the more finely dissected will be the landscape. Hence, a key component of channel network growth and landscape evolution theories1–7, as well as models for topographically controlled catch-ment runoff8, should be the prediction of where channels begin. Little field data exist, however, either on channel head locations9–14 or on what processes act to initiate and maintain a channel14–17. Here we report observations from several soil-mantled regions of Oregon and California, which show that the source area above the channel head decreases with increasing local valley gradient for slopes ranging from 5 to 45 degrees. Our results support a predicted relationship between source area and slope for steep humid landscapes where channel initiation is by landsliding, but they contradict theoretical predictions for channel initiation by overland flow in gentle valleys. Our data also suggest that, for the same gradient, drier regions tend to have larger source areas.

Pool Spacing in Forest Channels

Field surveys of stream channels in forested mountain drainage basins in southeast Alaska and Washington reveal that pool spacing depends on large woody debris (LWD) loading and channel type, slope, and width. Mean pool spacing in pool-riffle, plane-bed, and forced pool-riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing LWD loading, whereas pool spacing in generally steeper, step-pool channels is independent of LWD loading. Although plane-bed and pool-riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greater than 9 and 2–4 channel widths, respectively. Forced pool-riffle channels have high LWD loading, typical pool spacing of <2 channel widths, and slopes that overlap the ranges of free-formed pool-riffle and plane-bed channel types. While a forced pool-riffle morphology may mask either of these low-LWD-loading morphologies, channel slope provides an indicator of probable morphologic response to wood loss in forced pool-riffle reaches. At all study sites, less than 40% of the LWD pieces force the formation of a pool. We also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear-cut forests have lower LWD loading and hence fewer pools than reaches in pristine forests.

Climate, tectonics, and the morphology of the Andes

Large-scale topographic analyses show that hemisphere-scale climate variations are a first-order control on the morphology of the Andes. Zonal atmospheric circulation in the Southern Hemisphere creates strong latitudinal precipitation gradients that, when incorporated in a generalized index of erosion intensity, predict strong gradients in erosion rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial, glacial, and tectonic processes, and show that major morphologic features correlate with climatic regimes. Latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential, indicating that, like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes.

Geologic constraints on bedrock river incision using the stream power law

Denudation rate in unextended terranes is limited by the rate of bedrock channel incision, often modeled as work rate on the channel bed by water and sediment, or stream power. The latter can be generalized as KA m S n , where K represents the channel beds resistance to lowenng (whose variation with lithology is unknown), A is drainage area (a surrogate for discharge), S is local slope, and m and n are exponents whose values are debated. We address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method. We vary m, n, and K to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles. We find at least two end-member incision laws, KA 0.3-0.5 S for Australian rivers with stable base levels and K f A 0.1-0.2 S n for rivers in Kauai subject to abrupt base level change. The long-term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion, characterized by K f Incision patterns from Japan and California could follow either expression. If they follow the first expression with m = 0.4, K varies from 10 -7 -10 -6 m 0.2 /yr for granite and metamorphic rocks to 10 -5 -10 -4 m 0.2 /yr for volcaniclastic rocks and 10 -4 -10 -2 m 0.2 /yr for mudstones. This potentially large variation in K with lithology could drive strong variability in the rate of long-term landscape change, including denudation rate and sediment yield.